NameError: name 'self' is not defined IN EXEC/EVAL

Question

我正在编写一些代码，其中有一部分出错了。但是我找不到错误发生的原因。

代码（样例；与报错部分类似）：

class Test:
def __init__(self,a=0):
    self.x = a
    self.l = [2**x for x in range(a)]  #<--- self.l = [1,2,4,8,16]
    self.base()

def base(self):
    expr = "self.l{0} = [self.l[x]+{0} for x in range(self.x)]" #<--- when i=4, self.l4 = [5,6,8,12,20]
    for i in range(self.x):
        exec(expr.format(i))

w = Test(5)
print(w.l4)

所以我想我明白了：

[5, 6, 8, 12, 20]

但是，

File "D:/Documents and Settings/Desktop/py/py/test2.py", line 12, in <module>
  w = Test(5)
File "D:/Documents and Settings/Desktop/py/py/test2.py", line 5, in __init__
  self.base()
File "D:/Documents and Settings/Desktop/py/py/test2.py", line 10, in base
  exec(expr.format(i))
File "<string>", line 1, in <module>
File "<string>", line 1, in <listcomp>

NameError: name 'self' is not defined

（抱歉英语不好）

Answer 1

此处不需要 eval 或 exec。

for i in range(self.x):
    setattr(self, "l{}".format(i), [self.l[x]+i for x in range(self.x)])

虽然我不知道你为什么要这样做；最好将其保留为列表而不是动态设置属性。

Answer 2

如果您使用 for 循环而不是列表理解

它会起作用。

class Test:
    def __init__(self,a=0):
        self.x = a
        self.l = [2**x for x in range(a)]  #<--- self.l = [1,2,4,8,16]
        self.base()

    def base(self):
        # expr = "self.l{0} = [self.l[x]+{0} for x in range(self.x)]" #<--- when i=4, self.l4 = [5,6,8,12,20]
        expr = '''
self.l{0} = []
for x in range(self.x):
    self.l{0}.append(self.l[x]+{0})
'''
        for i in range(self.x):
            expr_formated = expr.format(i)
            print(expr_formated)
            exec(expr_formated)

w = Test(5)
print(w.l4)

列表推导实际上使用了 lamda 函数，正如您在 the python document. squares = [x**2 for x in range(a)] is actually squares = list(map(lambda x: x**2, range(a))). However, creating a function object (lamda function here) inside the exec() will results in problems. I post question 中看到的，这里解释了为什么创建函数对象不能按预期工作。简而言之，将定义的lamda函数的__closure__设置为None，使得调用lamda函数时变量a不可用。

如果你坚持使用lambda函数。

还有一个解决办法。有关更多信息，请参阅 my answer 前面提到的问题。

class Test:
    def __init__(self,a=0):
        self.x = a
        self.l = [2**x for x in range(a)]  #<--- self.l = [1,2,4,8,16]
        self.base()

    def base(self):
        expr = """
def closure_helper_func(self):
    self.l{0} = [self.l[x]+{0} for x in range(self.x)]
closure_helper_func(self)""" #<--- when i=4, self.l4 = [5,6,8,12,20]
        for i in range(self.x):
            expr_formated = expr.format(i)
            # print(expr_formated)
            exec(expr_formated)

w = Test(5)
print(w.l4)

Answer 3

'self'是一个特殊的class变量，只能在内部使用，不能在class外部读取。第一个示例需要一个 return 值。

NameError: name 'self' is not defined IN EXEC/EVAL