blocks|key|1818107|text|在Python中，class-+code中的代码有些混乱。与其说它是一个bug，不如说它是一个“变得奇怪的实现问题”。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1818108|问题是:代码通常在模块级运行，或者在函数内部运行-当在函数内部时，有定义良好的“局部”变量。|1818109|类体中的代码也在“局部”作用域中运行--但是如果创建的函数是在处理类体时运行的，那么这些函数就不会“看到”外部级别的变量。和生成器表达式，以及理解(在Python3中，Python2是另一种语言，它即将被淘汰，让我们不要把事情复杂化)。用于在生成器内部创建for迭代器的表达式在“看到”外部变量的作用域中运行。主表达式和if表达式本身都在生成器中，无法“看到”这些变量。|offset|length|style|CODE|1818110|因此，如果在类体中需要理解，一种变通方法是在类体中有一个中间函数，只需生成所需的值和变量，并使用该内部函数的本地命名空间调用它并更新类自己的变量：|1818111|class+A:
+++def+create_vals():
+++++++numbers+=+[1,+2,+3]
+++++++numbers_copy+=+numbers[:]
+++++++values+=+list(a+for+a+in+numbers+if+a+in+numbers_copy)
+++++++return+locals()
+++locals().update(create_vals())
+++del+create_vals|code-block|syntax|javascript|1818112|因此，在临时create_vals函数(它不是一个方法)中，通常的作用域-嵌套规则适用-在最后两行中，我们将创建的变量复制到类本身，并删除临时函数。|1818113|entityMap^0|0|0|3K|3|0|0|0|6|B|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|W|8|@$F|X|G|Y|H|I]]|9|@]|A|$]]|$1|J|3|K|5|6|7|Z|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|10|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|6|7|11|8|@$F|12|G|13|H|I]]|9|@]|A|$]]|$1|S|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|T|$]]

The code inside class-bodies is somewhat messed in Python. It is not so much a bug, but an "implementation problem that got weird".

The problem being that: code usually runs at module level, or inside functions - when inside functions, there are well-defined "local" variables.

Code inside class bodies also run with a "local" scope - but if one creates functions that are run while the class body is being processed, these do not
"see" the outer-level variables. And generator expressions, as do comprehensions (in Python 3, Python 2 is another language, which is on its way out, let's not complicate stuff). The expression used to create the iterator to the <code>for</code> inside the generator is run in a scope where it "sees" the outer variables. The main expression and if expressions themselves are inside the generator, and can't "see" those variables.

So, a workaround, if comprehensions are needed inside a class body is to have an intermediary function inside the class body, just to generate the values and variables you need, and have line to call that and update the class's own variables with the local namespace of that inner function:

<pre><code>class A:
 def create_vals():
 numbers = [1, 2, 3]
 numbers_copy = numbers[:]
 values = list(a for a in numbers if a in numbers_copy)
 return locals()
 locals().update(create_vals())
 del create_vals
</code></pre>

So, inside the temporary <code>create_vals</code> function (it is not a method), usual scope-nesting rules apply - and with the last two lines we copy the created variables to the class itself, and remove the temporary function.

Take this simple code:

<pre class="lang-py prettyprint-override"><code>class A(object):
 numbers = [1, 2, 3]
 numberscopy = numbers[:]
 print(*(a for a in numberscopy))
 print(*(a for a in numberscopy if a in numbers))
</code></pre>

I define the <code>numbers</code> variable inside the class. I can then use it to do other things, like make a copy, iterate over it, and print its contents.

But the last line, with the <code>for</code>-<code>if</code> statement, fails with <code>NameError: global name 'numbers' is not defined</code>. Not <code>numberscopy</code>, just <code>numbers</code>.

I tried on both python 2.7.14+ (with <code>print_function</code> imported) and 3.7.0, with the same result.

Why does this happen? Is it intended to work this way?

Class variable in `for`...`if` expression fails in python

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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看下面这段简单的代码：class A(object):  numbers = [1, 2, 3]  numberscopy = numbers[:]  print(*(a for a in numberscopy))  print(*(a for a in numberscopy if a in numbers))我在...

在python中，`for`...`if`表达式中的Class变量失败-腾讯云开发者社区-腾讯云

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