myfirstapp.py

Sample code in python 3

Sample code in python 3

Sample code in python 3

Comments in Python:

Sample code in python 3

Docstrings are also comments:

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Creating Variables

Unlike other programming languages, Python has no command for declaring a variable.

A variable is created the moment you first assign a value to it.

Variables do not need to be declared with any particular type and can even change type after they have been set.

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Variable Names

A variable can have a short name (like a and b) or a more descriptive name (name, amount, address). Rules for Python variables:

• A variable name must start with a letter or the underscore character
• A variable name cannot start with a number
• A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
• Variable names are case-sensitive (pcds, Pcds and PCDS are three different variables)

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The Python print statement is often used to output variables.

To combine both text and a variable, Python uses the + character:

You can also use the + character to add a variable to another variable:

For numbers, the + character works as a mathematical operator:

If you try to combine a string and a number, Python will give you an error:

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Python Numbers

There are three numeric types in Python:

• int
• float
• complex

Variables of numeric types are created when you assign a value to them:

To verify the type of any object in Python, use the type() function:

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Int

Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length.

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Float

Float, or "floating point number" is a number, positive or negative, containing one or more decimals.

Float can also be scientific numbers with an "e" to indicate the power of 10.

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Complex

Complex numbers are written with a "j" as the imaginary part:

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Python Casting

Specify a Variable Type

There may be times when you want to specify a type on to a variable. This can be done with casting. Python is an object-orientated language, and as such it uses classes to define data types, including its primitive types.

Casting in python is therefore done using constructor functions:

• int() - constructs an integer number from an integer literal, a float literal (by rounding down to the previous whole number), or a string literal (providing the string represents a whole number)
• float() - constructs a float number from an integer literal, a float literal or a string literal (providing the string represents a float or an integer)
• str() - constructs a string from a wide variety of data types, including strings, integer literals and float literals

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Python Strings

String Literals

String literals in python are surrounded by either single quotation marks, or double quotation marks.

'hello' is the same as "hello".

Strings can be output to screen using the print function. For example: print("hello").

Like many other popular programming languages, strings in Python are arrays of bytes representing unicode characters. However, Python does not have a character data type, a single character is simply a string with a length of 1. Square brackets can be used to access elements of the string.

Command-line String Input

Python allows for command line input.

That means we are able to ask the user for input.

The following example asks for the user's name, then, by using the input() method, the program prints the name to the screen:

Save this file as pcdsinput.py, and load it through the command line:

Our program will prompt the user for a string:

The user now enters a name:

Then, the program prints it to screen with a little message:

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Python Operators

Operators are used to perform operations on variables and values.

Python divides the operators in the following groups:

• Arithmetic operators
• Assignment operators
• Comparison operators
• Logical operators
• Identity operators
• Membership operators
• Bitwise operators

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Python Arithmetic Operators

Arithmetic operators are used with numeric values to perform common mathematical operations:

Operator	Name	Example	Try it
+	Addition	x + y	Try it »
-	Subtraction	x - y	Try it »
*	Multiplication	x * y	Try it »
/	Division	x / y	Try it »
%	Modulus	x % y	Try it »
**	Exponentiation	x ** y
//	Floor division	x // y

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Python Assignment Operators

Assignment operators are used to assign values to variables:

Operator	Example	Same As	Try it
=	x = 5	x = 5	Try it »
+=	x += 3	x = x + 3	Try it »
-=	x -= 3	x = x - 3	Try it »
*=	x *= 3	x = x * 3	Try it »
/=	x /= 3	x = x / 3	Try it »
%=	x %= 3	x = x % 3	Try it »
//=	x //= 3	x = x // 3	Try it »
**=	x **= 3	x = x ** 3	Try it »
&=	x &= 3	x = x & 3	Try it »
|=	x |= 3	x = x | 3	Try it »
^=	x ^= 3	x = x ^ 3	Try it »
>>=	x >>= 3	x = x >> 3	Try it »
<<=	x <<= 3	x = x << 3	Try it »

Python Comparison Operators

Comparison operators are used to compare two values:

Operator	Name	Example	Try it
==	Equal	x == y	Try it »
!=	Not equal	x != y	Try it »
>	Greater than	x > y	Try it »
<	Less than	x < y	Try it »
>=	Greater than or equal to	x >= y	Try it »
<=	Less than or equal to	x <= y	Try it »

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Python Logical Operators

Logical operators are used to combine conditional statements:

Operator	Description	Example	Try it
and	Returns True if both statements are true	x < 5 and  x < 10	Try it »
or	Returns True if one of the statements is true	x < 5 or x < 4	Try it »
not	Reverse the result, returns False if the result is true	not(x < 5 and x < 10)	Try it »

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Python Identity Operators

Identity operators are used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location:

