Failure to use typing.NewType as the type for an option

[mthorpe7]

typing.NewType is a cheap way to create an alias type for type checking only (at runtime it is the underlying type). I was hoping to use this one in a @click.option parameter/type list, but this does something unexpected (although I can understand why it behaves like this).

from typing import NewType, reveal_type
import click

UserId = NewType("UserId", int)

@click.command
@click.option("--user-id", type=UserId, default=UserId(1))
def main(user_id: UserId):
    reveal_type(UserId)
    reveal_type(user_id)
    assert isinstance(user_id, int)

if __name__ == "__main__":
    main()

With the default, this is happy:

$ python cli.py 
Runtime type is 'NewType'
Runtime type is 'int'

when passing a value, it isn't:

$ python cli.py --user-id 10
Runtime type is 'NewType'
Runtime type is 'str'
Traceback (most recent call last):
  File "/work/cli.py", line 17, in <module>
    main()
  File "/work/.venv/lib/python3.12/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/work/.venv/lib/python3.12/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "/work/.venv/lib/python3.12/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/work/.venv/lib/python3.12/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/work/cli.py", line 13, in main
    assert isinstance(user_id, int)
           ^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Environment:

• Python version: Python 3.12.5
• Click version: click-8.1.7

[davidism]

type is the Click type, not the Python type. You'd pass click.Integer (or int as a hard-coded shortcut) there. If you know of a way to introspect that something is a NewType at runtime, and find out what it aliases (int) then that can be added to the automatic conversions. But in general, you're expecting a level of typing support that we just don't do.

click/src/click/types.py

Lines 1001 to 1058 in 4a758f5

	def convert_type(ty: t.Any | None, default: t.Any | None = None) -> ParamType:
	"""Find the most appropriate :class:`ParamType` for the given Python
	type. If the type isn't provided, it can be inferred from a default
	value.
	"""
	guessed_type = False
	if ty is None and default is not None:
	if isinstance(default, (tuple, list)):
	# If the default is empty, ty will remain None and will
	# return STRING.
	if default:
	item = default[0]
	# A tuple of tuples needs to detect the inner types.
	# Can't call convert recursively because that would
	# incorrectly unwind the tuple to a single type.
	if isinstance(item, (tuple, list)):
	ty = tuple(map(type, item))
	else:
	ty = type(item)
	else:
	ty = type(default)
	guessed_type = True
	if isinstance(ty, tuple):
	return Tuple(ty)
	if isinstance(ty, ParamType):
	return ty
	if ty is str or ty is None:
	return STRING
	if ty is int:
	return INT
	if ty is float:
	return FLOAT
	if ty is bool:
	return BOOL
	if guessed_type:
	return STRING
	if __debug__:
	try:
	if issubclass(ty, ParamType):
	raise AssertionError(
	f"Attempted to use an uninstantiated parameter type ({ty})."
	)
	except TypeError:
	# ty is an instance (correct), so issubclass fails.
	pass
	return FuncParamType(ty)