param.pprint breaks if HoloViews is imported

[hoxbro]

Running the following:

import holoviews as hv
import param


class T(param.Parameterized):
    a = param.Number(1)


t = T()
t.param.pprint()

Will give the following traceback:

Traceback (most recent call last):
  File "/home/shh/Development/holoviz/development/tmp.py", line 10, in <module>
    t.param.pprint()
  File "/home/shh/Repos/holoviz/param/param/parameterized.py", line 2894, in pprint
    return partial(self_._pprint, self, imports=imports, prefix=prefix,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shh/Repos/holoviz/param/param/_utils.py", line 105, in wrapper
    result = user_function(self, *args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/shh/Repos/holoviz/param/param/parameterized.py", line 2914, in _pprint
    args = spec.args[1:] if spec.args[0] == 'self' else spec.args
                            ~~~~~~~~~^^^
IndexError: list index out of range

If I add __init__ to the class, it works or remove the holoviews import it works again.

class T(param.Parameterized):
    a = param.Number(1)
    
    def __init__(self):
        super().__init__()

[maximlt]

It can be reproduced with:

import inspect
import param


class A(param.Parameterized): pass

A.__init__.__signature__ = inspect.Signature([inspect.Parameter('test', inspect.Parameter.KEYWORD_ONLY)])


class T(param.Parameterized):
    a = param.Number(1)


t = T()
print(t.param.pprint())

This is indeed what HoloViews is doing:
https://github.com/holoviz/holoviews/blob/a64cfc544acc94b3806d427912047ca660253587/holoviews/util/__init__.py#L626

But I don't understand yet why updating the docstring of a class (A) affects another one (T).

[hoxbro]

Isn't it because A's __init__ is param.parameterized.Parameterized.__init__

[maximlt]

Yep indeed I also just figured this out. So I think there's two workaround?

• param could just check that 'self' is in spec.args, that should be sufficient to avoid this breakage
• HoloViews could add an __init__ method to its output ParameterizedFunction to avoid overriding the __signature__ of all the Parameterized objects :)

The real issue to fix has been well defined in #371.

[hoxbro]

I heavily favor number two. I think it is "correct" that params raise an error, as there is something "wrong" with what HoloViews does.

[maximlt]

Ok I got slightly confused by inspect.signature that doesn't return the same output on the class or instance: