bpo-33967: singledispatch raises TypeError when no positional arguments

[corona10]

https://bugs.python.org/issue33967

[methane]

I think this is too much.

I think this is enough:

if not args:
    raise TypeError("singledispatch requires at least 1 positional argument")

[doerwalter]

True, both f(x=3) and g(*args) can be called with one argument, so shouldn't be rejected. I think @methane's solution does the right thing.

[corona10]

Okay can I update this PR into suggested one?

[corona10]

@doerwalter @methane

Updated! Please take a look!

[methane]

Test seems too redundant for now.
TypeError happens always when no positional arguments.
How the generic function defined is not important.

[methane]

f"{func.__name__} requires at least 1 positional argument" may be better, if we can assume func.__name__ is available always.

I believe singledispatch requires positional argument by design, intentionally.
But I want @ambv review before merge.

[corona10]

@methane
Thanks for the quick review.
Updated!
One more thing, we need to add tags needs backport to 3.7 / 3.6.

[methane]

Error message is not important here. (We won't backport such improvements)

functools.singledispatch now raises TypeError instead of IndexError when no
positional arguments are passed.

[corona10]

@methane I've updated news

[serhiy-storchaka]

func can be not having the __name__ attribute in general case. This PR can break a code which use singledispatch() with custom callables.

[serhiy-storchaka]

Also this change creates a new reference to func linked to from wrapper. It may be better to keep a reference just to the name. E.g.

funcname = getattr(func, '__name__', 'singledispatch function')
def wrapper(*args, **kw):
    # use funcname

[corona10]

I fixed it

[miss-islington]

Thanks @corona10 for the PR, and @methane for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6, 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-8220 is a backport of this pull request to the 3.7 branch.

[bedevere-bot]

GH-8221 is a backport of this pull request to the 3.6 branch.

[ZackerySpytz]

assertRaisesRegexp() is deprecated, so this test fails when test_functools is run with -Werror (see #8261).