Python 类：NameError：名称“foo”未定义

Question

为什么在构建仅具有非零长度的元组时访问类属性

foo

会失败？

class Foo:
    foo = 42
    
    bar_ok1 = (foo for _ in range(10))
    bar_ok2 = tuple(foo for _ in [])
    
    bar_fail1 = tuple(foo for _ in range(10))
    bar_fail2 = tuple(foo for _ in [0, 1, 2])

Traceback (most recent call last):
  File "main.py", line 1, in <module>
    class Foo:
  File "main.py", line 7, in Foo
    bar_fail1 = tuple(foo for _ in range(10))
  File "main.py", line 7, in <genexpr>
    bar_fail1 = tuple(foo for _ in range(10))
NameError: name 'foo' is not defined

Answer 1

那是因为 `bar_ok1 尚未评估，因为它定义了一个生成器，并且在类定义中未访问 foo 值。 bar_ok2 也是如此，因为它根本不执行迭代（空列表）。

相比之下，bar_fail* 属性是根据类定义进行评估的。

运行以下命令，您会发现 bar_ok1 失败：

class Foo:
    foo = 42
    
    bar_ok1 = (foo for _ in range(10))
    bar_ok2 = tuple(foo for _ in [])
    
    #bar_fail1 = tuple(foo for _ in range(10))
    #bar_fail2 = tuple(foo for _ in [0, 1, 2])

list(Foo.bar_ok1)