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Confusing Operations in Python

Learn the differences between Python operations and operators that are often confused by beginners and frequently asked in interviews.

1. append() vs extend()

append() and extend() are methods of the Python list type. Both are used to add elements to a list, but they behave differently.

Method Available On Purpose
append() list only Adds a single object to the end of the list.
extend() list only Adds each element from an iterable to the end of the list.

append()

append() adds one object to the end of the list.

numbers = [1, 2, 3]

numbers.append(4)

print(numbers)

Output

[1, 2, 3, 4]

If the object is another list, the entire list is added as a single element.

numbers = [1, 2, 3]

numbers.append([4, 5])

print(numbers)

Output

[1, 2, 3, [4, 5]]

extend()

extend() expects an iterable and adds each of its elements individually.

numbers = [1, 2, 3]

numbers.extend([4, 5])

print(numbers)

Output

[1, 2, 3, 4, 5]

Since strings are iterable, extend() adds one character at a time.

letters = ["A"]

letters.extend("BC")

print(letters)

Output

['A', 'B', 'C']

Summary

append() extend()
Adds a single object Adds multiple elements
Accepts any object Expects an iterable
Nested lists remain nested Elements are added individually
Available only on list Available only on list

2. append() vs insert()

append() and insert() are methods of the Python list type. Both add elements to a list, but they differ in where the new element is placed.

Method Available On Purpose
append() list only Adds an element to the end of the list.
insert() list only Inserts an element at a specified index.

append()

append() always adds the new element at the end of the list.

numbers = [1, 2, 3]

numbers.append(4)

print(numbers)

Output

[1, 2, 3, 4]

insert()

insert(index, object) inserts the object before the specified index.

numbers = [1, 2, 3]

numbers.insert(1, 10)

print(numbers)

Output

[1, 10, 2, 3]

If the index is greater than the list length, the element is added at the end.

numbers = [1, 2, 3]

numbers.insert(100, 4)

print(numbers)

Output

[1, 2, 3, 4]

3. remove() vs pop() vs del

remove() and pop() are methods of the Python list type, whereas del is a Python statement that can remove items from many types or even delete entire variables.

Operation Available On Purpose
remove() list only Removes the first occurrence of a specified value.
pop() list only Removes and returns an element by index.
del Lists, dictionaries, variables, and more Deletes items, slices, or entire objects.

remove()

remove(value) removes the first matching value from the list.

numbers = [1, 2, 3, 2]

numbers.remove(2)

print(numbers)

Output

[1, 3, 2]

pop()

pop(index) removes and returns the element at the given index.

numbers = [1, 2, 3]

item = numbers.pop(1)

print(numbers)
print(item)

Output

[1, 3]
2

If no index is given, the last element is removed.

numbers = [1, 2, 3]

numbers.pop()

print(numbers)

Output

[1, 2]

del

del can remove an item by index, delete a slice, or delete an entire variable.

Delete an item:

numbers = [1, 2, 3]

del numbers[1]

print(numbers)

Output

[1, 3]

Delete a slice:

numbers = [1, 2, 3, 4, 5]

del numbers[1:4]

print(numbers)

Output

[1, 5]

Delete a variable:

x = 10

del x

4. sort() vs sorted()

sort() and sorted() are both used to sort data, but they differ in where they are available and whether they modify the original object.

Operation Available On Purpose
sort() list only Sorts the list in place.
sorted() Any iterable Returns a new sorted list without modifying the original object.

sort()

sort() modifies the original list.

numbers = [3, 1, 2]

numbers.sort()

print(numbers)

Output

[1, 2, 3]

sorted()

sorted() returns a new sorted list, leaving the original object unchanged.

numbers = [3, 1, 2]

result = sorted(numbers)

print(numbers)
print(result)

Output

[3, 1, 2]
[1, 2, 3]

sorted() also works with other iterables such as tuples, sets, and strings.

letters = ('c', 'a', 'b')

print(sorted(letters))

Output

['a', 'b', 'c']

Summary

sort() sorted()
List method Built-in function
Available only on list Works with any iterable
Modifies the original list Returns a new sorted list
Returns None Returns the sorted list

What type of functions are they?

Operation Type Available On
sort() Instance method list objects only
sorted() Built-in function Any iterable

Summary

remove() pop() del
Removes by value Removes by index Deletes by index, slice, or entire object
Returns nothing Returns the removed element Returns nothing
List method List method Python statement
Available only on list Available only on list Works with many object types

Summary

append() insert()
Adds an element at the end Adds an element at a specified position
Takes one argument: object Takes two arguments: index, object
Faster for adding to the end Existing elements may be shifted
Available only on list Available only on list

5. is vs ==

is and == are both used to compare objects, but they compare different things.

  • == checks whether two objects have the same value.
  • is checks whether two variables refer to the same object in memory.
Operator Purpose
== Compares the values of two objects.
is Compares the identity (memory reference) of two objects.

==

Use == when you want to check whether two objects contain the same value.

list1 = [1, 2, 3]
list2 = [1, 2, 3]

print(list1 == list2)

Output

True

Although list1 and list2 are different objects, they contain the same values.

is

Use is when you want to check whether two variables refer to the same object.

list1 = [1, 2, 3]
list2 = list1

print(list1 is list2)

Output

True

Both variables refer to the same list object.

If two variables refer to different objects, is returns False, even if their values are equal.

list1 = [1, 2, 3]
list2 = [1, 2, 3]

print(list1 is list2)

Output

False

When should you use is?

The most common use of is is to compare an object with None.

name = None

if name is None:
    print("No name provided")

Using is None is the recommended Python style.

Summary

== is
Compares values Compares object identity
Checks if values are equal Checks if both variables refer to the same object
Used for most comparisons Commonly used with None
May return True for different objects with equal values Returns True only for the exact same object

6. = vs == vs is

=, ==, and is look similar, but they serve completely different purposes in Python.

Operator Type Purpose
= Assignment operator Assigns a value to a variable.
== Comparison operator Checks whether two values are equal.
is Identity operator Checks whether two variables refer to the same object.

=

The assignment operator stores a reference to an object in a variable. See Variables are references

x = 10
y = x

print(x)
print(y)

Output

10
10

Here, = does not compare anything. It simply assigns a value (or object reference) to a variable.

==

The equality operator compares the values of two objects.

a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)

Output

True

Although a and b are different objects, they contain the same values.

is

The identity operator compares whether two variables refer to the same object.

a = [1, 2, 3]
b = a

print(a is b)

Output

True

If the variables refer to different objects, is returns False, even if their values are equal.

a = [1, 2, 3]
b = [1, 2, 3]

print(a is b)

Output

False

Key Takeaways

  • append() and extend() are list methods used to add elements, but append() adds a single object while extend() adds elements from an iterable.
  • append() always adds to the end of a list, whereas insert() adds an element at a specified position.
  • remove() deletes an element by value, pop() removes and returns an element by index, and del deletes items, slices, or entire objects.
  • sort() is a list instance method that sorts the original list, while sorted() is a built-in function that returns a new sorted list from any iterable.
  • == compares the values of two objects, whereas is compares whether two variables refer to the same object in memory.
  • = assigns a value or object reference to a variable, == compares values, and is compares object identity.

Understanding these differences will help you write clearer Python code, avoid common beginner mistakes, and confidently answer many Python interview questions.

Summary

= == is
Assigns a value to a variable Compares values Compares object identity
Assignment operator Comparison operator Identity operator
Does not return True or False Returns True or False Returns True or False
Used to create or update variables Used for value comparison Commonly used with None

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