Better error message when trying to use Optional[...] in a type parameter

[angus-lherrou]

Click should raise a clear error when a user tries to pass a generic type such as Optional[int] as the type parameter to an option, for example:

@click.command()
@click.option("--number", "-n", type=Optional[int], default=None)
def cmd(number):
    ...

This is incorrect code, but a very reasonable mistake to make.

Currently, code like this emits a rather cryptic error like so:

Traceback (most recent call last):
  File "cmd.py", line 3, in <module>
    def cmd(number):
  File ".../lib/python3.8/site-packages/click/decorators.py", line 308, in decorator
    _param_memo(f, OptionClass(param_decls, **option_attrs))
  File ".../lib/python3.8/site-packages/click/core.py", line 2495, in __init__
    super().__init__(param_decls, type=type, multiple=multiple, **attrs)
  File ".../lib/python3.8/site-packages/click/core.py", line 2075, in __init__
    self.type = types.convert_type(type, default)
  File ".../lib/python3.8/site-packages/click/types.py", line 1040, in convert_type
    return FuncParamType(ty)
  File ".../lib/python3.8/site-packages/click/types.py", line 165, in __init__
    self.name = func.__name__
  File ".../lib/python3.8/typing.py", line 760, in __getattr__
    raise AttributeError(attr)
AttributeError: __name__

This is a fairly cryptic error, and it would be better to intercept this and raise a more descriptive error to explain the problem.

[davidism]

Type checking should be done using a static type checking tool. I don't plan to add more setup/runtime checks.

[angus-lherrou]

@davidism this is not really about type checking. This is about emitting a better error message in a case of very reasonable incorrect usage of the library. Could you look at my example again and reconsider?