Operator	Description	Example	Try it
is	Returns true if both variables are the same object	x is y	Try it »
is not	Returns true if both variables are not the same object	x is not y	Try it »

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Python Membership Operators

Membership operators are used to test if a sequence is presented in an object:

Operator	Description	Example	Try it
in	Returns True if a sequence with the specified value is present in the object	x in y	Try it »
not in	Returns True if a sequence with the specified value is not present in the object	x not in y	Try it »

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Python Bitwise Operators

Bitwise operators are used to compare (binary) numbers:

Operator	Name	Description
&	AND	Sets each bit to 1 if both bits are 1
|	OR	Sets each bit to 1 if one of two bits is 1
^	XOR	Sets each bit to 1 if only one of two bits is 1
~	NOT	Inverts all the bits
<<	Zero fill left shift	Shift left by pushing zeros in from the right and let the leftmost bits fall off
>>	Signed right shift	Shift right by pushing copies of the leftmost bit in from the left, and let the rightmost bits fall off

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Python Lists

Python Collections (Arrays)

There are four collection data types in the Python programming language:

• List is a collection which is ordered and changeable. Allows duplicate members.
• Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
• Set is a collection which is unordered and unindexed. No duplicate members.
• Dictionary is a collection which is unordered, changeable and indexed. No duplicate members.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

List

A list is a collection which is ordered and changeable. In Python lists are written with square brackets.

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Access Items

You access the list items by referring to the index number:

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Change Item Value

To change the value of a specific item, refer to the index number:

Loop Through a List

You can loop through the list items by using a for loop:

You will learn more about for loops in out Python For Loops Chapter.

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Check if Item Exists

To determine if a specified item is present in a list use the in keyword:

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List Length

To determine how many items a list has, use the len() method:

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Add Items

To add an item to the end of the list, use the append() method:

To add an item at the specified index, use the insert() method:

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Remove Item

There are several methods to remove items from a list:

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Copy a List

You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.

There are ways to make a copy, one way is to use the built-in List method copy().

Another way to make a copy is to use the built-in method list().

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The list() Constructor

It is also possible to use the list() constructor to make a new list.

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List Methods

Python has a set of built-in methods that you can use on lists.

Method	Description
append()	Adds an element at the end of the list
clear()	Removes all the elements from the list
copy()	Returns a copy of the list
count()	Returns the number of elements with the specified value
extend()	Add the elements of a list (or any iterable), to the end of the current list
index()	Returns the index of the first element with the specified value
insert()	Adds an element at the specified position
pop()	Removes the element at the specified position
remove()	Removes the item with the specified value
reverse()	Reverses the order of the list
sort()	Sorts the list

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Python Tuples

Tuple

A tuple is a collection which is ordered and unchangeable. In Python tuples are written with round brackets.

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Access Tuple Items

You can access tuple items by referring to the index number, inside square brackets:

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Change Tuple Values

Once a tuple is created, you cannot change its values. Tuples are unchangeable.

Loop Through a Tuple

You can loop through the tuple items by using a for loop.

You will learn more about for loops in out Python For Loops Chapter.

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Check if Item Exists

To determine if a specified item is present in a tuple use the in keyword:

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Tuple Length

To determine how many items a tuple has, use the len() method:

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Add Items

Once a tuple is created, you cannot add items to it. Tuples are unchangeable.

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Remove Items

 Tuples are unchangeable, so you cannot remove items from it, but you can delete the tuple completely:

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The tuple() Constructor

It is also possible to use the tuple() constructor to make a tuple.

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Tuple Methods

Python has two built-in methods that you can use on tuples.

Method	Description
count()	Returns the number of times a specified value occurs in a tuple
index()	Searches the tuple for a specified value and returns the position of where it was found

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Python Sets

Set

A set is a collection which is unordered and unindexed. In Python sets are written with curly brackets.

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Access Items

You cannot access items in a set by referring to an index, since sets are unordered the items has no index.

But you can loop through the set items using a for loop, or ask if a specified value is present in a set, by using the in keyword.

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Change Items

Once a set is created, you cannot change its items, but you can add new items.

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Add Items

To add one item to a set use the add() method.

To add more than one item to a set use the update() method.

Get the Length of a Set

To determine how many items a set has, use the len() method.

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Remove Item

To remove an item in a set, use the remove(), or the discard() method.

You can also use the pop(), method to remove an item, but this method will remove the last item. Remember that sets are unordered, so you will not know what item that gets removed.

The return value of the pop() method is the removed item.

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The set() Constructor

It is also possible to use the set() constructor to make a set.

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Set Methods

Python has a set of built-in methods that you can use on sets.

