Del Keyword In Python

Overview:

  • del statement removes a name from a scope.
  • After del statement is applied on a name, identity is lost for that name. Any reference to it afterwards will raise an exception of type NameError.
  • del statement can be applied in several contexts:
    • To delete a name from the local scope
    • To delete a name from the global scope
    • To delete an item from a list, set, dictionary or any other mutable Python iterable
    • To delete an attribute of a class instance. Remember, a class method cannot be deleted this way.
    • To delete a slicing from an iterable

Example - Removing a name from local and global scopes:

# Python example program to remove names from global and local

# scopes using the Python keyword del

today = "Sunday";

 

def oddSequence():

    sum = 0;

    for i in range(1, 10, 2):

        print(i);

        sum = sum + i;  

 

    # Prints the last value again

    print(i);

   

    # Print sum

    print("Sum %d"%sum);

 

    # Remove sum from local scope

    try:

        del sum;

        print(sum);

    except Exception as Ex:

        print(Ex);

 

    # Remove today from global scope

    global today;

    del today;

       

    # Remove i from local scope

    del i;

 

    # Already i has been unbound

    try:

        print(i);

    except Exception as Ex:

        print(Ex);

       

oddSequence();

 

# Today no longer available at global scope

print(today);

 

Output:

1

3

5

7

9

9

Sum 25

local variable 'sum' referenced before assignment

local variable 'i' referenced before assignment

Traceback (most recent call last):

  File "del0.py", line 40, in <module>

    print(today);

NameError: name 'today' is not defined

 

Example - Removing a reference to a class instance from global scope:

# Example program that removes an object from

# the global scope

class X:

    def __init__(self, val):

        self.val = val

 

class Y:

    def __init__(self, val):

        self.val = val

       

class A:

    def __init__(self, x, y):

        self.x = x

        self.y = y

 

# Create instances of X and Y

x = X("one");

y = Y("two");

 

# Create instances of A.

# Instances of A have references to x and y instances.

a1 = A(x,y);

a2 = A(x,y);

 

# Removes a1 recursively. It will not remove object x from the scope.

del a1;

print(a2.x.val);

# Raises NameError

print(a1);

 

Overview:

one

Traceback (most recent call last):

  File "del1.py", line 29, in <module>

    print(a1);

NameError: name 'a1' is not defined

 

Example - Removing a class attribute:

class Robot:

    def __init__(self, id):

        self.id = id;

       

    def greet(self):

        print("Hello");

 

r1 = Robot("R1");

r1.greet();

 

# Delete the id attribute...possible

del r1.id;

try:

    print(r1.id); # Now id is no longer part of r1 instance

except Exception as Ex:

    print(Ex)

   

print("hi");

# Delete the function object - greet(), not possible

del r1.greet;

 

Output:

Hello

'Robot' object has no attribute 'id'

hi

Traceback (most recent call last):

  File "del2.py", line 20, in <module>

    del r1.greet;

AttributeError: greet

 

Example - Removing elements from Python iterables - list, dictionary using del:

# Python example program to delete elements from iterables

listObject  = [5, 10, 15, 20, 25];

tupleObject = (2, 4, 6, 8, 10);

mapObject   = {1:"One", 2:"Two", 3:"Three", 4:"Four", 5:"Five", 6:"Six"};

 

# Remove an element from a Python list

print("Length of the list before removal of an element:%d"%len(listObject));

del listObject[0];

print("Length of the list after removal of an element:%d"%len(listObject));

 

# Remove a key and the corresponding value from a Python dictionary

print("Length of the dictionary before removal of an element:%d"%len(mapObject));

del mapObject[2];

print("Length of the dictionary after removal of an element:%d"%len(mapObject));

 

# Removal of an element from an immutable sequence is not possible

del tupleObject[1];

 

Output:

Length of the list before removal of an element:5

Length of the list after removal of an element:4

Length of the dictionary before removal of an element:6

Length of the dictionary after removal of an element:5

Traceback (most recent call last):

  File "del_elem.py", line 17, in <module>

    del tupleObject[1];

TypeError: 'tuple' object doesn't support item deletion

 


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