Python’s lambda with underscore for an argument?

The _ is variable name. Try it.
(This variable name is usually a name for an ignored variable. A placeholder so to speak.)

Python:

>>> l = lambda _: True
>>> l()
<lambda>() missing 1 required positional argument: '_'

>>> l("foo")
True

So this lambda does require one argument. If you want a lambda with no argument that always returns True, do this:

>>> m = lambda: True
>>> m()
True