Have you ever used `list.sort()` with the `key` or `reverse` parameters?

I bet you have!

Here's the signature for `list.sort()`

sort(self, /, *, key=None, reverse=False)

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>>> numbers = [4, 6, 7, 2]
>>> numbers.sort(reverse=True)
>>> numbers
[7, 6, 4, 2]

All fine here. But try to use a positional argument instead:

>>> numbers.sort(True)
Traceback (most recent call last):
...
TypeError: sort() takes no positional arguments

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`key` and `reverse` are keyword-only in `list.sort()`

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How about `str.replace()`? You've used that too?

Here's the signature:

replace(self, old, new, count=-1, /)

`old`, `new`, and `count` are all positional-only

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Let's check:

>>> "hello".replace("l", '-')
'he--o'

>>> "hello".replace(old="l", new='-')
Traceback (most recent call last):
...
TypeError: str.replace() takes no keyword arguments

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Maybe you use NumPy a lot? Positional-only and keyword-only arguments appear very often in the NumPy codebase

Here's just one example, NumPy's `mean()` method:

a.mean(axis=None, dtype=None, out=None, keepdims=False, *, where=True)

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Or maybe you use Matplotlib? Here's `plt.scatter()`

Wait for it

scatter(x, y, s=None, c=None, marker=None, cmap=None, norm=None, vmin=None, vmax=None, alpha=None, linewidths=None, *, edgecolors=None, plotnonfinite=False, data=None, **kwargs)

Did you spot the lone * ?

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They're easy to miss and ignore. But if you've read documentation, you will definitely have seen a * or a / in many function signatures

Now, when you see them next, you'll know perfectly well what they are and what they do

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We'll return to the Python functions series tomorrow!

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