问题解决：SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame

转载：y小川

SettingWithCopyWarning 解决方案

问题场景：我在读取csv文件之后，因为要新增一个特征列并根据已有特征修改新增列的值，结果在修改的时候就碰到了SettingWithCopyWarning这个警告，花了很长时间才解决这个问题。

一个简易版的范例

import pandas as pd
import numpy as np

aa = np.array([1, 0, 1, 0])
bb = pd.DataFrame(aa.T, columns=['one'])
print(bb)

输出为：

添加一个新列后在输出

bb['two'] = 0
print(bb)

output[]:
   one  two
0    1    0
1    0    0
2    1    0
3    0    0

按条件修改新列再输出就报错了：

for i in range(bb.shape[0]):
    if bb['one'][i] == 0:
        bb['two'][i] = 1
print(bb)

output[]:
C:/PycharmProjects/NaiveBayesProduct/pandas/try_index.py:22: 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/indexing.html#indexing-view-versus-copy
  bb['two'][i] = 1
   one  two
0    1    0
1    0    1
2    1    0
3    0    1

这个问题怎么解决呢，我查了stackoverflow上的很多帖子，试了loc/iloc等函数都不管用，最后才发现是顺序错了。正确方案应该是生成好正确的数组再插入dataframe中。下面我把上面的例子用正确地方法再重新生成一遍。

import pandas as pd
import numpy as np

aa = np.array([1, 0, 1, 0])
bb = pd.DataFrame(aa.T, columns=['one'])
# 生成一个ndarray，装要插入的值
two = np.zeros(bb.shape[0])
# 按条件修改two
for i in range(bb.shape[0]):
    if bb['one'][i] == 0:
        two[i] = 1
# 完成后将two插入dataframe中
bb.insert(1,'two', two)
print(bb)

output[]:
   one  two
0    1  0.0
1    0  1.0
2    1  0.0
3    0  1.0