type() and isinstance() in Python - Time & Space Complexity

Time Complexity: type() and isinstance()

O(n)

Understanding Time Complexity

We want to understand how checking an object's type affects the time it takes to run a program.

Specifically, how does using type() or isinstance() grow with input size?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

def count_strings(items):
    count = 0
    for item in items:
        if isinstance(item, str):
            count += 1
    return count

This code counts how many strings are in a list of items.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each item in the list.
• How many times: Once for every item in the list.

How Execution Grows With Input

As the list gets bigger, the program checks more items one by one.

Input Size (n)	Approx. Operations
10	About 10 type checks
100	About 100 type checks
1000	About 1000 type checks

Pattern observation: The number of checks grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to count strings grows in a straight line as the list gets longer.

Common Mistake

[X] Wrong: "Using isinstance() or type() is slow and makes the program much slower as the list grows."

[OK] Correct: Each type check is very fast and only happens once per item, so the total time grows steadily, not wildly.

Interview Connect

Understanding how simple checks like isinstance() scale helps you explain your code's efficiency clearly and confidently.

Self-Check

"What if we checked types inside a nested loop? How would the time complexity change?"

Practice

1. What does the type() function do in Python?

easy

A. It creates a new type.

B. It checks if an object is an instance of a class or its subclass.

C. It converts an object to a string.

D. It returns the exact type of an object.

Solution

1. Step 1: Understand the purpose of type()

   The type() function returns the exact type of the given object, like int, str, or list.

2. Step 2: Compare with other options

   isinstance() checks type inheritance, not exact type. Conversion or creation of types is not done by type().

3. Final Answer:

   It returns the exact type of an object. -> Option D

4. Quick Check:

   type(obj) = exact type [OK]

Hint: Remember: type() gives exact type, isinstance() checks inheritance [OK]

Common Mistakes:

• Confusing type() with isinstance()
• Thinking type() converts or creates types
• Assuming type() checks subclass relationships

2. Which of the following is the correct syntax to check if variable x is an instance of int?

easy

A. type(x) == isinstance(int)

B. isinstance(int, x)

C. isinstance(x, int)

D. type(x) is int()

Solution

1. Step 1: Recall the syntax of isinstance()

   The correct syntax is isinstance(object, class_or_tuple). So, isinstance(x, int) checks if x is an int or subclass.

2. Step 2: Analyze other options

   type(x) == isinstance(int) mixes type() and isinstance() wrongly. isinstance(int, x) reverses arguments. type(x) is int() incorrectly calls int as a function.

3. Final Answer:

   isinstance(x, int) -> Option C

4. Quick Check:

   isinstance(x, int) correct syntax [OK]

Hint: Use isinstance(obj, Type) to check type safely [OK]

Common Mistakes:

• Swapping arguments in isinstance()
• Using type() with wrong comparison
• Calling type names as functions in checks

3. What will be the output of this code?

class Animal:
    pass

class Dog(Animal):
    pass

x = Dog()
print(isinstance(x, Animal))
print(type(x) == Animal)

medium

A. True\nFalse

B. False\nTrue

C. True\nTrue

D. False\nFalse

Solution

1. Step 1: Understand isinstance() with inheritance

   x is an instance of Dog, which is a subclass of Animal. So, isinstance(x, Animal) returns True.

2. Step 2: Understand type() equality check

   type(x) returns Dog, which is not equal to Animal. So, type(x) == Animal returns False.

3. Final Answer:

   True\nFalse -> Option A

4. Quick Check:

   isinstance() checks inheritance, type() exact type [OK]

Hint: isinstance() True if subclass; type() True only if exact type [OK]

Common Mistakes:

• Assuming type() checks inheritance
• Confusing output order
• Thinking isinstance() returns False for subclass

4. Find the error in this code snippet:

def check_number(n):
    if type(n, int):
        return True
    else:
        return False

print(check_number(5))

medium

A. type() is used with two arguments incorrectly.

B. The function should use isinstance() instead of type().

C. The return statements are missing parentheses.

D. The print statement syntax is wrong.

Solution

1. Step 1: Check type() usage

   type() takes one argument to return the type. Using two arguments like type(n, int) is invalid and causes a TypeError.

2. Step 2: Identify correct usage

   To check if n is an int, use isinstance(n, int). The return statements and print syntax are correct.

3. Final Answer:

   type() is used with two arguments incorrectly. -> Option A

4. Quick Check:

   type() needs one argument [OK]

Hint: type() takes one argument; use isinstance() to check type [OK]

Common Mistakes:

• Passing two arguments to type()
• Confusing type() and isinstance() syntax
• Thinking return needs parentheses

5. You have a list of mixed data:

class MyInt(int):
    pass
items = [123, 'hello', 45.6, True, MyInt(456)]

. You want to create a new list containing only integers (including booleans, since they are subclasses of int). Which code correctly uses isinstance() to do this?

hard

A. ints = [x for x in items if type(x) is int or type(x) is bool]

B. ints = [x for x in items if isinstance(x, int)]

C. ints = [x for x in items if isinstance(x, bool)]

D. ints = [x for x in items if type(x) == int]

Solution

1. Step 1: Understand subclass behavior with isinstance()

   Booleans are subclasses of int in Python, so isinstance(True, int) returns True. Using isinstance(x, int) includes both ints and bools.

2. Step 2: Compare with other options

   ints = [x for x in items if type(x) == int] excludes bools because type(True) is bool, not int. ints = [x for x in items if isinstance(x, bool)] selects only bools, not ints. ints = [x for x in items if type(x) is int or type(x) is bool] is verbose and misses subclass flexibility.

3. Final Answer:

   ints = [x for x in items if isinstance(x, int)] -> Option B

4. Quick Check:

   isinstance() includes subclasses [OK]

Hint: Use isinstance(x, int) to include bools as ints [OK]

Common Mistakes:

• Using type() == int excludes bools
• Selecting only bools instead of ints
• Using complex or incorrect type checks