ValueError while doing merge with DataFrames using Pandas

Question

I am trying to merge three DataFrames using Pandas. Why do I get this error message?

ValueError: The truth value of a DataFrame is ambiguous. Use a.empty, a.bool(), a.item(), a.any() or a.all().

This is the code:

df=pd.merge(energy,GDP,ScimEn,on="Country")

Here energy,GDP,ScimEn are the three DataFrames I was trying to merge.

Answer 1

The call signature of pd.merge accepts only 2 DataFrames to merge;

pd.merge(left, right, how='inner', on=None, left_on=None, right_on=None, left_index=False, right_index=False, sort=False, suffixes=('_x', '_y'), copy=True, indicator=False, validate=None)

So you'd need to do it in 2 steps:

df = pd.merge(energy, GDP, on="Country")
df = pd.merge(df, ScimEn, on="Country")

Or chain the 2 steps together with DataFrame.merge which allows for the immediate use of the new DataFrame:

df = energy.merge(GDP, on="Country").merge(ScimEn, on="Country")

Also it's a good idea to set the how keyword explicitly. If there are countries that are in one of the DataFrames but not in the others, then merging using the default how=inner would result in lost data, and should be changed to outer if you want to keep all data from all DataFrames.