Merging two dataframes in a loop change original dataframe in loop-CodePudding

Assuming I have a list of dataframes

dataframes = [a, b, c, d]

and one dataframe additionalInformation containing information I need for the merge.

Is there a way to join the dataframes in a loop and overwrite the original dataframe?

for index, df in enumerate(dataframes):
    dataframes[index] = pd.merge(df, additionalInformation, how="left", left_on="cat", right_on="cat")

However, this is not updating the dataframes. When I do a

a.columns

The columns from additionalInformation are not merged... When I perform a

a = pd.merge(a, additionalInformation, how="left", left_on="cat", right_on="cat")

It works. How would I merge dataframes in a loop and overwrite the original dataframe?

CodePudding user response：

If you want to modify the DataFrame, you have to assign to the dataframe's content, not the variable.

If the cat values in additionalInformation are unique (thus making the left-merge keep the same number of row), you can use:

for index, df in enumerate(dataframes):
    merged = pd.merge(df, additionalInformation, how="left", on="cat")
    dataframes[index].loc[:, merged.columns] = merged

If the values are not unique, this will truncate the output