Method	Description
add()	Adds an element to the set
clear()	Removes all the elements from the set
copy()	Returns a copy of the set
difference()	Returns a set containing the difference between two or more sets
difference_update()	Removes the items in this set that are also included in another, specified set
discard()	Remove the specified item
intersection()	Returns a set, that is the intersection of two other sets
intersection_update()	Removes the items in this set that are not present in other, specified set(s)
isdisjoint()	Returns whether two sets have a intersection or not
issubset()	Returns whether another set contains this set or not
issuperset()	Returns whether this set contains another set or not
pop()	Removes an element from the set
remove()	Removes the specified element
symmetric_difference()	Returns a set with the symmetric differences of two sets
symmetric_difference_update()	inserts the symmetric differences from this set and another
union()	Return a set containing the union of sets
update()	Update the set with the union of this set and others

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Python Dictionaries

Dictionary

A dictionary is a collection which is unordered, changeable and indexed. In Python dictionaries are written with curly brackets, and they have keys and values.

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Accessing Items

You can access the items of a dictionary by referring to its key name, inside square brackets:

There is also a method called get() that will give you the same result:

Change Values

You can change the value of a specific item by referring to its key name:

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Loop Through a Dictionary

You can loop through a dictionary by using a for loop.

When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.

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Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword:

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Dictionary Length

To determine how many items (key-value pairs) a dictionary has, use the len() method.

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Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

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Removing Items

There are several methods to remove items from a dictionary:

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Copy a Dictionary

You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be a reference to dict1, and changes made in dict1 will automatically also be made in dict2.

There are ways to make a copy, one way is to use the built-in Dictionary method copy().

Another way to make a copy is to use the built-in method dict().

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The dict() Constructor

It is also possible to use the dict() constructor to make a new dictionary:

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Dictionary Methods

Python has a set of built-in methods that you can use on dictionaries.

Method	Description
clear()	Removes all the elements from the dictionary
copy()	Returns a copy of the dictionary
fromkeys()	Returns a dictionary with the specified keys and values
get(	Returns the value of the specified key
items()	Returns a list containing the a tuple for each key value pair
keys()	Returns a list containing the dictionary's keys
pop()	Removes the element with the specified key
popitem()	Removes the last inserted key-value pair
setdefault()	Returns the value of the specified key. If the key does not exist: insert the key, with the specified value
update()	Updates the dictionary with the specified key-value pairs
values()	Returns a list of all the values in the dictionary

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Python If ... Else

Python Conditions and If statements

Python supports the usual logical conditions from mathematics:

• Equals: a == b
• Not Equals: a != b
• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in "if statements" and loops.

An "if statement" is written by using the if keyword.

In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".

Indentation

Python relies on indentation, using whitespace, to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Elif

The elif keyword is pythons way of saying "if the previous conditions were not true, then try this condition".

In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that "a and b are equal".

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Else

The else keyword catches anything which isn't caught by the preceding conditions.

In this example a is greater to b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".

You can also have an else without the elif:

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Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

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Short Hand If ... Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

You can also have multiple else statements on the same line:

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And

The and keyword is a logical operator, and is used to combine conditional statements:

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Or

The or keyword is a logical operator, and is used to combine conditional statements:

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Python While Loops

Python Loops

Python has two primitive loop commands:

• while loops
• for loops

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The while Loop

With the while loop we can execute a set of statements as long as a condition is true.

The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.

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The break Statement

With the break statement we can stop the loop even if the while condition is true:

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

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Python For Loops

Python For Loops

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the for keyword in other programming language, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

The for loop does not require an indexing variable to set beforehand.

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Looping Through a String

Even strings are iterable objects, they contain a sequence of characters:

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The break Statement

With the break statement we can stop the loop before it has looped through all the items:

The continue Statement

With the continue statement we can stop the current iteration of the loop, and continue with the next:

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The range() Function

To loop through a set of code a specified number of times, we can use the range() function,

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):

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Else in For Loop

The else keyword in a for loop specifies a block of code to be executed when the loop is finished:

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Nested Loops

A nested loop is a loop inside a loop.

The "inner loop" will be executed one time for each iteration of the "outer loop":

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Python Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.

Creating a Function

In Python a function is defined using the def keyword:

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Calling a Function

To call a function, use the function name followed by parenthesis:

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Parameters

Information can be passed to functions as parameter.

Parameters are specified after the function name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma.

The following example has a function with one parameter (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without parameter, it uses the default value:

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Passing a List as a Parameter

You can send any data types of parameter to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as a parameter, it will still be a List when it reaches the function:

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Return Values

To let a function return a value, use the return statement:

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Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself ("recurse"). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

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Python Lambda

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

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Syntax

The expression is executed and the result is returned:

Lambda functions can take any number of arguments:

Why Use Lambda Functions?

The power of lambda is better shown when you use them as an anonymous function inside another function.

Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

Use that function definition to make a function that always doubles the number you send in:

Or, use the same function definition to make a function that always triples the number you send in:

Or, use the same function definition to make both functions, in the same program:

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Python Arrays

Arrays

Arrays are used to store multiple values in one single variable:

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What is an Array?

An array is a special variable, which can hold more than one value at a time.

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

The solution is an array!

An array can hold many values under a single name, and you can access the values by referring to an index number.

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Access the Elements of an Array

You refer to an array element by referring to the index number.

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The Length of an Array

Use the len() method to return the length of an array (the number of elements in an array).

Looping Array Elements

You can use the for in loop to loop through all the elements of an array.

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Adding Array Elements

You can use the append() method to add an element to an array.

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Removing Array Elements

You can use the pop() method to remove an element from the array.

You can also use the remove() method to remove an element from the array.

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Array Methods

Python has a set of built-in methods that you can use on lists/arrays.

Method	Description
append()	Adds an element at the end of the list
clear()	Removes all the elements from the list
copy()	Returns a copy of the list
count()	Returns the number of elements with the specified value
extend()	Add the elements of a list (or any iterable), to the end of the current list
index()	Returns the index of the first element with the specified value
insert()	Adds an element at the specified position
pop()	Removes the element at the specified position
remove()	Removes the first item with the specified value
reverse()	Reverses the order of the list
sort()	Sorts the list

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Python Classes and Objects

Python Classes/Objects

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a "blueprint" for creating objects.

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Create a Class

To create a class, use the keyword class:

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Create Object

Now we can use the class named myClass to create objects:

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The __init__() Function

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() function.

All classes have a function called __init__(), which is always executed when the class is being initiated.

Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:

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Object Methods

Objects can also contain methods. Methods in objects are functions that belongs to the object.

Let us create a method in the Person class:

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The self Parameter

The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.

It does not have to be named self , you can call it whatever you like, but it has to be the first parameter of any function in the class:

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Modify Object Properties

You can modify properties on objects like this:

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Delete Object Properties

You can delete properties on objects by using the del keyword:

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Delete Objects

You can delete objects by using the del keyword:

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Python Inheritance

Python Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

Parent class is the class being inherited from, also called base class.

Child class is the class that inherits from another class, also called derived class.

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Create a Parent Class

Any class can be a parent class, so the syntax is the same as creating any other class:

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Create a Child Class

To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:

Now the Student class has the same properties and methods as the Person class.

Add the __init__() Function

So far we have created a child class that inherits the properties and methods from its parent.

We want to add the __init__() function to the child class (instead of the pass keyword).

When you add the __init__() function, the child class will no longer inherit the parent's __init__() function.

To keep the inheritance of the parent's __init__() function, add a call to the parent's __init__() function:

Now we have successfully added the __init__() function, and kept the inheritance of the parent class, and we are ready to add functionality in the __init__() function.

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Add Properties

In the example below, the year 2019 should be a variable, and passed into the Student class when creating student objects. To do so, add another parameter in the __init__() function:

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Add Methods

If you add a method in the child class with the same name as a function in the parent class, the inheritance of the parent method will be overridden.

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Python Iterators

Python Iterators

An iterator is an object that contains a countable number of values.

An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.

Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().

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Iterator vs Iterable

Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.

All these objects have a iter() method which is used to get an iterator:

Even strings are iterable objects, and can return an iterator:

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Looping Through an Iterator

We can also use a for loop to iterate through an iterable object:

The for loop actually creates an iterator object and executes the next() method for each loop.

Create an Iterator

To create an object/class as an iterator you have to implement the methods __iter__() and __next__() to your object.

As you have learned in the Python Classes/Objects chapter, all classes have a function called __init__(), which allows you do some initializing when the object is being created.

The __iter__() method acts similar, you can do operations (initializing etc.), but must always return the iterator object itself.

The __next__() method also allows you to do operations, and must return the next item in the sequence.

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StopIteration

The example above would continue forever if you had enough next() statements, or if it was used in a for loop.

To prevent the iteration to go on forever, we can use the StopIteration statement.

In the __next__() method, we can add a terminating condition to raise an error if the iteration is done a specified number of times:

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Python Modules

What is a Module?

Consider a module to be the same as a code library.

A file containing a set of functions you want to include in your application.

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Create a Module

To create a module just save the code you want in a file with the file extension .py:

Use a Module

Now we can use the module we just created, by using the import statement:

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Variables in Module

The module can contain functions, as already described, but also variables of all types (arrays, dictionaries, objects etc):

Naming a Module

You can name the module file whatever you like, but it must have the file extension .py

Re-naming a Module

You can create an alias when you import a module, by using the as keyword:

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Built-in Modules

There are several built-in modules in Python, which you can import whenever you like.

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Using the dir() Function

There is a built-in function to list all the function names (or variable names) in a module. The dir() function:

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Import From Module

You can choose to import only parts from a module, by using the from keyword.

