python
TypeError: Parameter list cannot be empty for TypeVarTuple
There is a problem with typing_extensions that doesn't happen with typing. Running the program with python3.11 works, but with python3.10 with typing_extensions throws:
Traceback (most recent call last):
File ".../main.py", line 48, in <module>
main()
File ".../main.py", line 40, in main
signal: Signal[()] = Signal[()]()
File "/usr/lib/python3.10/typing.py", line 312, in inner
return func(*args, **kwds)
File "/usr/lib/python3.10/typing.py", line 1328, in __class_getitem__
raise TypeError(
TypeError: Parameter list to Signal[...] cannot be empty
from __future__ import annotations
from typing_extensions import Callable, TypeVarTuple, Generic, Unpack, List, TypeVar
VarArgs = TypeVarTuple('VarArgs')
class Signal(Generic[Unpack[VarArgs]]):
def __init__(self):
self.functions: List[Callable[..., None]] = []
"""
Simple mechanism that allows abstracted invocation of callbacks. Multiple callbacks can be attached to a signal
so that they are all called when the signal is emitted.
"""
def connect(self, function: Callable[..., None]):
"""
Add a callback to this Signal
:param function: callback to call when emited
"""
self.functions.append(function)
def emit(self, *args: Unpack[VarArgs]):
"""
Call all callbacks with the arguments passed
:param args: arguments for the signal, must be the same type as type parameter
"""
for function in self.functions:
if args:
function(*args)
else:
function()
def main():
def signalEmptyCall():
print("Hello!")
signal: Signal[()] = Signal[()]()
signal.connect(signalEmptyCall)
signal.emit()
if __name__ == '__main__':
main()
I leave a quick comment here.
The problem is that in 3.10 typing.Generic(and earlier?) there is a check:
if not params and cls is not Tuple:
raise TypeError(f"Parameter list to {cls}[...] cannot be empty")
As params is an empty tuple the TypeError is raised. We either need to monkeypatch typing or create a custom version of Generic - as there are multiple is Generic tests in typing I am not sure how easy it is to create a stable alternative version.
Afaik, the only workaround that can be used before 3.11 is to use the equivalent form of () ≡ Unpack[Tuple[()]]:
signal: Signal[Unpack[Tuple[()]]]
We could make a typing_extensions.Generic that is different from typing.Generic and fixes this issue.
