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The error messages of def func(*args='Hello'): pass and def func(**kwargs='Hello'): pass should explain more clearly #137786
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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
# ErrorSyntaxError: var-positional argument cannot have default value
def func(**kwargs='Hello'):
pass
# ErrorSyntaxError: 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
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hnnynh commentedon Aug 15, 2025
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 commentedon Aug 15, 2025
Yeah please go a head!
corona10 commentedon Aug 15, 2025
@hnnynh Maybe worth to start here:
cpython/Grammar/python.gram
Line 1338 in 8665769
skirpichev commentedon Aug 15, 2025
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:
I suggest to close this.
skirpichev commentedon Aug 15, 2025
CC @pablogsal, as this related to parser
hnnynh commentedon Aug 15, 2025
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 commentedon Aug 15, 2025
I think that the current error messages are pretty clear. The proposed error messages are confusing and simply incorrect.
serhiy-storchaka commentedon Aug 15, 2025
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.
corona10 commentedon Aug 15, 2025
Looks like it is not worth to fixing it. Let's close the issue :) Thanks for the effort @hnnynh