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 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.
