Figure factory broken in release 4.7.0

Describe the bug

Related to https://github.com/plotly/plotly.py/issues/1111, import plotly.figure_factory fails with the most recent Plotly release (4.7) without a helpful error message.

To reproduce

First noticed with

conda create -n plotly_test python=3.7.7
conda activate plotly_test
pip install plotly_express
python
>>> import plotly.figure_factory

(which worked until this morning’s release), but also reproducible via:

conda create -n plotly_test python=3.7.7
conda activate plotly_test
pip install numpy==1.18.4 plotly==4.7.0
python
>>> import plotly.figure_factory

Expected behavior

It works, or at least gives a helpful error message.

Actual behavior

>>> import plotly.figure_factory
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/software/anaconda3/envs/plotly_test/lib/python3.7/site-packages/plotly/figure_factory/__init__.py", line 14, in <module>
    from plotly.figure_factory._2d_density import create_2d_density
  File "/software/anaconda3/envs/plotly_test/lib/python3.7/site-packages/plotly/figure_factory/_2d_density.py", line 8, in <module>
    from plotly.graph_objs import graph_objs
  File "/software/anaconda3/envs/plotly_test/lib/python3.7/site-packages/plotly/graph_objs/graph_objs.py", line 1, in <module>
    from plotly.graph_objs import *
  File "/software/anaconda3/envs/plotly_test/lib/python3.7/site-packages/plotly/graph_objs/__init__.py", line 290, in __getattr__
    return orig_getattr(import_name)
  File "/software/anaconda3/envs/plotly_test/lib/python3.7/site-packages/_plotly_utils/importers.py", line 41, in __getattr__
    name=import_name, __name__=parent_name
AttributeError: module 'plotly.graph_objs' has no attribute 'FigureWidget'

Workaround

Manually install ipywidgets (e.g., pip install ipywidgets plotly_express works).

Suggested solution

Add ipywidgets back to the plotly_express requirements, and make an ipywidgets error message similar to the one that exists if you try to use figure_factory without having numpy installed.

Platform:

• OS: Ubuntu 18.04
• Python: 3.7.7 (observed on Python 3.8 as well, but not on Python 3.6)

[nicolaskruchten]

@leo-smi this is intentional, yes.

[jonmmease]

Proposed fix in https://github.com/plotly/plotly.py/pull/2445. @emmanuelle would you have time to take a look?