python-3.x - 为什么这两个类在 python 3.x 中执行，它们只是定义了？

Question

为什么要执行这两个类？我刚刚定义了两个基类。我没用f = F ()和 g = G ()实例化对象。为什么要执行两次打印？

class F():
    print("F.__new__called")
    
class G:
    print("G.__new__called")

print("test")

输出:

F.__new__called
G.__new__called
test

Answer 1

如 Python 3 文档 9.3.1 "Class Definition Syntax" (强调我的):

The simplest form of class definition looks like this:

class ClassName:
  <statement-1>
  .
  .
  .
  <statement-N>

Class definitions, like function definitions (def statements) must be executed before they have any effect. (You could conceivably place a class definition in a branch of an if statement, or inside a function.)

In practice, the statements inside a class definition will usually be function definitions, but other statements are allowed, and sometimes useful — we’ll come back to this later. The function definitions inside a class normally have a peculiar form of argument list, dictated by the calling conventions for methods — again, this is explained later.

就 Python 而言， class 中允许任何语句。定义，甚至超出典型的属性分配和功能定义。这些语句立即运行，与 def 的含义相同。必须运行语句才能定义函数。
(文档显然没有回到 block 中其他语句的实用性，尽管其他示例在 9.6 代码块中交错了私有(private)变量引用的赋值。)
但是，与您的打印语句的内容不同，这些不对应于“F.__new__call”；它只对应于被定义的类。当您调用 f = F () , Python 将调用方法 __new__和 __init__ ，然后负责构造对象。