python / python/mypy

mypyc: Any subclass of BytesIO will always generate an exception when instantiated

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bug topic-mypyc
Dominant language
Python
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Description

Bug Report

Any subclass of BytesIO will always generate the following exception when instantiated:
BufferError: Existing exports of data: object cannot be re-sized

To Reproduce

1. Run TestSubclassingBytesIO.py and notice it prints two instances (one of BytesIO and the other of SubBytesIO)
2. Using mypyc compile the 4 line file: SubclassingBytesIO.py
3. Then again run the tiny TestSubclassingBytesIO.py
4. The run will crash with the mentioned exception.

TestSubclassingBytesIO.zip

Expected Behavior
Running TestSubclassingBytesIO.py after compiling with mypyc should work just like running it before compiling. It should print:
<_io.BytesIO object at 0x0000026903075BC0>
<SubclassingBytesIO.SubBytesIO object at 0x00000269030A0630>

Actual Behavior
<_io.BytesIO object at 0x00000238D1D95BC0>
Traceback (most recent call last):
File "c:\Projects\mypycbug\TestSubclassingBytesIO.py", line 7, in
x = SubBytesIO()
^^^^^^^^^^^^
BufferError: Existing exports of data: object cannot be re-sized

Your Environment

  • Mypy version used: 1.5.1
  • Mypy command-line flags: None
  • Mypy configuration options from mypy.ini (and other config files): None
  • Python version used: 3.11.4

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by extracting the attached TestSubclassingBytesIO.zip and running TestSubclassingBytesIO.py before and after compiling SubclassingBytesIO.py with mypyc. Compare the BytesIO and SubBytesIO instantiation paths; done means the compiled script prints both instances without raising BufferError.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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