在 pandas 中重新分配:复制还是查看? [英] Re-assignment in Pandas: Copy or view?
问题描述
假设我们有以下数据框:
Say we have the following dataframe:
df = pd.DataFrame({'A' : ['foo', 'bar', 'foo', 'bar',
'foo', 'bar', 'foo', 'foo'],
'B' : ['one', 'one', 'two', 'three',
'two', 'two', 'one', 'three'],
'C' : randn(8), 'D' : randn(8)})
如下所示:
> df
A B C D
0 foo one 0.846192 0.478651
1 bar one 2.352421 0.141416
2 foo two -1.413699 -0.577435
3 bar three 0.569572 -0.508984
4 foo two -1.384092 0.659098
5 bar two 0.845167 -0.381740
6 foo one 3.355336 -0.791471
7 foo three 0.303303 0.452966
然后我执行以下操作:
df2 = df
df = df[df['C']>0]
如果现在查看df和df2，您会看到df2保存了原始数据，而df已更新为仅保留C所在的值大于0.
If you now look at df and df2 you will see that df2 holds the original data, whereas df was updated to only keep the values where C was greater than 0.
我以为Pandas不应像df2 = df这样的作业来复制，而只能使用以下任何一种进行复制:
I thought Pandas wasn't supposed to make a copy in an assignment like df2 = df and that it would only make copies with either:
-
df2 = df.copy(deep=True) -
df2 = copy.deepcopy(df)
df2 = df.copy(deep=True)df2 = copy.deepcopy(df)
那上面发生了什么? df2 = df做了副本吗?我认为答案是否，因此肯定是df = df[df['C']>0]制作了副本，而且我假设，如果我上面没有df2=df，那肯定会有副本. 没有任何引用，它会在内存中浮动.正确吗?
What happened above then? Did df2 = df make a copy? I presume that the answer is no, so it must have been df = df[df['C']>0] that made a copy, and I presume that, if I didn't have df2=df above, there would have been a copy without any reference to it floating in memory. Is that correct?
注意:我已阅读返回视图而不是副本，我想知道是否存在以下情况:
Note: I read through Returning a view versus a copy and I wonder if the following:
每当索引操作涉及标签数组或布尔向量时，结果将是副本.
解释了此行为.
推荐答案
不是df2正在制作副本，而是df = df[df['C'] > 0]正在返回副本.
It's not that df2 is making the copy, it's that the df = df[df['C'] > 0] is returning a copy.
只需打印出ID，您就会看到:
Just print out the ids and you'll see:
print id(df)
df2 = df
print id(df2)
df = df[df['C'] > 0]
print id(df)
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