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Python Datetime

Python Dates

A date in Python is not a data type of its own, but we can import a module named datetime to work with dates as date objects.

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Date Output

When we execute the code from the example above the result will be:

The date contains year, month, day, hour, minute, second, and microsecond.

The datetime module has many methods to return information about the date object.

Here are a few examples, you will learn more about them later in this chapter:

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Creating Date Objects

To create a date, we can use the datetime() class (constructor) of the datetime module.

The datetime() class requires three parameters to create a date: year, month, day.

The datetime() class also takes parameters for time and timezone (hour, minute, second, microsecond, tzone), but they are optional, and has a default value of 0, (None for timezone).

The strftime() Method

The datetime object has a method for formatting date objects into readable strings.

The method is called strftime(), and takes one parameter, format, to specify the format of the returned string:

A reference of all the legal format codes:

Directive	Description	Example	Try it
%a	Weekday, short version	Wed	Try it »
%A	Weekday, full version	Wednesday	Try it »
%w	Weekday as a number 0-6, 0 is Sunday	3	Try it »
%d	Day of month 01-31	31	Try it »
%b	Month name, short version	Dec	Try it »
%B	Month name, full version	December	Try it »
%m	Month as a number 01-12	12	Try it »
%y	Year, short version, without century	18	Try it »
%Y	Year, full version	2018	Try it »
%H	Hour 00-23	17	Try it »
%I	Hour 00-12	05	Try it »
%p	AM/PM	PM	Try it »
%M	Minute 00-59	41	Try it »
%S	Second 00-59	08	Try it »
%f	Microsecond 000000-999999	548513	Try it »
%z	UTC offset	+0100
%Z	Timezone	CST
%j	Day number of year 001-366	365	Try it »
%U	Week number of year, Sunday as the first day of week, 00-53	52	Try it »
%W	Week number of year, Monday as the first day of week, 00-53	52	Try it »
%c	Local version of date and time	Mon Dec 31 17:41:00 2018	Try it »
%x	Local version of date	12/31/18	Try it »
%X	Local version of time	17:41:00	Try it »
%%	A % character	%	Try it »

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Python JSON

JSON is a syntax for storing and exchanging data.

JSON is text, written with JavaScript object notation.

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JSON in Python

Python has a built-in package called json, which can be used to work with JSON data.

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Parse JSON - Convert from JSON to Python

If you have a JSON string, you can parse it by using the json.loads() method.

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Convert from Python to JSON

If you have a Python object, you can convert it into a JSON string by using the json.dumps() method.

You can convert Python objects of the following types, into JSON strings:

• dict
• list
• tuple
• string
• int
• float
• True
• False
• None

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When you convert from Python to JSON, Python objects are converted into the JSON (JavaScript) equivalent:

Python	JSON
dict	Object
list	Array
tuple	Array
str	String
int	Number
float	Number
True	true
False	false
None	null

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Format the Result

The example above prints a JSON string, but it is not very easy to read, with no indentations and line breaks.

The json.dumps() method has parameters to make it easier to read the result:

You can also define the separators, default value is (", ", ": "), which means using a comma and a space to separate each object, and a colon and a space to separate keys from values:

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Order the Result

The json.dumps() method has parameters to order the keys in the result:

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Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

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RegEx Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

Import the re module:

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RegEx in Python

When you have imported the re module, you can start using regular expressions:

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RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Example	Try it
[]	A set of characters	"[a-m]"	Try it »
\	Signals a special sequence (can also be used to escape special characters)	"\d"	Try it »
.	Any character (except newline character)	"he..o"	Try it »
^	Starts with	"^hello"	Try it »
$	Ends with	"world$"	Try it »
*	Zero or more occurrences	"aix*"	Try it »
+	One or more occurrences	"aix+"	Try it »
{}	Exactly the specified number of occurrences	"al{2}"	Try it »
|	Either or	"falls|stays"	Try it »
()	Capture and group

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Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character	Description	Example	Try it
\A	Returns a match if the specified characters are at the beginning of the string	"\AThe"	Try it »
\b	Returns a match where the specified characters are at the beginning or at the end of a word	r"\bain" r"ain\b"	Try it » Try it »
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word	r"\Bain" r"ain\B"	Try it » Try it »
\d	Returns a match where the string contains digits (numbers from 0-9)	"\d"	Try it »
\D	Returns a match where the string DOES NOT contain digits	"\D"	Try it »
\s	Returns a match where the string contains a white space character	"\s"	Try it »
\S	Returns a match where the string DOES NOT contain a white space character	"\S"	Try it »
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)	"\w"	Try it »
\W	Returns a match where the string DOES NOT contain any word characters	"\W"	Try it »
\Z	Returns a match if the specified characters are at the end of the string	"Spain\Z"	Try it »

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Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Set	Description	Try it
[arn]	Returns a match where one of the specified characters (a, r, or n) are present	Try it »
[a-n]	Returns a match for any lower case character, alphabetically between a and n	Try it »
[^arn]	Returns a match for any character EXCEPT a, r, and n	Try it »
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present	Try it »
[0-9]	Returns a match for any digit between 0 and 9	Try it »
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59	Try it »
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case	Try it »
[+]	In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string	Try it »

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The findall() Function

The findall() function returns a list containing all matches.

