python / python/cpython

Method call on imported definition differs inside of a list comprehension

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#127,929 4 comments 0 reactions 0 assignees View on GitHub

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interpreter-core performance type-bug
Dominant language
Python
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Description

Bug report

Bug description:
import re
[x for x in [] if re.match(".*", x)]

vs:

import re
re.match(".*", x)

When you disassemble these you'll see the inlined one will be doing a LOAD_ATTR 3 (match + NULL|self) while the one outside of the comprehension will be doing a PUSH_NULL/CALL which is to disable the method call optimization on things which are likely not methods.

These should both have the same byte code. There's no perceivable code execution difference other than performance though.

CPython versions tested on:

3.12, CPython main branch

Operating systems tested on:

Linux

Contributor guide

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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 disassembling the two Python examples from the report on CPython 3.12 or main, then trace how the list-comprehension and top-level call paths are compiled. Done means both forms use consistent bytecode for the imported definition call without changing observable behavior.

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
Clearly specified
Newbie friendliness
42/100

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