This function is implemented in pandas, actually even in value_counts(). No need to calculate 🙂
just type:
df.sex.value_counts(normalize=True)
which gives exactly the desired output.
Please note that value_counts() excludes NA values, so numbers might not add up to 1.
See here: http://pandas-docs.github.io/pandas-docs-travis/generated/pandas.Series.value_counts.html
(A column of a DataFrame is a Series)