使用逻辑运算符构建 Pandas DataFrame
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Using logical operators in building a Pandas DataFrame
提问by Jamie Bull
I have two snippets of pandas code which I think should be equivalent, but the second one doesn't do what I expect.
我有两个 Pandas 代码片段，我认为它们应该是等效的，但是第二个片段没有达到我的预期。
# snippet 1
data = all_data[[((np.isfinite(all_data[self.design_metric][i])
and all_data['Source'][i] == 2))
or ((np.isfinite(all_data[self.actual_metric][i])
and all_data['Source'][i] != 2))
for i in range(len(all_data))]]
# snippet 2
data = all_data[(all_data['Source'] == 2 &
np.isfinite(all_data[self.design_metric])) |
(all_data['Source'] != 2 &
np.isfinite(all_data[self.actual_metric]))]
Each section (e.g. all_data['Source'] == 2) does what I expect on its own but it seems that I'm doing something wrong with the logical operators as the final result is coming out with a different result to the list comprehension version.
每个部分（例如all_data['Source'] == 2）都按照我自己的期望执行，但似乎我在逻辑运算符上做错了，因为最终结果与列表理解版本的结果不同。
回答by BrenBarn
The &operator binds more tightly than ==(or any comparison operator). See the documentation. A simpler example is:
的&操作者比结合更紧密地==（或任何比较操作符）。请参阅文档。一个更简单的例子是：
>>> 2 == 2 & 3 == 3
False
This is because it is grouped as 2 == (2 & 3) == 3, and then comparison chaining is invoked. This is what is happening in your case. You need to put parentheses around each comparison.
这是因为它被分组为2 == (2 & 3) == 3，然后调用比较链。这就是你的情况。您需要在每个比较周围加上括号。
data = all_data[((all_data['Source'] == 2) &
np.isfinite(all_data[self.design_metric])) |
((all_data['Source'] != 2) &
np.isfinite(all_data[self.actual_metric]))]
Note the extra parentheses around the ==and !=comparisons.
请注意==和!=比较周围的额外括号。
回答by alko
Along with priority, there is a difference between AND and & operators, first one being boolean and the latter being binary bitwise. Also, you must be aware of boolead expressions.
除了优先级之外，AND 和 & 运算符之间也存在差异，第一个是布尔值，后者是按位二进制。此外，您必须了解 boolead 表达式。
See examples in the following snippet:
请参阅以下代码段中的示例：
logical expressions
逻辑表达式
>>> 1 and 2
1
>>> '1' and '2'
'1'
>>> 0 == 1 and 2 == 0 or 0
0
bitwise operators
按位运算符
>>> 1 & 2
0
>>> '1' & '2'
Traceback (most recent call last):
...
TypeError: unsupported operand type(s) for &: 'str' and 'str'
>>> 0 == 1 & 2 == 0 | 0
True
