Answers for "pandas count rows by column value"

pandas count rows with value

len(df[df['score'] == 1.0])

pandas count values by column

df = pd.DataFrame({'a':list('abssbab')})
df.groupby('a').count()

pandas count

>>> df.set_index(["Person", "Single"]).count(level="Person")
        Age
Person
John      2
Lewis     1
Myla      1