Python - Variable Scope

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Python Variable Scope

A variable in Python is a symbols name to the object in computer's memory. Python works on the concept of namespaces to define the context for various identifiers such as functions, variables etc. A namespace is a collection of symbolic names defined in the current context.

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.

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