The unknown side of Python functions

Usually we define a function , Then the function is called. , The code related to the function begins to execute . But many people don't know , When we define a function , Some code starts to execute . Today, let's talk about the unknown side of functions .

Let's start with a piece of code ：

def do_something(opt: print(' Parameters  opt'), arg=print(' Parameters  arg')) -> print(' The return value of the function '):
    print("do something runing")

if __name__ == '__main__':
    pass
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In the above code, we set a function , But it was not called , Does it output information ？

It will be , In the parameter, whether it is the type prompt part , Or the default assignment part , Or the type prompt of the return value , The statements inside will be executed , give the result as follows ：

This practice is very rare for me , In fact print Open file instead of function , Connect to the database and other operations , Will also be executed .

But usually , Parameters are immutable types , If you pass in a variable type , Maybe every function call , The results will be changed , for example ：

def do_something(opt: print(" Parameters  opt"), arg=[]) -> print(" The return value of the function "):
    print("do something runing")
    print(f"{arg = }")
    arg.append(0)


if __name__ == "__main__":
    do_something(opt=1)
    do_something(opt=1)

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The operation results are as follows ：

You can find , Call the function twice do_something, Although not introduced arg Parameters ,arg The value of has sent a change . If you don't pay attention to this , There may be a bug happen . stay Pythcarm in , Will warn us arg It's a mutable object ：

If you want to get the type prompt of the function 、 The default value is , It can be like this ：

def do_something(opt: 1, arg=2) -> 3:
    print("do something runing")
    print(f"{arg = }")
    arg.append(0)


if __name__ == "__main__":
    print(f"{do_something.__annotations__ = }")
    print(f"{do_something.__defaults__ = }")

#do_something.__annotations__ = {'opt': 1, 'return': 3}
#do_something.__defaults__ = (2,)
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