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15 May 2023

Pandas joining and merging tables

I was trying to do a join based on two columns, one of which is a pd Timestamp.

What I learned: If you’re trying to join/merge two DataFrames not by their indexes,
pandas.DataFrame.merge is better (yay precise language) than pandas.DataFrame.join.

Or, for some reason I had issues with df.join(.. by=[col1,col2]), even with df.set_index([col1,col2]).join(df2.set_index...), then it went out of memory and I gave up.

Then a SO answer1 said

use merge if you are not joining on the index

I tried it and df.merge(..., by=col2) magically worked!

  1. python - Why does Pandas inner join give ValueError: len(left_on) must equal the number of levels in the index of “right”? - Stack Overflow ↩︎

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Nel mezzo del deserto posso dire tutto quello che voglio.