为什么对象不需要全局关键字

Question

in this example i need to include the global keyword. I have read elsewhere on stack exchange that immutable objects need the global keyword.

def foo(param):
    global message
    message += param

message = " hello "

foo("world")

print(message)

but in this example using lists which are mutable i also need the global keyword.

def foo(param):
    global message
    message += [ param ]

message = [ "hello" ]

foo("world")

print(message)

but in this example I create a 'container' object which allows me to fore go the global keyword.

class container:

    def __init__(self):
        self._message = ""

    def __str__(self):
        return self._message

    def add(self, param):
        self._message += param

def foo(param):
    message.add(param)

message = container()
message.add(" hello ")

foo("world")

print(message)

What are the rules for using the global keyword? This seems inconsistent

Answer 1

This frequently asked question has an answer in the official documentation:

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function's body, it's assumed to be a local unless explicitly declared as global.

Though a bit surprising at first, a moment's consideration explains this. On one hand, requiring global for assigned variables provides a bar against unintended side-effects. On the other hand, if global was required for all global references, you'd be using global all the time. You'd have to declare as global every reference to a built-in function or to a component of an imported module. This clutter would defeat the usefulness of the global declaration for identifying side-effects.

See also the documentation on global for more details.