Question

What is a global statement? And how is it used? I have read Python’s official definition;
however, it doesn’t make a lot of sense to me.

Answer 1

Every "variable" in python is limited to a certain scope. The scope of a python "file" is the module-scope. Consider the following:

#file test.py
myvariable = 5  # myvariable has module-level scope

def func():
    x = 3       # x has "local" or function level scope.

Objects with local scope die as soon as the function exits and can never be retrieved (unless you return them), but within a function, you can access variables in the module level scope (or any containing scope):

myvariable = 5
def func():
    print(myvariable)  # prints 5

def func2():
    x = 3
    def func3():
        print(x)       # will print 3 because it picks it up from `func2`'s scope
    
    func3()

However, you can’t use assignment on that reference and expect that it will be propagated to an outer scope:

myvariable = 5
def func():
    myvariable = 6     # creates a new "local" variable.  
                       # Doesn't affect the global version
    print(myvariable)  # prints 6

func()
print(myvariable)      # prints 5

Now, we’re finally to global. The global keyword is the way that you tell python that a particular variable in your function is defined at the global (module-level) scope.

myvariable = 5
def func():
    global myvariable
    myvariable = 6    # changes `myvariable` at the global scope
    print(myvariable) # prints 6

func()
print(myvariable)  # prints 6 now because we were able 
                   # to modify the reference in the function

In other words, you can change the value of myvariable in the module-scope from within func if you use the global keyword.

---

As an aside, scopes can be nested arbitrarily deeply:

def func1():
    x = 3
    def func2():
        print("x=",x,"func2")
        y = 4
        def func3():
            nonlocal x  # try it with nonlocal commented out as well.  See the difference.
            print("x=",x,"func3")
            print("y=",y,"func3")
            z = 5
            print("z=",z,"func3")
            x = 10

        func3()

    func2()
    print("x=",x,"func1")

func1()

Now in this case, none of the variables are declared at the global scope, and in python2, there is no (easy/clean) way to change the value of x in the scope of func1 from within func3. That’s why the nonlocal keyword was introduced in python3.x . nonlocal is an extension of global that allows you to modify a variable that you picked up from another scope in whatever scope it was pulled from.