Python slicing is a computationally fast way to methodically access parts of your data. In my opinion, to be even an intermediate Python programmer, it's one aspect of the language that it is necessary to be familiar with.
Important Definitions
To begin with, let's define a few terms:
start: the beginning index of the slice, it will include the element at this index unless it is the same as stop, defaults to 0, i.e. the first index. If it's negative, it means to start n items from the end.
stop: the ending index of the slice, it does not include the element at this index, defaults to length of the sequence being sliced, that is, up to and including the end.
step: the amount by which the index increases, defaults to 1. If it's negative, you're slicing over the iterable in reverse.
How Indexing Works
You can make any of these positive or negative numbers. The meaning of the positive numbers is straightforward, but for negative numbers, just like indexes in Python, you count backwards from the end for the start and stop, and for the step, you simply decrement your index. This example is from the documentation's tutorial, but I've modified it slightly to indicate which item in a sequence each index references:
+---+---+---+---+---+---+
| P | y | t | h | o | n |
+---+---+---+---+---+---+
0 1 2 3 4 5
-6 -5 -4 -3 -2 -1
How Slicing Works
To use slice notation with a sequence that supports it, you must include at least one colon in the square brackets that follow the sequence (which actually implement the __getitem__ method of the sequence, according to the Python data model.)
Slice notation works like this:
sequence[start:stop:step]
And recall that there are defaults for start, stop, and step, so to access the defaults, simply leave out the argument, n in the notation. Slice notation to get the every other elements from nine elements from a list (or any other sequence that supports it, like a string) would look like this:
my_list[-9:]
When I see this, I read the part in the brackets as "9th from the end, to the end." (Actually, I abbreviate it mentally as "-9, on")
Explanation:
The full notation is
my_list[-9:None:None]
and to substitute the defaults (actually when step is negative, stop's default is -len(my_list)-1), so None for stop really just means it goes to whichever end step takes it to):
my_list[-9:len(my_list):1]
But the colon, :, is what tells Python you're giving it a slice and not a regular index. That's why the idiomatic way of making a shallow copy of lists in Python 2 is
list_copy = sequence[:]
And clearing them is with:
del my_list[:]
(Python 3 gets a list.copy and list.clear method.)
Give your slices a descriptive name!
You may find it useful to separate forming the slice from passing it to the list.__getitem__ method (that's what the square brackets do). Even if you're not new to it, it keeps your code more readable so that others that may have to read your code can more readily understand what you're doing.
However, you can't just assign some integers separated by colons to a variable. You need to use the slice object:
last_nine_slice = slice(-9, None)
The second argument, None, is required, so that the first argument is interpreted as the start argument otherwise it would be the stop argument.
You can then pass the slice object to your sequence:
>>> list(range(100))[last_nine_slice]
[91, 92, 93, 94, 95, 96, 97, 98, 99]
Memory Considerations:
Since slices of Python lists create new objects in memory, another important function to be aware of is itertools.islice. Typically you'll want to iterate over a slice, not just have it created statically in memory. islice is perfect for this. A caveat, it doesn't support negative arguments to start, stop, or step, so if that's an issue you may need to calculate indices or reverse the iterable in advance.
>>> length = 100
>>> last_nine_iter = itertools.islice(list(range(length)), length-9, None, 1)
>>> list_last_nine = list(last_nine)
>>> list_last_nine
[91, 92, 93, 94, 95, 96, 97, 98, 99]
The fact that list slices make a copy is a feature of lists themselves. If you're slicing advanced objects like a Pandas DataFrame, it may return a view on the original, and not a copy.