plotly / plotly/plotly.py

Plotly Express constructor: in wide mode, lists passed to `x` and `y` params are mutated

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bug P3
Dominant language
Python
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Description

When either the x or y parameters to the Plotly Express constructor are passed lists of values, they are mutated such that values are converted to string.

Expected behaviour: Plotly Express should not mutate objects supplied as arguments by the user.

Minimal example (tested on Plotly 5.13.1 and master branch):

from random import randrange
import pandas as pd
import plotly.express as px

cols, rows = list(range(3)), list(range(10))
print(cols)
df = pd.DataFrame({col: [randrange(100) for _row in rows] for col in cols})
px.bar(df, y=cols, barmode="group")
print(cols)

Before the px.bar() call, cols has the value [1, 2, 3] and afterwards it is ["1", "2", "3"]. The same behaviour occurs for the x param and also when using other plotting functions than pxbar() (I tried px.histogram()).

I did some poking around, and can see that in the function build_dataframe in plotly/express/_core.py, the some of the fields in the user supplied args have copies made (which would fix this issue), but that is only applied to args found in the array_attrables list, which is ["dimensions", "custom_data", "hover_data", "path", "wide_variable"], which does not include x and y.

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start in plotly/express/_core.py and read build_dataframe, especially how array_attrables controls copying of user-supplied arguments. Reproduce the minimal example with wide-mode x or y lists, then verify that the original lists retain their values and are not converted to strings after the constructor call.

Written by the indexing model from the issue text.

Assessment

Tech stack
pandas, python
Domain
data-visualization
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
35/100

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