Python NameError：名称未定义
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Python NameError: name is not defined
提问by user1899679
I have a python script and I am receiving the following error:
我有一个 python 脚本，我收到以下错误：
Traceback (most recent call last):
File "C:\Users\Tim\Desktop\pop-erp\test.py", line 1, in <module>
s = Something()
NameError: name 'Something' is not defined
Here is the code that causes the problem:
这是导致问题的代码：
s = Something()
s.out()
class Something:
def out():
print("it works")
This is being run with Python 3.3.0 under Windows 7 x86-64.
这是在 Windows 7 x86-64 下使用 Python 3.3.0 运行的。
Why can't the Somethingclass be found?
为什么找不到Something类？
采纳答案by Blender
Define the class before you use it:
在使用之前定义类：
class Something:
def out(self):
print("it works")
s = Something()
s.out()
You need to pass selfas the first argument to all instance methods.
您需要self将第一个参数作为第一个参数传递给所有实例方法。
回答by user574435
You must define the class before creating an instance of the class. Move the invocation of Somethingto the end of the script.
您必须在创建类的实例之前定义类。将 的调用移动Something到脚本的末尾。
You can try to put the cart before the horse and invoke procedures before they are defined, but it will be an ugly hack and you will have to roll your own as defined here:
您可以尝试将手推车放在马之前并在定义它们之前调用程序，但这将是一个丑陋的黑客，您将不得不按照此处定义的方式推出自己的：
回答by Tomasz Bartkowiak
Note that sometimes you will want to use the class type name inside its own definition, for example when using Python Typingmodule, e.g.
请注意，有时您会希望在其自己的定义中使用类类型名称，例如在使用 Python Typing模块时，例如
class Tree:
def __init__(self, left: Tree, right: Tree):
self.left = left
self.right = right
This will also result in
这也会导致
NameError: name 'Tree' is not defined
That's because the class has not been defined yet at this point. The workaround is using so called Forward Reference, i.e. wrapping a class name in a string, i.e.
这是因为此时尚未定义该类。解决方法是使用所谓的Forward Reference，即将类名包装在一个字符串中，即
class Tree:
def __init__(self, left: 'Tree', right: 'Tree'):
self.left = left
self.right = right
