生成器表达式评估，在...中有多个... for ...部分 [英] Generator expression evaluation with several ... for ... in ... parts

问题描述

问题:当Python看到这种表达时，它在幕后做什么?

Question: What does Python do under the hood when it sees this kind of expression?

sum(sum(i) for j in arr for i in j)

---

我的想法:上面的表达式有效.但是它是用

生成器表达式是使用函数作用域实现的

generator expressions are implemented using a function scope

不必太冗长:)我有一个具有以下布局的数组(以为例):

Not to be verbose :) I have an array with the following layout (as an example):

>>> arr = [
           [[1,2,3], [4,5,6]],
           [[7,8,9],[10,11,12]]
          ]

首先，我尝试使用以下表达式对 arr 的所有元素求和:

At first, I try to sum all elements of arr with the following expression:

>>> sum(sum(i) for i in j for j in arr)
NameError: name 'j' is not defined

它引发 NameError ，但是为什么不 UnboundLocalError:赋值之前引用的本地变量'j'(如果使用函数作用域实现)， ...这个生成器表达式的等效生成器函数是什么?

It raises NameError, but why not UnboundLocalError: local variable 'j' referenced before assignment if it is implemented using a function scope, what is evaluation rules for for ... in ... from left-to-right or from right-to-left? And what is an equivalent generator function for this generator expression?

编辑:

我明白了.感谢@vaultah的一些见识.在这种情况下， j 是发送到生成器表达式的参数:

I catch the idea. Thanks @vaultah for some insight. In this case j is the argument that is send to generator expression:

>>> sum(sum(i) for i in j for j in arr)  # NameError

这就是为什么我得到这个奇怪的 NameError 的原因.

that's why I get this weird NameError.

@Eric答案显示生成器表达式:

>>> sum(sum(i) for j in arr for i in j)

等效于:

>>> def __gen(arr):
        for j in arr:
            for i in j:
                yield sum(i)

>>> sum(__gen(arr))

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推荐答案

无论是生成器还是列表推导，推导嵌套都是相同的.很容易看到列表理解发生了什么，这就是我将在下面的示例中使用的内容.

Whether it is a generator or a list comprehension, the comprehension nesting is the same. It is easier to see what is going on with a list comprehension and that is what I will use in the examples below.

给出:

>>> arr
[[[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]]

您可以使用嵌套列表推导(或生成器)将整数列表的列表展平1级:

You can flatten the List of Lists of Ints by 1 level using a nested list comprehension (or generator):

>>> [e for sl in arr for e in sl]
[[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]]

在给定该结构的情况下，您可以通过再次嵌套来完全展平(仅作为示例；有更好的方法来展平深度嵌套的列表):

You can flatten completely, given that structure, by nesting again (example only; there are better ways to flatten a deeply nested list):

>>> [e2 for sl2 in [e for sl in arr for e in sl] for e2 in sl2]
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

由于 sum 需要迭代，因此在您的示例中无需进行第二次展平:

Since sum takes an iterable, the second flattening is not necessary in your example:

>>> [sum(e) for sl in arr for e in sl]
[6, 15, 24, 33]   # sum of those is 78...

---

理解的一般形式是:

---

The general form of a comprehension is:

[ expression      for a_variable in a_DEFINED_sequence     optional_predicate ]

通过使用未定义的名称，您可以获得在嵌套理解中看到的相同的 NameError :

You can get the same NameError you are seeing on your nested comprehension by using a non defined name:

>>> [c for c in not_defined]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'not_defined' is not defined

所以您在 sum(sum(i)表示i在arr的j中的i的sum(i))上看到的错误是因为尚未定义 j .理解是从左到右，从内到外的.尝试将 j 定义为序列的权利.

So the error you see on sum(sum(i) for i in j for j in arr) is because j has not been defined yet. Comprehensions are evaluated left to right, inner to outer. The definition of j as a sequence is to the right of its attempted use.

要将列表理解展开为嵌套循环，内部(或左手)部分将变为外部循环:

To unroll the list comprehension into nested loops, the inner (or left hand) section becomes the outer loop:

for sl in arr:
    for sl2 in sl:
        for e in sl2:
           # now you have each int in the LoLoInts...
           # you could use yield e for a generator here

---

您的最后一个问题:为什么得到 TypeError 且 gen =(j代表arr中的j)?

该生成器表达式不执行任何操作.示例:

That generator expression does nothing. Example:

>>> [j for j in arr]
[[[1, 2, 3], [4, 5, 6]], [[7, 8, 9], [10, 11, 12]]]
>>> [j for j in arr] == arr
True

因此表达式(j代表arr中的j)只是通过 arr 返回一个生成器.

So the expression (j for j in arr) just returns a generator over arr.

sum 也不知道如何添加或添加arr:

And sum does not know how to add that or arr either:

>>> sum(arr)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'list'

由于示例中的 gen 返回相同的数据结构，因此出现了错误.

Since gen in your example is returning the same data structure, that is your error.

要修复它:

>>> gen=(e for sl in arr for e in sl)
>>> sum(sum(li) for li in gen)
78

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