TemporaryFile gets closed somehow after text I/O but only when passed through function

[kbolino]

Bug report

Bug description:

Issue

I cannot explain this difference in behavior. I have reduced this code to what I believe are the minimal elements necessary to demonstrate the issue. Notably, this is not reproducible without using TextIOWrapper, but that gets me no closer to an explanation.

First, no function call:

import io
import tempfile

with tempfile.TemporaryFile() as out_file:   
    out_writer = io.TextIOWrapper(out_file)
    out_writer.write("hello,world")
    out_writer.flush()
    out_file.seek(0)
    print(out_file.read())

Result:

b'hello,world'

Next, a function call is introduced, but otherwise the program should behave identically:

import io
import tempfile

def handle_file(out_file):
    out_writer = io.TextIOWrapper(out_file)
    out_writer.write("hello,world")
    out_writer.flush()

with tempfile.TemporaryFile() as out_file:
    handle_file(out_file)
    out_file.seek(0)
    print(out_file.read())

Result:

Traceback (most recent call last):
  File "/Users/.../tempfile_close_test.py", line 11, in <module>
    out_file.seek(0)
ValueError: seek of closed file

Yet, the file was not closed anywhere, and the code is functionally identical.

Environment

Tested with macOS Sequoia 15.3.1 (Darwin 24.3.0) on Apple Silicon (M4 MacBook Pro)

Reproduced with all of these Python versions:

• 3.9.6 (system interpreter at /usr/bin/python3)
• 3.10.16 (from homebrew)
• 3.12.9 (from homebrew)
• 3.13.2 (from homebrew)

CPython versions tested on:

3.13

Operating systems tested on:

macOS

[kbolino]

I can also repro this on Linux (x86-64, Ubuntu 24.04 LTS, 6.8.0-54-generic) with Python 3.12.3.

[kbolino]

I get the same exception ("seek of closed file") using try-finally instead of with, but again only if there's a function call involved.

import io
import tempfile

def handle_file(out_file):
    out_writer = io.TextIOWrapper(out_file)
    out_writer.write("hello,world")
    out_writer.flush()

out_file = tempfile.TemporaryFile()
try:
    handle_file(out_file)
    out_file.seek(0)
    print(out_file.read())
finally:
    out_file.close()

Also, out_writer.flush() is not necessary to repro

[dolfinus]

Probably TextIOWrapper has __del__ method implemented which calls .close(), and it is being triggered by GC while returning from function.

[ordinary-jamie]

You need to detach your TextIOWrapper from your buffer before it closes (when it falls out of scope in handle_file)

See: https://docs.python.org/3/library/io.html#io.IOBase.__del__

import io
import tempfile


def handle_file(out_file):
    out_writer = io.TextIOWrapper(out_file)
    out_writer.write("hello,world")
    out_writer.flush()
    out_writer.detach()   # <---- add this


with tempfile.TemporaryFile() as out_file:
    handle_file(out_file)
    out_file.seek(0)
    print(out_file.read())

[kbolino]

Ok, this behavior seems strange to me as a default for what is explicitly a wrapper class, but at least it is explainable now.

Given that there is an explanation which lends itself to workarounds, it probably makes sense to close this issue, though it might be good to start a discussion (probably elsewhere) about whether this default behavior should exist in the first place.

Also, at least for my use case, it's simpler to just open the TemporaryFile in text mode with an appropriate encoding (like 'utf-8') rather than use a wrapper to go from binary to text mode, but I didn't know that at the time.

[ordinary-jamie]

Glad we could help @kbolino :)

it might be good to start a discussion (probably elsewhere) about whether this default behavior should exist in the first place.

I don't know enough to answer this sorry 😅; the Python Discourse forum is a good spot for these discussions: https://discuss.python.org

[kbolino]

Discussion: https://discuss.python.org/t/textiowrapper-probably-shouldnt-close-its-stream-when-gced/84346