How should a NamedTemporaryFile be annotated?

Question

I tried typing.IO as suggested in Type hint for a file or file-like object?, but it doesn't work:

from __future__ import annotations
from tempfile import NamedTemporaryFile
from typing import IO

def example(tmp: IO) -> str:
    print(tmp.file)
    return tmp.name


print(example(NamedTemporaryFile()))

for this, mypy tells me:

test.py:6: error: "IO[Any]" has no attribute "file"; maybe "fileno"?

and Python runs fine. So the code is ok.

Answer 1

I don't think this can be easily type hinted.

If you check the definition of NamedTemporaryFile, you'll see that it's a function that ends in:

return _TemporaryFileWrapper(file, name, delete)

And _TemporaryFileWrapper is defined as:

class _TemporaryFileWrapper:

Which means there isn't a super-class that can be indicated, and _TemporaryFileWrapper is "module-private". It also doesn't look like it has any members that make it a part of an existing Protocol * (except for Iterable and ContextManager; but you aren't using those methods here).

I think you'll need to use _TemporaryFileWrapper and ignore the warnings:

from tempfile import _TemporaryFileWrapper  # Weak error

def example(tmp: _TemporaryFileWrapper) -> str:
    print(tmp.file)
    return tmp.name

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If you really want a clean solution, you could implement your own Protocol that includes the attributes you need, and have it also inherit from Iterable and ContextManager. Then you can type-hint using your custom Protocol.

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* It was later pointed out that it does fulfil IO, but the OP requires attributes that aren't in IO, so that can't be used.