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pdb.line_prefix assignment ignored due to early binding #141781
Description
Bug Description
, in pdb.print_stack_entry, line_prefix is used as a default argument:
def print_stack_entry(self, frame_lineno, prompt_prefix=line_prefix):
This causes the value to be bound at definition time (when the module is imported). Any subsequent assignment to pdb.line_prefix by the user is ignored by this function.
Fix
Change the default argument to None and assign it inside the function body.
CPython versions tested on:
CPython main branch
Operating systems tested on:
No response
Activity
gaogaotiantian commentedon Nov 21, 2025
I actually can't find this in pdb documentation. Where did you find it?
pareshjoshij commentedon Nov 21, 2025
@gaogaotiantian You're completely right — that first sentence is wrong and should never have been there. The official documentation does not say anything about customizing
pdb.line_prefix. It was an undocumented implementation detail that some people (including me) mistakenly treated as a supported feature. I apologize for the misleading wording in the issue; that’s on me.I'will edited the issue description to remove the incorrect claim.
The actual bug (the default-argument binding in
print_stack_entrycapturing the value ofline_prefixat import time instead of at call time) is still real and reproducible. After this fix, assigning topdb.line_prefixat runtime finally affects the arrow prefix inlist,where, etc., which is the expected behavior for anyone who was already monkey-patching that variable.Thank you for pointing it out — much appreciated!
Add regression test for pythongh-141781
Fix regression test spacing and add pythongh-141781 test
gh-141781: Fix pdb.line_prefix binding (#141779)
pythongh-141781: Fix pdb.line_prefix binding (python#141779)
pythongh-141781: Fix pdb.line_prefix binding (python#141779)