Thread by @gusthema

I was telling a friend that one cool feature from Python is list slice notation

So instead of just posting the link I decided to do a brief explanation.

[5 min]

Python's regular array indexing as in multiple languages is: a[index]

a = [0, 1, 2, 3, 4]
a[0] == 0

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Python has negative indexing too:

a = [0, 1, 2, 3, 4]
a[-1] == 4
a[-2] == 3

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Python also enables creating sub-lists from a list, or a slice:

-> a[start_index:last_index]

a = [0, 1, 2, 3, 4]
a[1:3] == [1, 2]

start_index is inclusive
last_index is exclusive

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→ The results of a slice are a copy not a reference!

a = [0, 1, 2, 3, 4]
b = a[1:3] # b == [1, 2]
b[0] = 10
a == [0, 1, 2, 3, 4]
b == [10, 2]

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Slicing also works with negative index:

a = [0, 1, 2, 3, 4]
a[1:-2] == [1, 2]

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You can (and you SHOULD) omit the first item if it's a zero, or the last if it's the length:

a = [0, 1, 2, 3, 4]
a[:3] == a[0:3] == [0, 1, 2]
‍a[3:] == a[3:len(a)] == [3, 4]
a[:] == a

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Python Slicing has one last optional parameter that is the step:

-> a[start_index:last_index:step]

a = [0, 1, 2, 3, 4]
a[1:4:2] == [1, 3]
a[::2] == [0, 2, 4]

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The stride can be a negative number, and this can be used to invert a list:

a = [0, 1, 2, 3, 4]
a[::-1] == [4, 3, 2, 1, 0]

Tip 💡: avoid using stride and slicing in the same expression, it can be confusing. Prefer breaking in two expressions

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Python Slicing works on lists, strings, bytes and any class that implements __getitem__ and __setitem__

a = "file.zip"
a[1:4] == "ile"
a[:-4] == "file"
a[::-1] == "piz.elif"

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Python Slicing is forgiving for start and end indexes! 👍

a = [0, 1, 2, 3, 4]
a[20:] == []
a[:20] == [0, 1, 2, 3, 4]

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Do you want to learn these concepts? Open your python interpreter or go to Colab and try/experiment with all these lines! (that's why I didn't put images of code this time) 🧠💪

It might take 20 minutes but I guarantee that's well worth your time!

11/12🧵

Slicing is a very important concept in Python and understanding it will make you a better developer and make it easier to understand other people's code!

Do you have any cool tips related to slicing?
Which other languages also have slicing?

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