pandas 如何修改熊猫数据框的一个“单元格”中的值？
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How to modify a value in one "cell" of a pandas data frame?
提问by Roman
I have a very simple problem. I would like to change a value in a given column of a given row of a pandas data frame. I try to do it in the following way:
我有一个非常简单的问题。我想更改 Pandas 数据框给定行的给定列中的值。我尝试通过以下方式做到这一点：
df['column3'].loc[this_date] = val
As a result I get the following warning:
结果我收到以下警告：
SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame
My interpretation of this warning is that by using columns name ('column3') and locI do not really access (refer to) the desired cell of the data frame. Instead, I create an object which is a copy of the "cell" object and then I try to change the value associated with this "copy-object".
我对这个警告的解释是，通过使用列名 ( 'column3')，loc我并没有真正访问（参考）数据框的所需单元格。相反，我创建了一个对象，它是“单元格”对象的副本，然后我尝试更改与此“复制对象”关联的值。
What I do not understand is that it seems to work. In spite on the fact that pandas writes me that I try to modify the copy, I do modify the original data frame.
我不明白的是它似乎有效。尽管Pandas写信给我说我尝试修改副本，但我确实修改了原始数据框。
My question is how to make sure that I am really doing what I would like to do and how to do it in a "correct" way so that the pandas does not complain?
我的问题是如何确保我真的在做我想做的事情以及如何以“正确”的方式做到这一点，以便Pandas不会抱怨？
回答by Yang
The reason of getting the warning is that dfitself is a copy of some other dataframe object. I guess that you have some original dataframe df_originand you get dffrom df_originby some operation such as slicing. So dfis a copy of df_origin. Then you try to set some value to dfand the warning raises to tell you that this would not change the value in the original dataframe df_origin.
One solution is to use a single variable to point to the dataframe object before and after slicing if you don't care for df_origin. Otherwise you can suppress the warning by pd.set_option('mode.chained_assignment', None)
收到警告的原因是它df本身是某个其他数据帧对象的副本。我想，你有一些原始的数据帧df_origin，你会得到df从df_origin一些操作，如切片。所以，df是的副本df_origin。然后，您尝试将某个值设置为 ，df并且会出现警告以告诉您这不会更改原始数据帧中的值df_origin。如果您不关心 df_origin，一种解决方案是在切片前后使用单个变量指向数据帧对象。否则，您可以通过以下方式抑制警告pd.set_option('mode.chained_assignment', None)
Your way of setting value is fine, along with the following ones:
您设置值的方式很好，还有以下几种：
df.ix[this_date, 'column3'] = val
df.loc[this_date, 'column3'] = val
df.at[this_date, 'column3'] = val
