Signature.bind TypeErrors could be more helpful

[RazerM]

BPO	39720
Nosy	@RazerM

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

Show more details

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-02-21.22:41:33.023>
labels = ['type-feature', 'library']
title = 'Signature.bind TypeErrors could be more helpful'
updated_at = <Date 2020-02-21.22:41:33.023>
user = 'https://github.com/RazerM'

bugs.python.org fields:

activity = <Date 2020-02-21.22:41:33.023>
actor = 'RazerM'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-02-21.22:41:33.023>
creator = 'RazerM'
dependencies = []
files = []
hgrepos = []
issue_num = 39720
keywords = []
message_count = 1.0
messages = ['362439']
nosy_count = 1.0
nosy_names = ['RazerM']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39720'
versions = []

[RazerM]

Signature.bind does not tell you if a missing argument is keyword-only for example. I created the following snippet to examine the differences:

    import inspect
    
    def run(f):
        try:
            f()
        except TypeError as exc:
            print(exc.args[0])
        else:
            raise RuntimeError('Expected to raise!')
        sig = inspect.signature(f)
        try:
            sig.bind()
        except TypeError as exc:
            print(exc.args[0])
        else:
            raise RuntimeError('Expected to raise!')
        print()
    
    @run
    def f1(pos_only, /): ...
    
    @run
    def f2(pos_or_kw): ...
    
    @run
    def f3(*, kw_only): ...

Output on current 3.9 master:

f1() missing 1 required positional argument: 'pos_only'
missing a required argument: 'pos_only'

f2() missing 1 required positional argument: 'pos_or_kw'
missing a required argument: 'pos_or_kw'

f3() missing 1 required keyword-only argument: 'kw_only'
missing a required argument: 'kw_only'

I am willing to create a PR so that the TypeError for f3 says "missing a required keyword-only argument: 'kw_only'", if this would be accepted.