How to prevent: "UserWarning: Boolean Series key will be reindexed to match DataFrame index"

Question

I am having some issues with my Python code. When working with DataFrames I get a UserWarning and I'm not really sure of how to prevent it.

for index in matplatsID:
      mask = (kameraData["Tid"].dt.hour >= timme) & (kameraData["Tid"].dt.hour < timme+1)
      matplatsSummaHastigheter += kameraData[mask][kameraData["MätplatsID"] == index]["Hastighet"].sum()
      matplatsAntalFordon += kameraData[mask][kameraData["MätplatsID"] == index]["Hastighet"].count()

UserWarning: Boolean Series key will be reindexed to match DataFrame index.
  matplatsSummaHastigheter += kameraData[mask][kameraData["MätplatsID"] == index]["Hastighet"].sum()
UserWarning: Boolean Series key will be reindexed to match DataFrame index.
  matplatsAntalFordon += kameraData[mask][kameraData["MätplatsID"] == index]["Hastighet"].count()

It generates these warnings, and I am dumbfounded of how to avoid them, any ideas of what to do?

score 0 · Accepted Answer · answered May 20 '22 at 09:33

You need chain all 3 conditions for testing original DataFrame:

for index in matplatsID:
     mask = (kameraData["Tid"].dt.hour >= timme) & 
            (kameraData["Tid"].dt.hour < timme+1) & 
            (kameraData["MätplatsID"] == index)
     matplatsSummaHastigheter += kameraData.loc[mask, "Hastighet"].sum()
     matplatsAntalFordon += kameraData.loc[mask, "Hastighet"].count()

In your solution first filter by mask and then by kameraData["MätplatsID"] == index, so raise error, because filter of filtered DataFrame.

Perfect thank you! It works flawlessly now :) – RasmusOberg May 20 '22 at 09:43 — RasmusOberg, May 20 '22 at 09:43

How to prevent: "UserWarning: Boolean Series key will be reindexed to match DataFrame index"

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