pyjanitor-devs/pyjanitor

[ENH] expand_columns should support drop_first

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#368 opened on May 19, 2019

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 (5 comments) (3 reactions) (0 assignees)Python (1,217 stars) (164 forks)batch import
being worked onenhancementgood first issuegood intermediate issue

Description

Brief Description

In general, when you create dummy variables it is a good idea to drop one of the resultant columns as it is a linear combination of the other columns. See https://datascience.stackexchange.com/questions/27957/why-do-we-need-to-discard-one-dummy-variable

Pandas has the drop_first option in get_dummies

I would like to propose that drop_first be added as a parameter to expand_columns

Example API

Please modify the example API below to illustrate your proposed API, and then delete this sentence.

>>> X_cat2 = pd.DataFrame({'A': [1, None, 3],
...     'names': ['Fred,George', 'George', 'John,Paul']})
>>> jn.expand_column(X_cat2, 'names', sep=',')
     A        names  Fred  George  John  Paul
0  1.0  Fred,George     1       1     0     0
1  NaN       George     0       1     0     0
2  3.0    John,Paul     0       0     1     1

to

>>> X_cat2 = pd.DataFrame({'A': [1, None, 3],
...     'names': ['Fred,George', 'George', 'John,Paul']})
>>> jn.expand_column(X_cat2, 'names', sep=',', drop_first=True)
     A        names  Fred  George  John
0  1.0  Fred,George     1       1     0     
1  NaN       George     0       1     0     
2  3.0    John,Paul     0       0     1

Contributor guide

Tech stack
pythonpandas
Domain
data
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with pandas get dummiesBasic PythonUnderstanding of one hot encoding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
75
Research direction
The expand column function is implemented in pyjanitor's codebase (likely in functions.py). Review the current implementation to understand how it creates dummy variables. Add a drop first parameter that excludes the first category when set to True. Ensure to update documentation and tests accordingly. Check the issue comments for any additional requirements or suggestions from maintainers.