Conditional expression in "with" statement inferred as 'object', lacking __enter__ and __exit__

from tempfile import TemporaryDirectory, TemporaryFile
import sys

use_file = 'directory' not in sys.argv

with (TemporaryFile() if use_file else TemporaryDirectory()) as thing:
    print(thing)

Mypy infers the type of the conditional expression as object rather than Union[IO,TemporaryDirectory]

$ mypy multi_with.py

multi_with.py:6: error: "object" has no attribute "__enter__"
multi_with.py:6: error: "object" has no attribute "__exit__"

Explicitly typing the result of the conditional expression makes the error messages go away:

named_intermediate: Union[IO, TemporaryDirectory] = (TemporaryFile() if use_file else TemporaryDirectory())

with named_intermediate as thing:
    print(thing)

I observe this in both mypy versions I tried: 0.600 and 0.620

These past issues and pull requests look relevant:
#3487 Type of conditional expression is join instead of union
#5041 Fix type of conditional statement to Union and not Join.
#5095 Don't always infer unions for conditional expressions

Yes, one option is to infer a union for conditional expression. Another option however is to still use join, but fix #3830 (at leat for the situation where we have a protocol context, like ContextManager in this case, which should be pretty straightforward)

@ilevkivskyi

This bug still seems to be open in mypy 0.812: https://mypy-play.net/?mypy=latest&python=3.9&gist=35daf188cbaa119b95c77522250a6081

I don't quite understand why

typing the expression as ContextManager[None] doesn't suffice/work
why assigning in an if block instead of the if else expression is different

Anyway, @ilevkivskyi's #5512 (comment) doesn't seem to be relevant any longer.

Still open as of 0.942.

For reference of contemporary Googlers, the specific error message has changed:

error: "Iterator[None]" has no attribute "__enter__"; maybe "__iter__"?

The original example now works on master.

Variation that still fails, with mypy 1.19.1:

with TemporaryDirectory() if is_foo else nullcontext() as temp_dir:

has to be

named_intermediate: nullcontext | TemporaryDirectory = TemporaryDirectory() if is_foo else nullcontext()
    with named_intermediate as temp_dir:

@ar4hc

@ar4hc I can't reproduce this with mypy 1.19.1. I just tried

from tempfile import TemporaryDirectory, TemporaryFile
from contextlib import nullcontext
import sys

use_file = 'directory' not in sys.argv

with TemporaryDirectory() if use_file else nullcontext() as temp_dir:
    reveal_type(temp_dir)

and I am getting

tstfoo.py:8: note: Revealed type is "builtins.str | None"
Success: no issues found in 1 source file

which is what I would expect.

Navigation Menu

Repository navigation

Conditional expression in "with" statement inferred as 'object', lacking enter and exit #5512

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Conditional expression in "with" statement inferred as 'object', lacking __enter__ and __exit__ #5512

Description

Activity

ilevkivskyi commented on Aug 23, 2018

indigoviolet commented on Mar 21, 2021

MattiasMartens commented on May 3, 2022

ilevkivskyi commented on Jul 14, 2024

ar4hc commented on Jan 15, 2026

ilevkivskyi commented on Jan 16, 2026

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions

Conditional expression in "with" statement inferred as 'object', lacking enter and exit #5512