In Python what is a global statement?

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.

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As an aside, scopes can be nested arbitrarily deep:

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.