df['column'] = df['column'].fillna(0)
# based on another column value
df['column_1'].fillna(df['column_2'], inplace=True)
df.fillna(np.nan).replace([np.nan], [None])
df.fillna(np.nan).replace([np.nan], [None])
# selecting your desired columns
df[['a', 'b']] = df[['a', 'b']].fillna(df['c'], inplace=True)
df = df.fillna(method = 'ffill')
df[['a', 'b']] = df[['a','b']].fillna(value=0)