Converting a Panda DF List into a string

Question

I have a pandas data frame. One of the columns contains a list. I want that column to be a single string.

For example my list ['one','two','three'] should simply be 'one, two, three'

df['col'] = df['col'].astype(str).apply(lambda x: ', '.join(df['col'].astype(str))) 

gives me ['one, two, three],['four','five','six'] where the second list is from the next row. Needless to say with millions of rows this concatenation across rows is not only incorrect, it kills my memory.

Answer 1

You should certainly not convert to string before you transform the list. Try:

df['col'].apply(', '.join) 

Also note that apply applies the function to the elements of the series, so using df['col'] in the lambda function is probably not what you want.

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Or, there is a native .str.join method, but it is (surprisingly) a bit slower than apply.

Answer 2

When you cast col to str with astype, you get a string representation of a python list, brackets and all. You do not need to do that, just apply join directly:

import pandas as pd  df = pd.DataFrame({     'A': [['a', 'b', 'c'], ['A', 'B', 'C']]     })  # Out[8]:  #            A # 0  [a, b, c] # 1  [A, B, C]  df['Joined'] = df.A.apply(', '.join)  #            A   Joined # 0  [a, b, c]  a, b, c # 1  [A, B, C]  A, B, C