The list contains the matches in the order they are found.

If no matches are found, an empty list is returned:

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The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

If no matches are found, the value None is returned:

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The split() Function

The split() function returns a list where the string has been split at each match:

You can control the number of occurrences by specifying the maxsplit parameter:

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The sub() Function

The sub() function replaces the matches with the text of your choice:

You can control the number of replacements by specifying the count parameter:

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Match Object

A Match Object is an object containing information about the search and the result.

The Match object has properties and methods used to retrieve information about the search, and the result:

.span() returns a tuple containing the start-, and end positions of the match.
.string returns the string passed into the function
.group() returns the part of the string where there was a match

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Python PIP

What is PIP?

PIP is a package manager for Python packages, or modules if you like.

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What is a Package?

A package contains all the files you need for a module.

Modules are Python code libraries you can include in your project.

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Check if PIP is Installed

Navigate your command line to the location of Python's script directory, and type the following:

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Install PIP

If you do not have PIP installed, you can download and install it from this page: https://pypi.org/project/pip/

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Download a Package

Downloading a package is very easy.

Open the command line interface and tell PIP to download the package you want.

Navigate your command line to the location of Python's script directory, and type the following:

Now you have downloaded and installed your first package!

Using a Package

Once the package is installed, it is ready to use.

Import the "camelcase" package into your project.

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Find Packages

Find more packages at https://pypi.org/.

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Remove a Package

Use the uninstall command to remove a package:

The PIP Package Manager will ask you to confirm that you want to remove the camelcase package:

Press y and the package will be removed.

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List Packages

Use the list command to list all the packages installed on your system:

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Python Try Except

The try block lets you test a block of code for errors.

The except block lets you handle the error.

The finally block lets you execute code, regardless of the result of the try- and except blocks.

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Exception Handling

When an error occurs, or exception as we call it, Python will normally stop and generate an error message.

These exceptions can be handled using the try statement:

Since the try block raises an error, the except block will be executed.

Without the try block, the program will crash and raise an error:

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Many Exceptions

You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:

Else

You can use the else keyword to define a block of code to be executed if no errors were raised:

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Finally

The finally block, if specified, will be executed regardless if the try block raises an error or not.

This can be useful to close objects and clean up resources:

The program can continue, without leaving the file object open.

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File Handling

Python File Open

File handling is an important part of any web application.

Python has several functions for creating, reading, updating, and deleting files.

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File Handling

The key function for working with files in Python is the open() function.

The open() function takes two parameters; filename, and mode.

There are four different methods (modes) for opening a file:

In addition you can specify if the file should be handled as binary or text mode

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Syntax

To open a file for reading it is enough to specify the name of the file:

The code above is the same as:

Because "r" for read, and "t" for text are the default values, you do not need to specify them.

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Python File Open

Open a File on the Server

Assume we have the following file, located in the same folder as Python:

To open the file, use the built-in open() function.

The open() function returns a file object, which has a read() method for reading the content of the file:

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Read Only Parts of the File

By default the read() method returns the whole text, but you can also specify how many characters you want to return:

Read Lines

You can return one line by using the readline() method:

By calling readline() two times, you can read the two first lines:

By looping through the lines of the file, you can read the whole file, line by line:

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Close Files

It is a good practice to always close the file when you are done with it.

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Python File Write

Write to an Existing File

To write to an existing file, you must add a parameter to the open() function:

"a" - Append - will append to the end of the file

"w" - Write - will overwrite any existing content

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Create a New File

To create a new file in Python, use the open() method, with one of the following parameters:

"x" - Create - will create a file, returns an error if the file exist

"a" - Append - will create a file if the specified file does not exist

"w" - Write - will create a file if the specified file does not exist

Result: a new empty file is created!

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Python Delete File

Delete a File

To delete a file, you must import the OS module, and run its os.remove() function:

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Check if File exist:

To avoid getting an error, you might want to check if the file exists before you try to delete it:

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Delete Folder

To delete an entire folder, use the os.rmdir() method:

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Python MySQL

Python can be used in database applications.

One of the most popular databases is MySQL.

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MySQL Database

To be able to experiment with the code examples in this tutorial, you should have MySQL installed on your computer.

