没有在具有多个循环的列表理解中定义名称

问题描述

我正在尝试打开一个复杂的字典，并使用多个循环在列表理解表达式中得到一个NameError:

I'm trying to unpack a complex dictionary and I'm getting a NameError in a list comprehension expression using multiple loops:

a={
  1: [{'n': 1}, {'n': 2}],
  2: [{'n': 3}, {'n': 4}],
  3: [{'n': 5}],
}
good = [1,2]
print [r['n'] for r in a[g] for g in good]
# NameError: name 'g' is not defined

推荐答案

您将循环的顺序混为一谈；它们被认为是从左到右嵌套的，因此for r in a[g]是 outer 循环并首先执行.交换循环:

You have the order of your loops mixed up; they are considered nested from left to right, so for r in a[g] is the outer loop and executed first. Swap out the loops:

print [r['n'] for g in good for r in a[g]]

现在为下一个循环for r in a[g]定义了g，表达式不再引发异常:

Now g is defined for the next loop, for r in a[g], and the expression no longer raises an exception:

>>> a={
...   1: [{'n': 1}, {'n': 2}],
...   2: [{'n': 3}, {'n': 4}],
...   3: [{'n': 5}],
... }
>>> good = [1,2]
>>> [r['n'] for g in good for r in a[g]]
[1, 2, 3, 4]

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