help() output of lambda with manually set __annotations__ is one char off

[Dutcho]

Bug report

help() output of lambda with manually set __annotations__ is slightly scrambled.

f = lambda a, b, c: 0              # lambdas cannot have annotations
f.__annotations__['return'] = int  # but you can set them manually

help(f)                            # superfluous ")" and missed last char
# <lambda> lambda a, b, c) -> in

import inspect
print(inspect.signature(f))        # correctly displays
# (a, b, c) -> int

Note

This is a minor glitch for an atypical (perhaps even unsupported) use of annotations.

Hypothesis

Perhaps help() uses (the same logic as) inspect.signature() and (to reflect lambda syntax) wants to drop the brackets "(...)", but then has a one-off error for its text slice (picking ") -> in" instead of " -> int")

Environment

• CPython versions tested on: 3.6.8 - 3.11.4, 3.12.0b4
• Operating system and architecture: Windows 11

Linked PRs

• gh-107155: Fix help(lambda_func) when lambda_func has `__annotati… #107401
• [3.12] gh-107155: Fix help() for lambda function with return annotati… #115612
• [3.11] gh-107155: Fix help() for lambda function with return annotati… #115613

[Eclips4]

Looks like a known issue :)

cpython/Lib/pydoc.py

Lines 1503 to 1506 in f443b54

	# XXX lambda's won't usually have func_annotations['return']
	# since the syntax doesn't support but it is possible.
	# So removing parentheses isn't truly safe.
	argspec = argspec[1:-1] # remove parentheses

[Eclips4]

Probably, we can just replace boundaries, argspec.replace("(", "").replace(")", ""). Can there be any parenthesis in signature of lambda?

[AA-Turner]

f = lambda a, b: (a, b)
f.__annotations__['return'] = (int, int)
help(f)
# Help on function <lambda> in module __main__:
#
# <lambda> lambda a, b) -> (<class 'int'>, <class 'int'>

This isn't currently a legal type annotation, but using tuple literals was proposed in PEP-677 and something similar could come up again -- a cleaner fix could be to handle the string conversion manually. I've suggested something on the PR.

A

[serhiy-storchaka]

The parentheses were removed because lambda (a, b, c) is not a valid Python syntax. But neither lambda a, b, c -> int is. So why simply not keep parentheses if there are annotations? lambda (a, b, c) -> int. It will also be less ambiguous if parameters are annotated: lambda (a: int, b: int, c: int) -> int.