Python:SettingWithCopyWarning:试图在 DataFrame 的切片副本上设置一个值

Question

我的 Pandas 数据框:

dframe = pd.DataFrame({"A":list("abcde"), "B":list("aabbc"), "C":[1,2,3,4,5]},  index=[10,11,12,13,14])

    A   B   C
10  a   a   1
11  b   a   2
12  c   b   3
13  d   b   4
14  e   c   5

我想要的输出:

    A   B   C   a   b   c
10  a   a   1   1   None    None
11  b   a   2   2   None    None
12  c   b   3   None    3   None
13  d   b   4   None    4   None
14  e   c   5   None    None    5

想法是根据“B”列中的值创建新列，复制“C”列中的相应值并将它们粘贴到新创建的列中。 这是我的代码:

lis = sorted(list(dframe.B.unique()))

#creating empty columns
for items in lis:
   dframe[items] = None


 #here copy and pasting
    for items in range(0, len(dframe)):
        slot = dframe.B.iloc[items]
        dframe[slot][items] = dframe.C.iloc[items]

我遇到了这个错误:

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
  app.launch_new_instance()

此代码在 Python 2.7 中运行良好，但在 3.x 中运行不佳。我哪里出错了？

Answer 1

开始于

to_be_appended = pd.get_dummies(dframe.B).replace(0, np.nan).mul(dframe.C, axis=0)

然后连接

dframe = pd.concat([dframe, to_be_appended], axis=1)

看起来像:

print dframe

    A  B  C    a    b    c
10  a  a  1  1.0  NaN  NaN
11  b  a  2  2.0  NaN  NaN
12  c  b  3  NaN  3.0  NaN
13  d  b  4  NaN  4.0  NaN
14  e  c  5  NaN  NaN  5.0

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