python count null values in dataframe

#Python, pandas
#Count missing values for each column of the dataframe df

df.isnull().sum()

In [5]: df = pd.DataFrame({'a':[1,2,np.nan], 'b':[np.nan,1,np.nan]})

In [6]: df.isna().sum()
Out[6]:
a    1
b    2
dtype: int64