Interactive fails for non-zero decimal default argument

[ianhi]

Description

On master if you have a default argument that has a non-zero decimal then interactive will fail to generate a slider.
This appears to be a regression from the 7.x branch.

Reproduce

from ipywidgets import interactive

# only a non-zero decimal will fail
def f(tau=1, beta=1.0,  zeta=1.5):
    print(beta, tau)
interactive(f)

Jupyter Log Console

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~/mambaforge/envs/widgets-2/lib/python3.9/site-packages/traitlets/traitlets.py in validate(self, obj, value)
   2093         try:
-> 2094             value = float(value)
   2095         except Exception:
TypeError: float() argument must be a string or a number, not 'NoneType'
During handling of the above exception, another exception occurred:
TraitError                                Traceback (most recent call last)

~/Documents/oss/jupyter/ipywidgets/ipywidgets/widgets/widget.py in _handle_msg(self, msg)

628                 if 'buffer_paths' in data:

629                     _put_buffers(state, data['buffer_paths'], msg['buffers'])

--> 630                 self.set_state(state)

631

632         # Handle a state request.
~/Documents/oss/jupyter/ipywidgets/ipywidgets/widgets/widget.py in set_state(self, sync_data)

512                     from_json = self.trait_metadata(name, 'from_json',

513                                                     self._trait_from_json)

--> 514                     self.set_trait(name, from_json(sync_data[name], self))

515

516     def send(self, content, buffers=None):
~/mambaforge/envs/widgets-2/lib/python3.9/site-packages/traitlets/traitlets.py in set_trait(self, name, value)

1434                                 (cls.name, name))

1435         else:

-> 1436             getattr(cls, name).set(self, value)

1437

1438     @classmethod
~/mambaforge/envs/widgets-2/lib/python3.9/site-packages/traitlets/traitlets.py in set(self, obj, value)

576

577     def set(self, obj, value):

--> 578         new_value = self._validate(obj, value)

579         try:

580             old_value = obj._trait_values[self.name]
~/mambaforge/envs/widgets-2/lib/python3.9/site-packages/traitlets/traitlets.py in _validate(self, obj, value)

608             return value

609         if hasattr(self, 'validate'):

--> 610             value = self.validate(obj, value)

611         if obj._cross_validation_lock is False:

612             value = self._cross_validate(obj, value)
~/mambaforge/envs/widgets-2/lib/python3.9/site-packages/traitlets/traitlets.py in validate(self, obj, value)

2094             value = float(value)

2095         except Exception:

-> 2096             self.error(obj, value)

2097         return _validate_bounds(self, obj, value)

2098
~/mambaforge/envs/widgets-2/lib/python3.9/site-packages/traitlets/traitlets.py in error(self, obj, value, error, info)

688                     e = "The '%s' trait expected %s, not %s." % (

689                         self.name, self.info(), describe("the", value))

--> 690                 raise TraitError(e)

691

692     def get_metadata(self, key, default=None):
TraitError: The 'value' trait of a FloatSlider instance expected a float, not the NoneType None.

Expected behavior

A working slider is generated

Context

• ipywidgets version 8.0.0a0 21c2dc6

[davidbrochart]

Also, the initial values of the sliders are not the default values of the function's parameters. Is that expected?

[SylvainCorlay]

cc @ibdafna

[davidbrochart]

It's not specific to interactive, it also affects a FloatSlider:

from ipywidgets import FloatSlider
FloatSlider(value=1.5, description='zeta', min=-1.5, max=4.5)

[ibdafna]

@davidbrochart I see you self assigned, let me know if you need help!

[davidbrochart]

For now I have no idea what's going on, so no problem if you want to look into it too 😄

[davidbrochart]

I can see that here:

ipywidgets/packages/controls/src/widget_int.ts

Line 229 in c46e5dc

this.handleSliderChanged(values, handle);

we have values = [NaN]. Where is this value set?

[ibdafna]

I've done some tests and this combination of parameters seems to fail noUiSlider at the source😱. You can see the issue here; open the console and try dragging the slider: https://jsfiddle.net/itayd/aoL43yn8/2/

Removing the built-in format function does seem to solve the problem, but we rely on it for float precision...I'll need to dig deeper to find a suitable fix.

[davidbrochart]

It is because here:

ipywidgets/packages/controls/src/widget_int.ts

Lines 217 to 220 in c46e5dc

	format: {
	from: (value: number): number => value,
	to: (value: number): number => value
	}

from receives a string and should return a number. If you replace with the following it works fine:

from: (value) => Number(value),

I'll open a PR.