pythonpandasdataframeapply

Pandas apply does not modify DataFrame

I am just starting pandas so please forgive if this is something stupid.

I am trying to apply a function to a column but its not working and i don't see any errors also.

capitalizer = lambda x: x.upper()
for df in pd.read_csv(downloaded_file, chunksize=2, compression='gzip', low_memory=False):
    df['level1'].apply(capitalizer)
    print df
    exit(1)

This print shows the level1 column values same as the original csv not doing upper. Am i missing something here?

Solution

  • apply is not an inplace function - it does not modify values in the original object, so you need to assign it back:

    df['level1'] = df['level1'].apply(capitalizer)

    Alternatively, you can use str.upper, it should be much faster.

    df['level1'] = df['level1'].str.upper()