python - 名称未在具有多个循环的列表理解中定义

Question

我正在尝试解压一个复杂的字典，但我在使用多个循环的列表理解表达式中得到了一个 NameError:

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

Answer 1

你的循环顺序搞混了；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:

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

现在 g 为下一个循环定义，for r in a[g]，表达式不再引发异常:

>>> 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]