Python Variable Scope

The Python variables allow us to store data values. They are a symbolic name that refers to the object in computer's memory. For instance, we can create a varibale named "age" that stores age of a person. We assign a value to this variable as shown below − 

age = 23

Now, whenever we use the "age" variable, it will refer to the value "23". However, whether we can access it depends on its scope.

Scope of Variables in Python

The scope of a variable in Python is defined as the specific area or region where the variable is accessible to the user. The scope of a variable depends on where and how it is defined. In Python, a variable can have either a global or a local scope.

On the basis of scope, the Python variables are classified in three categories −

  • Local Variables

  • Global Variables

  • Nonlocal Variables

Local Variables

A local variable is defined within a specific function or block of code. It can only be accessed by the function or block where it was defined, and it has a limited scope. In other words, the scope of local variables is limited to the function they are defined in and attempting to access them outside of this function will result in an error. Always remember, multiple local variables can exist with the same name.

Example

The following example shows the scope of local variables.

def myfunction():
   a = 10
   b = 20
   print("variable a:", a)
   print("variable b:", b)
   return a+b
   
print (myfunction())

Output

In the above code, we have accessed the local variables through its function. Hence, the code will produce the following output −

variable a: 10
variable b: 20
30

Global Variables

A global variable can be accessed from any part of the program, and it is defined outside any function or block of code. It is not specific to any block or function.

Example

The following example shows the scope of global variable. We can access them inside as well as outside of the function scope.

#global variables
name = 'TutorialsPoint'
marks = 50
def myfunction():
   # accessing inside the function
   print("name:", name)
   print("marks:", marks)
# function call   
myfunction()

Output

The above code will produce the following output −

name: TutorialsPoint
marks: 50

Nonlocal Variables

The Python variables that are not defined in either local or global scope are called nonlocal variables. They are used in nested functions.

Example

The following example demonstrates the how nonlocal variables works.

def yourfunction():
   a = 5
   b = 6 
   # nested function
   def myfunction():
      # nonlocal function 
      nonlocal a
      nonlocal b
      a = 10
      b = 20 
      print("variable a:", a)
      print("variable b:", b)
      return a+b
   print (myfunction())
yourfunction()

Output

The above code will produce the below output −

variable a: 10
variable b: 20
30

Namespace and Scope of Python Variables

A namespace is a collection of identifiers, such as variable names, function names, class names, etc. In Python, namespace is used to manage the scope of variables and to prevent naming conflicts.

Python provides the following types of namespaces −

  • Built-in namespace contains built-in functions and built-in exceptions. They are loaded in the memory as soon as Python interpreter is loaded and remain till the interpreter is running.

  • Global namespace contains any names defined in the main program. These names remain in memory till the program is running.

  • Local namespace contains names defined inside a function. They are available till the function is running.

These namespaces are nested one inside the other. Following diagram shows relationship between namespaces.

Types Of Namespace

The life of a certain variable is restricted to the namespace in which it is defined. As a result, it is not possible to access a variable present in the inner namespace from any outer namespace.

Python globals() Function

Python's standard library includes a built-in function globals(). It returns a dictionary of symbols currently available in global namespace.

Run the globals() function directly from the Python prompt.

>>> globals()
{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>}

It can be seen that the builtins module which contains definitions of all built-in functions and built-in exceptions is loaded.

Save the following code that contains few variables and a function with few more variables inside it.

name = 'TutorialsPoint'
marks = 50
result = True
def myfunction():
   a = 10
   b = 20
   return a+b
   
print (globals())

Calling globals() from inside this script returns following dictionary object −

{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x00000263E7255250>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, '__file__': 'C:\\Users\\user\\examples\\main.py', '__cached__': None, 'name': 'TutorialsPoint', 'marks': 50, 'result': True, 'myfunction': <function myfunction at 0x00000263E72004A0>}

The global namespace now contains variables in the program and their values and the function object in it (and not the variables in the function).

Any variable created outside a function can be accessed within any function and so they have global scope. Following is an example to show the usage of global variable in Python:

x = 5
y = 10
def sum():
   sum = x + y
   return sum
print(sum())

This will produce the following result:

15

Python locals() Function

Python's standard library includes a built-in function locals(). It returns a dictionary of symbols currently available in the local namespace of the function.

Modify the above script to print dictionary of global and local namespaces from within the function.

name = 'TutorialsPoint'
marks = 50
result = True
def myfunction():
   a = 10
   b = 20
   c = a+b
   print ("globals():", globals())
   print ("locals():", locals())
   return c
myfunction()

The output shows that locals() returns a dictionary of variables and their values currently available in the function.

globals(): {'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <_frozen_importlib_external.SourceFileLoader object at 0x00000169AE265250>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>, '__file__': 'C:\\Users\\mlath\\examples\\main.py', '__cached__': None, 'name': 'TutorialsPoint', 'marks': 50, 'result': True, 'myfunction': <function myfunction at 0x00000169AE2104A0>}
locals(): {'a': 10, 'b': 20, 'c': 30}

Since both globals() and locals functions return dictionary, you can access value of a variable from respective namespace with dictionary get() method or index operator.

print (globals()['name']) # displays TutorialsPoint
print (locals().get('a')) # displays 10

Following is a simple example to show the usage of local variables in Python:

def sum(x,y):
   sum = x + y
   return sum
print(sum(5, 10))

15

Namespace Conflict in Python

If a variable of same name is present in global as well as local scope, Python interpreter gives priority to the one in local namespace.

marks = 50 # this is a global variable
def myfunction():
   marks = 70 # this is a local variable
   print (marks)
   
myfunction()
print (marks) # prints global value

It will produce the following output −

70
50

If you try to manipulate value of a global variable from inside a function, Python raises UnboundLocalError.

marks = 50 # this is a global variable
def myfunction():
   marks = marks + 20
   print (marks)

myfunction()
print (marks) # prints global value

It will produce the following output −

   marks = marks + 20
           ^^^^^
UnboundLocalError: cannot access local variable 'marks' where it is not associated with a value

To modify a global variable, you can either update it with a dictionary syntax, or use the global keyword to refer it before modifying.

var1 = 50 # this is a global variable
var2 = 60 # this is a global variable
def myfunction():
   "Change values of global variables"
   globals()['var1'] = globals()['var1']+10
   global var2
   var2 = var2 + 20

myfunction()
print ("var1:",var1, "var2:",var2) #shows global variables with changed values

It will produce the following output −

var1: 60 var2: 80

Lastly, if you try to access a local variable in global scope, Python raises NameError as the variable in local scope can't be accessed outside it.

var1 = 50 # this is a global variable
var2 = 60 # this is a global variable
def myfunction(x, y):
   total = x+y
   print ("Total is a local variable: ", total)

myfunction(var1, var2)
print (total) # This gives NameError

It will produce the following output −

Total is a local variable: 110
Traceback (most recent call last):
   File "C:\Users\user\examples\main.py", line 9, in <module>
   print (total) # This gives NameError
          ^^^^^
NameError: name 'total' is not defined
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