我正在编写一些代码，但有一部分出错了。 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)
请您参考如下方法:
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)
列表理解实际上是使用lamda函数，正如你在the python document中看到的那样. squares = [x**2 for x in range(a)] 实际上是 squares = list(map(lambda x: x**2, range(a)))。 (编辑:最近我发现这不准确，参见 my question ，它们不一样，但它们的工作方式相似；也许仍然可以使用 lambda 来理解this if doesn't want to refer to python assembly) 但是，在 exec() 中创建函数对象(此处为 lamda 函数)会导致问题。我发question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。
还有另一种解决方案。请引用my answer到前面提到的问题以获取更多信息。
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)
