python - 为什么 pandas 逻辑运算符没有像它应该的那样在索引上对齐？

Question

考虑这个简单的设置:

x = pd.Series([1, 2, 3], index=list('abc'))
y = pd.Series([2, 3, 3], index=list('bca'))

x

a    1
b    2
c    3
dtype: int64

y

b    2
c    3
a    3
dtype: int64

正如您所看到的，索引是相同的，只是顺序不同。

现在，考虑使用相等 (==) 运算符进行简单的逻辑比较:

x == y
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

这会引发 ValueError，很可能是因为索引不匹配。另一方面，调用等效的 eq 运算符可以工作:

x.eq(y)

a    False
b     True
c     True
dtype: bool

OTOH，如果 y 首先重新排序，操作符方法就可以工作...

x == y.reindex_like(x)

a    False
b     True
c     True
dtype: bool

我的理解是，函数和运算符比较应该做同样的事情，所有其他事情都相同。 eq 做了哪些操作符比较没有做的事情？

Answer 1

查看与不匹配索引的系列比较的整个回溯，特别关注异常消息:

In [1]: import pandas as pd
In [2]: x = pd.Series([1, 2, 3], index=list('abc'))
In [3]: y = pd.Series([2, 3, 3], index=list('bca'))
In [4]: x == y
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-4-73b2790c1e5e> in <module>()
----> 1 x == y
/usr/lib/python3.7/site-packages/pandas/core/ops.py in wrapper(self, other, axis)
   1188 
   1189         elif isinstance(other, ABCSeries) and not self._indexed_same(othe
r):
-> 1190             raise ValueError("Can only compare identically-labeled "
   1191                              "Series objects")
   1192 
ValueError: Can only compare identically-labeled Series objects

我们看到这是一个经过深思熟虑的实现决定。此外，这并不是 Series 对象所独有的 - DataFrames 也会引发类似的错误。

深入挖掘相关行的 Git 责任最终会发现一些相关的提交和问题跟踪线程。例如，Series.__eq__ 用于完全忽略 RHS 的索引，并且在 comment 中在有关该行为的错误报告中，Pandas 作者 Wes McKinney 说道:

This is actually a feature / deliberate choice and not a bug-- it's related to #652. Back in January I changed the comparison methods to do auto-alignment, but found that it led to a large amount of bugs / breakage for users and, in particular, many NumPy functions (which regularly do things like arr[1:] == arr[:-1]; example: np.unique) stopped working.

This gets back to the issue that Series isn't quite ndarray-like enough and should probably not be a subclass of ndarray.

So, I haven't got a good answer for you except for that; auto-alignment would be ideal but I don't think I can do it unless I make Series not a subclass of ndarray. I think this is probably a good idea but not likely to happen until 0.9 or 0.10 (several months down the road).

这就是当时的changed到 pandas 0.19.0 中的当前行为。引用 "what's new" page :

Following Series operators have been changed to make all operators consistent, including DataFrame (GH1134, GH4581, GH13538)

• Series comparison operators now raise ValueError when index are different.
• Series logical operators align both index of left and right hand side.

这使得 Series 行为与 DataFrame 的行为相匹配，DataFrame 已经在比较中拒绝了不匹配的索引。

总之，让比较运算符自动对齐索引会破坏太多东西，所以这是最好的选择。