[Easy fix] plotly/express/_core.py:271 FutureWarning

[Getslow6]

Hi all,

I've got an online Jupyter notebook where I came across this error, but not sure how to fix and test this bug. I think it might be an easy fix:

/usr/local/lib/python3.9/dist-packages/plotly/express/_core.py:271: FutureWarning:

The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.

Example code to reproduce the issue:

df = pd.DataFrame({'Weekday': ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday', 'Monday', 'Thursday', 'Sunday'],
                   'Temperature': [10., 12., 14., 16., 12., 14., 16., 12., 14., 16.]})

temperatures = df.groupby('Weekday', as_index=False)['Temperature'].mean()
fig = px.line_polar(temperatures, r='Temperature', theta='Weekday', line_close=True)
fig.show()

The code that needs to be changed can be found here, on line 271:
https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/plotly/express/_core.py

I think the change will need to be:
trace_data = trace_data.append(trace_data.iloc[0]) to trace_data = pd.concat([trace_data, trace_data.iloc[0]])

[AaronStiff]

Hi @Getslow6. It appears this deprecation issue is being worked on in #4025, but it hasn't been merged yet