Python generic type annotations

python-generic-type-annotations

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There's a minor conflict within Python where mypy's strict typing is
incompatible with its current default syntax.  Specifically, if you have a
generic type like `_TemporaryFileWrapper`, mypy will tell you to add type
parameters which will convert it to something like
`_TemporaryFileWrapper[bytes]`.  However, python 3 syntax will error out
with a `TypeError: 'type' object is not subscriptable`.  This can be fixed
by importing the [annotations future](https://peps.python.org/pep-0563/):
`from __future__ import annotations`.

A working example that passes both strict mypy typing and is syntactically
correct:

```python
from __future__ import annotations

import tempfile


def get_tempfile() -> tempfile._TemporaryFileWrapper[bytes]:
    return tempfile.NamedTemporaryFile()


temp = get_tempfile()
with open(temp.name, 'wb') as f:
    f.write(b'data')
temp.close()
```

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