[New-bugs-announce] [issue42317] Docs of `typing.get_args`: Mention that due to caching of typing generics the order of arguments for Unions can be different from the one of the returned tuple

[Dominik V.]

New submission from Dominik V. <dominik.vilsmeier1123 at gmail.com>:

Due to caching of `__getitem__` for generic types, the order of arguments as returned by `get_args` might be different for Union:

```python
>>> from typing import List, Union, get_args
>>> get_args(get_args(List[Union[int, str]])[0])
(<class 'int'>, <class 'str'>)
>>> get_args(get_args(List[Union[str, int]])[0])
(<class 'int'>, <class 'str'>)
```

This is because `List[Union[int, str]] is List[Union[str, int]]`.

I understand that caching is useful to reduce the memory footprint of type hints, so I suggest to update the documentation of `get_args`. At the moment it reads:

> For a typing object of the form X[Y, Z, ...] these functions return X and (Y, Z, ...).

This seems to imply that the returned objects are identical to the ones in the form `X[Y, Z, ...]`. However that's not the case:

```python
>>> U1 = Union[int, str]
>>> U2 = Union[str, int]
>>> get_args(List[U1])[0] is U1
True
>>> get_args(List[U2])[0] is U2
False
```

I'm not so much concerned about the identity, but the fact that a subsequent call to `get_args` on the Union returns a different type seems to be relevant.

So I propose to add the following sentence to the `get_args` docs:

> [...], it gets normalized to the original class.
> If `X` is a `Union`, the order of `(Y, Z, ...)` can be different from the one of the original arguments `[Y, Z, ...]`.

Or alternatively:

> [...], it gets normalized to the original class.
> If `X` is a `Union`, the order of `(Y, Z, ...)` is arbitrary.

The second version is shorter but it's not completely accurate (since the order is actually not arbitrary).

----------
assignee: docs at python
components: Documentation
messages: 380699
nosy: Dominik V., docs at python
priority: normal
severity: normal
status: open
title: Docs of `typing.get_args`: Mention that due to caching of typing generics the order of arguments for Unions can be different from the one of the returned tuple
type: enhancement
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42317>
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