Pandas apply with swifter on axis 1 doesn't return

Question

I try to apply the following code (minimal example) to my 2 Million rows DataFrame, but for some reason .apply returns more than one row to the function and breaks my code. I'm not sure what changed, but the code did run before.

def function(row):

  return [row[clm1], row[clm2]]


res = pd.DataFrame()
res[["clm1", "clm2"]] = df.swifter.apply(function,axis=1)

Did anyone get an idea or a similar issue?

Important without swifter everything works fine, but too slow due to the amount of rows.

Answer 1

axis=1 means column, so it will insert it vertically. Is that what you want? Try removing axis=1

Answer 2

based on this previous answer what you are trying to do should work if you change it like this:

def function(row):
  return [row.swifter[clm1], row.swifter[clm2]]


res = pd.DataFrame()
res[["clm1", "clm2"]] = df.apply(function, axis=1, result_type='expand')

this because apply on a column lacks result_type as arg, while apply on a dataframe has it

Answer 3

This should work ==>

  def function(row_different_name):
        
        return [row_different_name[clm1], row_different_name[clm2]]
        
        
  res = pd.DataFrame()
  res[["clm1", "clm2"]] = df.swifter.apply(function,axis=1)

Try changing the name of function parameter rwo to some other name.