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>>> df = pd.DataFrame([[1, 2, 3], [4, None, None], [None, None, 9]])
>>> df.fillna(method='ffill')
0 1 2
0 1 2 3
1 4 2 3
2 4 2 9
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--fillna
product_mean = df['product'].mean()
df['product'] = df['product'].fillna(product_mean)
--replace method
col_mean = np.mean(df['col'])
df['col'] = df['col'].replace(np.nan, col_mean)