Plotting a DataFrame with duplicate columns should produce a more informative error

[schlich]

Some weird quirks in the Pandas API allow for columns with duplicate column names:

df=pd.DataFrame.from_records(
    {"Variable1": 1, "Variable2": 2}, index=[1]
).rename(columns={"Variable2": "Variable1"})

	Variable1	Variable1
1	1	2

attempting to plot this results in an uninformative ValueError:

>> px.scatter(df)
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-95-d52a0e44708e> in <module>
      1 import plotly.express as px
      2 df=pd.DataFrame.from_records({"Variable1": "Value", "Variable2": "Value"}, index=[1]).rename(columns={"Variable2": "Variable1"})
----> 3 px.scatter(df)
      4 # px.scatter(
      5 #     df,

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/plotly/express/_chart_types.py in scatter(data_frame, x, y, color, symbol, size, hover_name, hover_data, custom_data, text, facet_row, facet_col, facet_col_wrap, facet_row_spacing, facet_col_spacing, error_x, error_x_minus, error_y, error_y_minus, animation_frame, animation_group, category_orders, labels, orientation, color_discrete_sequence, color_discrete_map, color_continuous_scale, range_color, color_continuous_midpoint, symbol_sequence, symbol_map, opacity, size_max, marginal_x, marginal_y, trendline, trendline_color_override, log_x, log_y, range_x, range_y, render_mode, title, template, width, height)
     62     mark in 2D space.
     63     """
---> 64     return make_figure(args=locals(), constructor=go.Scatter)
     65 
     66 

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/plotly/express/_core.py in make_figure(args, constructor, trace_patch, layout_patch)
   1859     apply_default_cascade(args)
   1860 
-> 1861     args = build_dataframe(args, constructor)
   1862     if constructor in [go.Treemap, go.Sunburst] and args["path"] is not None:
   1863         args = process_dataframe_hierarchy(args)

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/plotly/express/_core.py in build_dataframe(args, constructor)
   1375     # now that things have been prepped, we do the systematic rewriting of `args`
   1376 
-> 1377     df_output, wide_id_vars = process_args_into_dataframe(
   1378         args, wide_mode, var_name, value_name
   1379     )

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/plotly/express/_core.py in process_args_into_dataframe(args, wide_mode, var_name, value_name)
   1196                 else:
   1197                     col_name = str(argument)
-> 1198                     df_output[col_name] = to_unindexed_series(df_input[argument])
   1199             # ----------------- argument is likely a column / array / list.... -------
   1200             else:

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/plotly/express/_core.py in to_unindexed_series(x)
   1047     required to get things to match up right in the new DataFrame we're building
   1048     """
-> 1049     return pd.Series(x).reset_index(drop=True)
   1050 
   1051 

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/pandas/core/series.py in __init__(self, data, index, dtype, name, copy, fastpath)
    266             name = ibase.maybe_extract_name(name, data, type(self))
    267 
--> 268             if is_empty_data(data) and dtype is None:
    269                 # gh-17261
    270                 warnings.warn(

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/pandas/core/construction.py in is_empty_data(data)
    626     is_none = data is None
    627     is_list_like_without_dtype = is_list_like(data) and not hasattr(data, "dtype")
--> 628     is_simple_empty = is_list_like_without_dtype and not data
    629     return is_none or is_simple_empty
    630 

~/.cache/pypoetry/virtualenvs/psychoanalyze-zEU_a_jI-py3.9/lib/python3.9/site-packages/pandas/core/generic.py in __nonzero__(self)
   1440     @final
   1441     def __nonzero__(self):
-> 1442         raise ValueError(
   1443             f"The truth value of a {type(self).__name__} is ambiguous. "
   1444             "Use a.empty, a.bool(), a.item(), a.any() or a.all()."

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

[nicolaskruchten]

Indeed, we need a better error message here! Sorry if it took you a while to figure out what was happening :) We'd love a pull request to make this clearer in the future :)

[dvfariaf-bops]

+1 on this!

[kb-]

+1 I lost quite some time on this tonight

[PhorstenkampFuzzy]

This is a super anoying bug.

[mohana-martin]

+1 It's been two years.. Please fix this...
Plotly is otherwise a great plotting tool, but I must say it is very annoying to use due to the inconsistencies and very obscure errors..

[gvwilson]

Hi - we are tidying up stale issues and PRs in Plotly's public repositories so that we can focus on things that are still important to our community. Since this one has been sitting for a while, I'm going to close it; if you'd like to submit a PR, we'd be happy to prioritize a review. Thank you - @gvwilson

[MarcoGorelli]

#4790 addresses this "for free", and produces the error message:

ValueError: Expected unique column names, got:
- 'Variable1' 2 times

[MarcoGorelli]

this is addressed by #4790, I think it can be closed

[turbotimon]

maybe similar to #4826