pandas DataFrame中经常出现SettingWithCopyWarning

如果你对原先的dataframe先进行slicing或者indexing，然后赋值，都会出现SettingWithCopyWarning。

情形一

d[d['col_1'] == 0]['col_2'] = 1

会出现警告

SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

解决方法，

使用loc或者iloc

d.loc[d['col_1'] == 0, 'col_2'] = 1

情形二

先从原dataframe取出一个子dataframe，然后再对其中的元素赋值，例如

s = d[d['col_1'] == 0]
s.loc[:, 'col_2'] = 1

就会出现

SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

解决方法，

对子datafame先进行copy，然后再赋值

s = d[d['col_1'] == 0].copy()
s.loc[:, 'col_2'] = 1

尝试用iloc或者loc来索引，比如

# 替换'norm'列
df.loc[:,'norm'] = np.random.randn(10)

# 替换第十列
df.iloc[:,10] = np.random.randn(10)