Python - Global Keyword

The global keyword is used inside a function to work with global variables.

By default, when we assign a value to a name inside a function, Python creates a local variable. The global keyword changes this behaviour.

We can use the global keyword for the following 2 purposes:

• To declare a global variable inside a function.
• To make a global variable available to the function so that we can perform required modifications.

Consider what happens when we assign to a name inside a function without using global:

777
10

Inside f1(), a = 777 creates a new local variable named a. It does not modify the global a.

So f1() prints the local value 777, while f2() still sees the original global value 10.

Now use the global keyword inside f1() so that the assignment changes the actual global variable:

777
777

The global a statement inside f1() tells Python that a refers to the global variable, not a local one.

Therefore a = 777 modifies the global variable. Now f1() and f2() both print 777.

Without global	With global
Creates a local variable.	Refers to the global variable.
Global value is unchanged.	Global value is modified.
Other functions still see the old value.	Other functions see the new value.

If a variable is declared only inside a function, it cannot be accessed from another function:

10
NameError: name 'a' is not defined

The variable a is local to f1(). When f2() tries to access it, Python cannot find a in any scope and raises a NameError.

Using global, we can declare a brand new global variable from inside a function:

10
10

There is no global a at module level. Inside f1(), the global a statement declares a as a global variable, and a = 10 gives it a value.

Now f2() can access the same global a and prints 10.

If a global variable and a local variable have the same name, we can access the global variable inside the function using globals()['name']:

777
10

Inside f1(), the plain name a refers to the local variable, so print(a) shows 777.

globals()['a'] returns the value of the global variable, which is 10.

• global is used only inside a function.
• It must be written before any use of the variable in that function.
• Without global, an assignment inside a function always creates a local variable.
• globals() returns a dictionary of all global names and their values.
• Use global only when a function really needs to change a module-level variable.