Count number of words per row

str.split + str.len

str.len works nicely for any non-numeric column.

df['totalwords'] = df['col'].str.split().str.len()

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str.count

If your words are single-space separated, you may simply count the spaces plus 1.

df['totalwords'] = df['col'].str.count(' ') + 1

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List Comprehension

This is faster than you think!

df['totalwords'] = [len(x.split()) for x in df['col'].tolist()]