python - Converting a Panda DF List into a string

Question

I have a panda 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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Edit: thanks Yakym for pointing out that there is no need for a lambda function.

Edit: as noted by Anton Protopopov, there is a native .str.join method, but it is (surprisingly) a bit slower than apply.