The error messages of `def func(*args='Hello'): pass` and `def func(**kwargs='Hello'): pass` should explain more clearly

[hyperkai]

Bug report

Bug description:

*args and **kwargs have an empty tuple and dictionary respectively as default values as shown below:

def func(*args):
    print(args, type(args))

func()
# () <class 'tuple'>

def func(**kwargs):
    print(kwargs, type(kwargs))

func()
{} <class 'dict'>

Then, setting a default value to *args and **kwargs gets the error messages as shown below. I think the error messages should explain more clearly because Python doc doesn't explain *args and **kwargs are the optional parameters which have the immutable default values () and {} respectively and var-keyword argument and var-keyword argument should be var-keyword parameter and var-keyword parameter:

def func(*args='Hello'):
    pass
# Error

SyntaxError: var-positional argument cannot have default value

def func(**kwargs='Hello'):
    pass
# Error

SyntaxError: var-keyword argument cannot have default value

So, the error messages should be like belolw:

SyntaxError: var-positional parameter's default value cannot be changed because it has the immutable default value ()

SyntaxError: var-keyword parameter's default value cannot be changed because it has the immutable default value {}

CPython versions tested on:

3.12

Operating systems tested on:

Windows

Linked PRs

• gh-137786: fix error msg for *args, **kwargs default value #137793

[hnnynh]

Looks Interesting! @corona10 Can you assign this issue to me? It may take some time as this is my first time, but I will do my best. 🚀

[corona10]

Yeah please go a head!

[corona10]

@hnnynh Maybe worth to start here:

cpython/Grammar/python.gram

Line 1338 in 8665769

| '**' param a='=' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "var-keyword parameter cannot have default value") }

[skirpichev]

Strictly speaking, error messages are correct. They are talking about default values for parameters, this has clear meaning in the grammar specification. See 8.7. Function definitions:

When one or more parameters have the form parameter = expression, the function is said to have “default parameter values.” For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter’s default value is substituted.

I suggest to close this.

[skirpichev]

CC @pablogsal, as this related to parser

[hnnynh]

I thought adding a reason to this message would help users better understand the error.
If the issue is closed, I will close the PR as well.

[serhiy-storchaka]

I think that the current error messages are pretty clear. The proposed error messages are confusing and simply incorrect.

[serhiy-storchaka]

There was an error in these error messages -- the term "argument" was misused instead of "parameter". This is perhaps the reason of your confusion. But this was fixed in #133379.