python中的全局和局部作用域

Question

Just a beginner question about local and global scope in python

X = 100
#is X a global variable?.We defined it outside the function scope
def foo():
    print(X)
    return X
#it prints 100 and even returns it
def foo():
    X = X + 10
#local error
#UnboundLocalError: local variable 'X' referenced before assignment
def foo():
    global X
    # if X is a global variable why specify again?
    X = X + 10
    return X

Answer 1

要修改变量的全局副本，您需要使用global关键字，但如果仅访问它，则不需要global 。

Answer 2

from the python website :

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.

this means that you can access a global variable inside a function without a global keyword. if you want to change it though, you must use the global keyword beforehand.

Answer 3

global and nonlocal are very strange things when I was a beginner.

Just think about it: why do we need them in Python?

It is because we don't need var , let and such similar things to declare variables.

Think about Javascript , it is dynamic script language too and very alike to python, but it needs var or let or const to declare variables.

The most important thing of declaring variables is to determine scope.

So, in Python, our variables have implicit default scope: current scope where they are defined, and if we want to change scope of some variables, we need use global or nonlocal explicitly .

All names on the left side of = mean to define variables.

Before executing code of some certain scope, Python will pre-compute all local variables , which are those on the left side of = . This is why you got UnboundLocalError: local variable 'X' referenced before assignment in:

def foo():
    X = X + 10

So, if we look up those names not in defined current scope, just follow the rules of scope chain: up, up, up and until built_in .

Remember : scope of any name on the left side of = is default current scope, and you have to assign it(bind something to it) before referring it.