Answers for "add numbers in dataframe where number may be nan"

count nan pandas

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

df.isnull().sum()

find the number of nan per column pandas

In [1]: s = pd.Series([1,2,3, np.nan, np.nan])

In [4]: s.isna().sum()   # or s.isnull().sum() for older pandas versions
Out[4]: 2