pandas的settingwithWaring报警

#  0 读取数据
import pandas as pd
df = pd.read_csv("beijing_tianqi_2018.csv")

# 换掉温度后面的后缀
df.loc[:,"bWendu"] = df["bWendu"].str.replace("℃","").astype("int32")
df.loc[:,"yWendu"] = df["yWendu"].str.replace("℃","").astype("int32")

#  1 复现

# 只选出3月份的数据用于分析
condition = df["ymd"].str.startswith("2018-03")
# 设置温差
df[condition]["wen_cha"] = df["bWendu"] - df["yWendu"]
# 查看是否修改成功
df[condition].head()

D:\Anaconda\lib\site-packages\ipykernel_launcher.py:6: 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

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy

# 2 原因
# 发出警告的代码 df[condition]["wen_cha"] = df["bWendu"] - df["yWendu"]
# 相当于： df.get(condition)set(wen_cha),第一步骤get发出了警报
# 链式操作其实就是两个步骤，先get后set，get得到的dataframe可能是view也可能是copy，pandas发出警告

# 核心要诀：pandas的dataframe的修改写操作，只允许在源dataframe上进行，一步到位

# 3 解决办法1
df.loc[condition,"wen_cha"]  = df["bWendu"] - df["yWendu"]
df[condition].head()

# 4 解决方法2
# 如果需要筛选数据做后续的处理分析，使用copy复制dataframe
df_month3 = df[condition].copy()
df_month3.head()
df_month3["wen_cha"] = df["bWendu"] - df["yWendu"]
df_month3.head()
# 总之，pandas不允许先筛选自dataframe，再进行修改写入
# 要么使用.loc实现一个步骤直接修改源dataframe
# 要么先复制一个子dataframe再一个步骤执行修改