New submission from Mark Grandi <markgra...@gmail.com>:
this is with python 3.8:
```plaintext
PS C:\Users\mark> py -3 --version --version
Python 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 23:03:10) [MSC v.1916 64 bit
(AMD64)]
```
So when using functools.singledispatch or functools.singledispatchmethod, you
need to provide at least 1 positional argument so it can dispatch based on the
type of said argument
however, with `functools.singledispatchmethod`, you get an error message that
is confusing and not obvious what the problem is.
Example with `functools.singledispatchmethod`:
```
class NegatorTwo:
@singledispatchmethod
def neg(arg):
raise NotImplementedError("Cannot negate a")
@neg.register
def _(self, arg: int):
return -arg
@neg.register
def _test(self, arg: bool):
return not arg
if __name__ == "__main__":
n = NegatorTwo()
print(n.neg(0))
print(n.neg(False))
print(n.neg(arg=0))
```
you end up getting:
```plaintext
PS C:\Users\mark> py -3 C:\Users\mark\Temp\singledisp.py
0
True
Traceback (most recent call last):
File "C:\Users\mark\Temp\singledisp.py", line 58, in <module>
print(n.neg(arg=0))
File "C:\Python38\lib\functools.py", line 910, in _method
method = self.dispatcher.dispatch(args[0].__class__)
IndexError: tuple index out of range
```
but with just regular `functools.singledispatch`:
```plaintext
@functools.singledispatch
def negate_func(arg):
raise NotImplementedError("can't negate")
@negate_func.register
def negate_int_func(arg:int):
return -arg
@negate_func.register
def negate_bool_func(arg:bool):
return not arg
if __name__ == "__main__":
print(negate_func(0))
print(negate_func(False))
print(negate_func(arg=0))
```
you get an error that tells you what actually is wrong:
```plaintext
PS C:\Users\mark> py -3 C:\Users\mark\Temp\singledisp.py
0
True
Traceback (most recent call last):
File "C:\Users\mark\Temp\singledisp.py", line 63, in <module>
print(negate_func(arg=0))
File "C:\Python38\lib\functools.py", line 871, in wrapper
raise TypeError(f'{funcname} requires at least '
TypeError: negate_func requires at least 1 positional argument
```
it seems that the code in `functools.singledispatchmethod` needs to check to
see if `args` is empty, and throw a similar (if not the same) exception as
`functools.singledispatch`
----------
components: Library (Lib)
messages: 372406
nosy: markgrandi
priority: normal
severity: normal
status: open
title: functools.singledispatchfunction has confusing error message if no
position arguments are passed in
type: behavior
versions: Python 3.8
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue41122>
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