#Replace NaN Values with Zeros in Pandas DataFrame
#For a single column using Pandas
df['column-name'] = df['column-name'].fillna(0)
f_num.strip().zfill(2)
>>> n = '4'
>>> print(n.zfill(3))
004
# apply to pandas dataframe column
df['column'].apply(lambda x: str(x).zfill(11))