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open() returns IO[str] instead of io.TextIOWrapper #1229
Description
Hello,
My env is Win7P+SP1 x64, Py 3.5.2 32b, Mypy 0.501.
I opened this issue (#3286) in mypy, but GVR told me to open it here because it was a "(complex) typeshed issue".
The var used in the with context is of type <class '_io.TextIOWrapper'>.
with open(pathname, encoding='utf-8') as f_in:
print(type(f_in))
<class '_io.TextIOWrapper'>
But when I try to set it's type, mypy returns the following error
from io import TextIOWrapper
with open(pathname, encoding='utf-8') as f_in: # type: TextIOWrapper
print(type(f_in))
data_processing.py:917: error: Incompatible types in assignment (expression has type IO[Any], variable has type "TextIOWrapper")
If I try to use module typing's own TextIO type it returns
from typing import TextIO
with open(pathname, encoding='utf-8') as f_in: # type: TextIO
print(type(f_in))
data_processing.py:917: error: Incompatible types in assignment (expression has type IO[Any], variable has type "TextIO")
It only works if I do
from typing import IO
with open(pathname, encoding='utf-8') as f_in: # type: IO[Any]
print(type(f_in))
Best regards,
JM
Activity
lincolnq commentedon May 30, 2017
The solution to your problem is probably to use IO[str], which is compatible with the IO[Any] returned by open() and has (most of) the right methods. It doesn't have the TextIO specific ones (like .encoding, .line_buffering, etc.), so if you want those you should use cast(TextIO, open(...)).
However, I agree that this is an interesting problem and worth thinking about. It means TextIO and BinaryIO are annoyingly impractical to use in real code (a cast is necessary whenever open() is called). The documentation doesn't give many examples or suggest that IO[str] is what most people should use.
gvanrossum commentedon May 30, 2017
Yeah, this is not one of the smoothest APIs. Note that in master,
open(fn, 'rb')will now returnIO[bytes]whileopen(fn, 'r')will returnIO[str]. (However, if the mode argument is a variable, it returnsIO[Any]; and in PY2 they all returnBinaryIOregardless of the mode.)I think this is still wrong though. open() with a recognized mode argument should return
BinaryIOorTextIO. That way you can writeSome classes used by the implementation in the
iomodule are subclasses of TextIO or BinaryIO, but you're better off not referencing them.(Another note: it's not great to use runtime inspection to come up with type names to put into annotations -- things at runtime are a bit different from how things are dine on the static typing side of things.)
srittau commentedon Sep 11, 2018
I think any further improvements in typeshed for this are dependent on implementing dependent types, see python/mypy#3062.
[-]Mypy assumes and only accepts type IO[Any] when it is _io.TextIOWrapper (GVR told me to open this here)[/-][+]open() returns IO[str] instead of io.TextIOWrapper[/+]srittau commentedon Oct 28, 2018
This is part of the IO classes restructuring.
Work around python/typeshed#1229
hauntsaninja commentedon Jun 11, 2020
This was fixed in #4146 and others!