在List Comprehensions中使用if，elif，else，Python [英] Using if, elif, else in List Comprehensions, Python

问题描述

我在python中创建了以下列表解析：

I created the following list comprehension in python:

[int(a[0].internal_value).lower() if type(a[0].internal_value) in (str,unicode) and a[0].internal_value.isdigit() == True
 else str(a[0].internal_value).lower() if type(a[0].internal_value) in (str,unicode)
 else int(a[0].internal_value) if type(a[0].internal_value) in (float,int)
 for a in ws.iter_rows() if a[0].internal_value <> None]

我在尝试构建最终的else时遇到问题，如果条件：

I'm having issues trying to construct the final else, if condition:

else int(a[0].internal_value) if type(a[0].internal_value) in (float,int)

如果我在该行中使用if条件，则语法无效。

I get an invalid syntax if I use the if conditional in that line.

 if type(a[0].internal_value) in (float,int)

如果我删除if语句

else int(a[0].internal_value)

然后它似乎运行正常。我需要在那里有if语句。

then it seems to run fine. I need to have that if statement in there.

给我其他的，如果条件是列表推导的方式做更简单的if，else条件：

To me the else, if conditions are list comprehensions way of doing the more simple if, else conditions:

if i == x:
  do something
elif i == y:
  do something
elif i == z:
  do something

按照规则，你并不总是需要'else'来关闭a一系列条件句。在我看来，我的代码想要理解中的最终其他。我是否正确说明如果是这样，有没有办法构建一个最终的else，如果在python列表中理解而不是最终的其他？

By rule, you do not always have to have an 'else' to close a series of conditional sentences. It seems to me, that my code wants a final 'else' in the comprehension. Am I correct in stating that and if so, is there a way to construct a final else, if in a python list comprehension instead of a final else?

推荐答案

你是（ab）使用条件表达式，如果测试其他false_expression ，则必须的格式为 true_expression。这些表达式总是产生一个值，不像 if 复合语句。

You are (ab)using conditional expressions, and they must be of the form true_expression if test else false_expression. These expressions always produce a value, unlike an if compound statement.

注意您应该不测试 == True ;没有该测试，布尔表达式已经为true或false。不要使用<> ，该运算符已被弃用并已从Python 3中删除。当测试无，一个单身时，你使用不是无然而。

Note that you should not test for == True; boolean expressions are already true or false without that test. Don't use <> either, that operator has been deprecated and has removed from Python 3 altogether. When testing for None, a singleton, you'd use is not None however.

您正在测试 type（）结果;看起来你想使用 isinstance（）测试。

You are testing against type() results; that looks like you want to use isinstance() tests instead.

您还在值上使用 int（），然后在结果上调用 .lower（）。没有 int.lower（）方法，因此这些调用将失败并带有 AttributeError 。

You are also using int() on values, then calling .lower() on the result. There is no int.lower() method, so those calls will fail with an AttributeError.

以下更接近工作正常，除非有比 int ， float ， str 或 unicode ：

The following is closer to working just fine, unless there are more types than int, float, str or unicode:

[int(a[0].internal_value) if isinstance(a[0].internal_value, (float, int)) or a[0].internal_value.isdigit() 
 else str(a[0].internal_value).lower()
 for a in ws.iter_rows() if a[0].internal_value is not None]

但是，我会将转换转换为过滤功能：

However, I'd farm out the conversion to filter function instead:

def conversion(value):
    if isinstance(value, (float, int)):
        return int(value)
    return str(value).lower()

然后在列表理解中使用 ：

then use that in a list comprehension:

[conversion(a[0].internal_value) for a in ws.iter_rows() if a[0].internal_value is not None]

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