pydata / pydata/patsy

Categorical does not work with nan

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Python
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Description

I have a columns whose unique looks like:

array([nan, 'CONFERENCE', 'ANALYST', 'FORUM', 'SEMINAR'], dtype=object)

I would expect that adding C(col_name) to the formula would create 4 dummy variables (5 values-1), bu in fact it only adds 3.

When I tried to explicitly set control to be nan, i get an exception:

C(col_name, Treatment(reference=nan)) 
PatsyError: specified level nan not found

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Research direction

Start by reproducing the reported C(col_name) behavior with the shown values, then try Treatment(reference=nan) and inspect the resulting exception. Trace how categorical levels and missing values are identified during formula processing. Done means the categorical term creates four dummy variables and nan can be selected as the reference level without the reported error.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
data
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
38/100

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