You can download a free MySQL database at https://www.mysql.com/downloads/.

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Install MySQL Driver

Python needs a MySQL driver to access the MySQL database.

In this tutorial we will use the driver "MySQL Connector".

We recommend that you use PIP to install "MySQL Connector".

PIP is most likely already installed in your Python environment.

Navigate your command line to the location of PIP, and type the following:

Now you have downloaded and installed a MySQL driver.

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Test MySQL Connector

To test if the installation was successful, or if you already have "MySQL Connector" installed, create a Python page with the following content:

If the above code was executed with no errors, "MySQL Connector" is installed and ready to be used.

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Create Connection

Start by creating a connection to the database.

Use the username and password from your MySQL database:

Now you can start querying the database using SQL statements.

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Python MySQL Create Database

Creating a Database

To create a database in MySQL, use the "CREATE DATABASE" statement:

If the above code was executed with no errors, you have successfully created a database.

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Check if Database Exists

You can check if a database exist by listing all databases in your system by using the "SHOW DATABASES" statement:

Or you can try to access the database when making the connection:

If the database does not exist, you will get an error.

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Python MySQL Create Table

Creating a Table

To create a table in MySQL, use the "CREATE TABLE" statement.

Make sure you define the name of the database when you create the connection

If the above code was executed with no errors, you have now successfully created a table.

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Check if Table Exists

You can check if a table exist by listing all tables in your database with the "SHOW TABLES" statement:

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Primary Key

When creating a table, you should also create a column with a unique key for each record.

This can be done by defining a PRIMARY KEY.

We use the statement "INT AUTO_INCREMENT PRIMARY KEY" which will insert a unique number for each record. Starting at 1, and increased by one for each record.

If the table already exists, use the ALTER TABLE keyword:

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Python MySQL Insert Into Table

Insert Into Table

To fill a table in MySQL, use the "INSERT INTO" statement.

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Insert Multiple Rows

To insert multiple rows into a table, use the executemany() method.

The second parameter of the executemany() method is a list of tuples, containing the data you want to insert:

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Get Inserted ID

You can get the id of the row you just inserted by asking the cursor object.

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Python MySQL Select From

Select From a Table

To select from a table in MySQL, use the "SELECT" statement:

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Selecting Columns

To select only some of the columns in a table, use the "SELECT" statement followed by the column name(s):

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Using the fetchone() Method

If you are only interested in one row, you can use the fetchone() method.

The fetchone() method will return the first row of the result:

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Python MySQL Where

Select With a Filter

When selecting records from a table, you can filter the selection by using the "WHERE" statement:

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Wildcard Characters

You can also select the records that starts, includes, or ends with a given letter or phrase.

Use the %  to represent wildcard characters:

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Prevent SQL Injection

When query values are provided by the user, you should escape the values.

This is to prevent SQL injections, which is a common web hacking technique to destroy or misuse your database.

The mysql.connector module has methods to escape query values:

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Python MySQL Order By

Sort the Result

Use the ORDER BY statement to sort the result in ascending or descending order.

The ORDER BY keyword sorts the result ascending by default. To sort the result in descending order, use the DESC keyword.

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ORDER BY DESC

Use the DESC keyword to sort the result in a descending order.

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Python MySQL Delete From By

Delete Record

You can delete records from an existing table by using the "DELETE FROM" statement:

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Prevent SQL Injection

It is considered a good practice to escape the values of any query, also in delete statements.

This is to prevent SQL injections, which is a common web hacking technique to destroy or misuse your database.

The mysql.connector module uses the placeholder %s to escape values in the delete statement:

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Python MySQL Drop Table

Delete a Table

You can delete an existing table by using the "DROP TABLE" statement:

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Drop Only if Exist

If the the table you want to delete is already deleted, or for any other reason does not exist, you can use the IF EXISTS keyword to avoid getting an error.

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Python MySQL Update Table

Update Table

You can update existing records in a table by using the "UPDATE" statement:

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Prevent SQL Injection

It is considered a good practice to escape the values of any query, also in update statements.

This is to prevent SQL injections, which is a common web hacking technique to destroy or misuse your database.

The mysql.connector module uses the placeholder %s to escape values in the delete statement:

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Python MySQL Limit

Limit the Result

You can limit the number of records returned from the query, by using the "LIMIT" statement:

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Start From Another Position

If you want to return five records, starting from the third record, you can use the "OFFSET" keyword:

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Python MySQL Join

Join Two or More Tables

You can combine rows from two or more tables, based on a related column between them, by using a JOIN statement.

Consider you have a "users" table and a "products" table:

These two tables can be combined by using users' fav field and products' id field.

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LEFT JOIN

In the example above, Hannah, and Michael were excluded from the result, that is because INNER JOIN only shows the records where there is a match.

