pandas_WTF.py

df = pd.DataFrame({'a': [5,1,6,2,23], 'b': [7,7,7,7,7]})
"""
Out:
    a  b
0   5  7
1   1  7
2   6  7
3   2  7
4  23  7
"""
df.loc[(df.a < 5), ['a']] = pd.Series([100,100])  # WAT
"""
Out:
    a  b
0   5  7
1   100  7
2   6  7
3 NaN  7
4  23  7
"""
df.loc[(df.a < 5), 'a'] = [100, 100]  # Good
"""
Out[103]:
    a  b
0   5  7
1   100  7
2   6  7
3   100  7
4  23  7
"""
df.loc[(df.a < 5), ['a', 'b']] = [[100, 100], [200, 300]]  # Good
"""
Out[111]:
     a    b
0    5    7
1  100  100
2    6    7
3  200  300
4   23    7
"""
series = df[df.a < 5].apply(lambda row: pd.Series([row['a']+100, row['b']+200]), axis=1)
"""
     0    1
1  101  207
3  102  207
"""
noseries = df[df.a < 5].apply(lambda row: [row['a']+100, row['b']+200], axis=1)
"""
     a    b
1  101  207
3  102  207
"""
df[df.a < 5] = series  # Done 
"""
     a    b
0    5    7
1  101  207
2    6    7
3  102  207
4   23    7
"""
df[df.a < 5] = noseries  # Done 
"""
     a    b
0    5    7
1  101  207
2    6    7
3  102  207
4   23    7
"""
series_one = df[df.a < 5].apply(lambda row: pd.Series([row['a']+100]), axis=1)  # only one column
"""
     0
1  101
3  102
"""
noseries_one = df[df.a < 5].apply(lambda row: [row['a']+100], axis=1)
"""
     a    b
1  101  101
3  102  102
"""
noseries = df[df.a < 5].apply(lambda row: row['a']+100, axis=1)
"""
1    101
3    102
"""

df.loc[(df.a < 5), ['a', 'b']] = [[100, 100], [200, 300]] # Good 로 결국 해결

Series 는 Index로 매칭