Incorrect error message: "missing 1 required positional argument"

[boxed]

BPO	46637
Nosy	@terryjreedy, @ericvsmith, @aroberge, @boxed, @AlexWaygood

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-02-04.15:54:15.436>
labels = ['interpreter-core', 'type-feature', '3.11']
title = 'Incorrect error message: "missing 1 required positional argument"'
updated_at = <Date 2022-02-05.00:40:55.384>
user = 'https://github.com/boxed'

bugs.python.org fields:

activity = <Date 2022-02-05.00:40:55.384>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2022-02-04.15:54:15.436>
creator = 'Anders.Hovm\xc3\xb6ller'
dependencies = []
files = []
hgrepos = []
issue_num = 46637
keywords = []
message_count = 6.0
messages = ['412510', '412511', '412514', '412519', '412520', '412530']
nosy_count = 5.0
nosy_names = ['terry.reedy', 'eric.smith', 'aroberge', 'Anders.Hovm\xc3\xb6ller', 'AlexWaygood']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46637'
versions = ['Python 3.11']

Linked PRs

• gh-90795: Fix error message incorrectly including "positional" #139678

[boxed]

>>> def foo(a):
...     pass
... 
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() missing 1 required positional argument: 'a'

This error is incorrect. It says "positional argument", but it's just "argument". The proof is that if you call it with

foo(a=3)

it works fine.

[ericvsmith]

I guess the technically correct term is positional-or-keyword, but that seems like too much: https://docs.python.org/3/glossary.html

I think dropping "positional" doesn't increase the precision of the error message, but I'll admit I can't think of anything better. And lacking something better I don't think we should change this.

[boxed]

Just dropping the word "positional" is very good. That word is a lie, and just removing it makes it true.

[ericvsmith]

Given this current behavior:

>>> def foo(a, /, b): pass
...
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() missing 2 required positional arguments: 'a' and 'b'

What would you suggest?

I agree the current messages aren't perfect.

[boxed]

For foo(a, /, b), it could be:

"TypeError: foo() missing 1 required argument 'a', and one required positional argument 'b'.

If we start on this road there are some more, like for def foo(a, *, b) you get the error "TypeError: foo() missing 1 required positional argument: 'a'" which leaves out that the keyword only argument is also required.

Another solution would be something like:

TypeError: foo() missing 3 required arguments: 'a' (positional only), 'b', 'c' (keyword only)

This solution scales to the worst complex cases, and is a lot clearer imo. Could even be further improved with some nicer formatting:

TypeError: foo() missing 3 required arguments:
'a' (positional only)
'b'
'c' (keyword only)

But that might be a bit overkill :)