If you want to show all users, even if they do not have a favorite product, use the LEFT JOIN statement:

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RIGHT JOIN

If you want to return all products, and the users who have them as their favorite, even if no user have them as their favorite, use the RIGHT JOIN statement:

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Python MongoDB

Python can be used in database applications.

One of the most popular NoSQL database is MongoDB.

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MongoDB

MongoDB stores data in JSON-like documents, which makes the database very flexible and scalable.

To be able to experiment with the code examples in this tutorial, you will need access to a MongoDB database.

You can download a free MongoDB database at https://www.mongodb.com.

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PyMongo

Python needs a MongoDB driver to access the MongoDB database.

In this tutorial we will use the MongoDB driver "PyMongo".

We recommend that you use PIP to install "PyMongo".

PIP is most likely already installed in your Python environment.

Navigate your command line to the location of PIP, and type the following:

Now you have downloaded and installed a mongoDB driver.

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Test PyMongo

To test if the installation was successful, or if you already have "pymongo" installed, create a Python page with the following content:

If the above code was executed with no errors, "pymongo" is installed and ready to be used.

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Python MongoDB Create Database

Creating a Database

To create a database in MongoDB, start by creating a MongoClient object, then specify a connection URL with the correct ip address and the name of the database you want to create.

MongoDB will create the database if it does not exist, and make a connection to it.

MongoDB waits until you have created a collection (table), with at least one document (record) before it actually creates the database (and collection).

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Check if Database Exists

You can check if a database exist by listing all databases in you system:

Or you can check a specific database by name:

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Python MongoDB Create Collection

Creating a Collection

To create a collection in MongoDB, use database object and specify the name of the collection you want to create.

MongoDB will create the collection if it does not exist.

MongoDB waits until you have inserted a document before it actually creates the collection.

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Check if Collection Exists

You can check if a collection exist in a database by listing all collections:

Or you can check a specific collection by name:

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Python MongoDB Insert Document

Insert Into Collection

To insert a record, or document as it is called in MongoDB, into a collection, we use the insert_one() method.

The first parameter of the insert_one() method is a dictionary containing the name(s) and value(s) of each field in the document you want to insert.

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Return the _id Field

The insert_one() method returns a InsertOneResult object, which has a property, inserted_id, that holds the id of the inserted document.

If you do not specify an _id field, then MongoDB will add one for you and assign a unique id for each document.

In the example above no _id field was specified, so MongoDB assigned a unique _id for the record (document).

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Insert Multiple Documents

To insert multiple documents into a collection in MongoDB, we use the insert_many() method.

The first parameter of the insert_many() method is a list containing dictionaries with the data you want to insert:

The insert_many() method returns a InsertManyResult object, which has a property, inserted_ids, that holds the ids of the inserted documents.

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Insert Multiple Documents, with Specified IDs

If you do not want MongoDB to assign unique ids for you document, you can specify the _id field when you insert the document(s).

Remember that the values has to be unique. Two documents cannot have the same _id.

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Python MongoDB Find

In MongoDB we use the find and findOne methods to find data in a collection.

Just like the SELECT statement is used to find data in a table in a MySQL database.

Sample code in python 3

Find the first document in the customers collection:

No parameters in the find() method gives you the same result as SELECT * in MySQL.

Sample code in python 3

Return all documents in the "customers" collection, and print each document:

Sample code in python 3

Return only the names and addresses, not the _ids:

You are not allowed to specify both 0 and 1 values in the same object (except if one of the fields is the _id field). If you specify a field with the value 0, all other fields get the value 1, and vice versa:

Sample code in python 3

This example will exclude "address" from the result:

Sample code in python 3

You get an error if you specify both 0 and 1 values in the same object (except if one of the fields is the _id field):

Sample code in python 3

Find document(s) with the address "Park Lane 38":

Sample code in python 3

Find documents where the address starts with the letter "S" or higher:

Regular expressions can only be used to query strings.

Sample code in python 3

Find documents where the address starts with the letter "S":

Sample code in python 3

Sort the result alphabetically by name:

sort("name", 1) #ascending
sort("name", -1) #descending

Sample code in python 3

Sort the result reverse alphabetically by name:

Note: If the query finds more than one document, only the first occurrence is deleted.

Sample code in python 3

Delete the document with the address "Mountain 21":

Sample code in python 3

Delete all documents were the address starts with the letter S:

Sample code in python 3

Delete all documents in the "customers" collection:

Sample code in python 3

Delete the "customers" collection:

Note: If the query finds more than one record, only the first occurrence is updated.

Sample code in python 3

Change the address from "Valley 345" to "Canyon 123":

Sample code in python 3

Update all documents where the address starts with the letter "S":

Customers

Sample code in python 3

Limit the result to only return 5 documents:

Python Tutorials:

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