python初学者类变量错误

Python beginner Class variable Error

这是我的第一个问题，对不起…我对python和一般的编码是个初学者，我想创建一个名为"map"的类，该类包含以下类变量：

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class Map:
height = 11
width = 21

top = [['#']*width]
middle = [['#']+[' ']*(width-2)+['#'] for i in range(height-2)]
field = top + middle + top

b = Map()

Shell:
>>> middle = [['#']+[' ']*(width-2)+['#'] for i in range(height-2)]
NameError: name 'width' is not defined

如果我把变量放在类之外，它就会工作。我做错什么了？？

谢谢你的帮助。

相关讨论

来自文档：

Names refer to objects. Names are introduced by name binding operations. Each occurrence of a name in the program text refers to the binding of that name established in the innermost function block containing the use.

A block is a piece of Python program text that is executed as a unit. The following are blocks: a module, a function body, and a class definition. Each command typed interactively is a block. A script file (a file given as standard input to the interpreter or specified as a command line argument to the interpreter) is a code block. A script command (a command specified on the interpreter command line with the ‘-c‘ option) is a code block. The string argument passed to the built-in functions eval() and exec() is a code block.

A code block is executed in an execution frame. A frame contains some administrative information (used for debugging) and determines where and how execution continues after the code block’s execution has completed.

A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one, unless a contained block introduces a different binding for the name. The scope of names defined in a class block is limited to the class block; it does not extend to the code blocks of methods – this includes comprehensions and generator expressions since they are implemented using a function scope. This means that the following will fail:

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class A:
a = 42
b = list(a + i for i in range(10))

python3中的列表comp有自己的作用域，而python2中的代码将按原样工作。

如果使用python2以下面的示例为例，可以看到泄漏列表comp范围的变量如何导致一些问题：

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class A:
a = 42
b = [a for a in range(10)]

a = A()

print(a.a)
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您有几个选项，可以使用__init__创建实例属性：

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class Map:
def __init__(self):
self.height = 11
self.width = 21
self.top = [['#']*self.width]
self.middle = [['#']+[' ']*(self.width-2)+['#'] for i in range(self.height-2)]
self.field = self.top + self.middle + self.top
m=Map()
print(m.field)

使用方法：

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class Map:
@staticmethod
def meth():
height = 11
width = 21
top = [['#']*width]
middle = [['#']+[' ']*(width-2)+['#'] for i in range(height-2)]
field = top + middle + top
return field

b = Map()

print(b.meth())

你的选择实际上取决于你想做什么。