pandas count number of rows with value

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

>>> df.count(axis='columns')
0    3
1    2
2    3
3    3
4    3
dtype: int64

In [37]:
df = pd.DataFrame({'a':list('abssbab')})
df.groupby('a').count()

Out[37]:

   a
a   
a  2
b  3
s  2

[3 rows x 1 columns]