Group code Prefecture name Municipality name Gender Total number 0-4 years old 5-9 10-14 15-19\
3      10006.0 Hokkaido*NaN total 5339539 181591 201119 213206 231870
4      10006.0 Hokkaido*NaN Man 2522526 93048 102850 108783 118414
5      10006.0 Hokkaido*NaN Woman 2817013 88543 98269 104423 113456
6      11002.0 Sapporo, Hokkaido Total 1952348 71281 75025 76342 82644
7      11002.0 Sapporo, Hokkaido Man 913077 36715 38250 38839 41729

df['City name'].fillna(df['Name of prefectures'], inplace=True)

#It's possible here, but it seems to be slower.
df.loc[pd.isnull(df['City name']), ['City name']] = df['Name of prefectures']

for i, (prefecture, city) in enumerate(zip(df['Name of prefectures'], df['City name'])):
    if pd.isnull(city):
        df['City name'][i] = prefecture

<ipython-input-59-8ff91f161741>:3: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  df['City name'][i] = prefecture
​

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