How variable type changes at runtime in Python - Performance & Efficiency

Time Complexity: How variable type changes at runtime

O(n)

Understanding Time Complexity

We want to see how the time it takes to run code changes when a variable changes its type during the program.

How does this affect the speed as the program runs?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

x = 10
x = "hello"
x = [1, 2, 3]
for item in x:
    print(item)

This code changes the variable x from a number to a string, then to a list, and finally loops over the list to print each item.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping over the list x to print each item.
• How many times: Once for each item in the list (3 times here).

How Execution Grows With Input

As the list x gets bigger, the loop runs more times, once per item.

Input Size (n)	Approx. Operations
10	10 print operations
100	100 print operations
1000	1000 print operations

Pattern observation: The time grows directly with the number of items in the list.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the loop grows in a straight line with the number of items in the list.

Common Mistake

[X] Wrong: "Changing the variable type multiple times makes the program slower in a way that depends on the number of changes."

[OK] Correct: Changing the variable type itself is a simple assignment and happens once each time; it does not repeat or grow with input size, so it does not affect the overall time complexity significantly.

Interview Connect

Understanding how variable types can change and how loops behave helps you explain how your code runs as data grows, a key skill in many programming tasks.

Self-Check

"What if the variable x was changed to a dictionary instead of a list? How would the time complexity change when looping over it?"

Practice

1. What happens to the type of a Python variable when you assign a new value of a different type to it?

easy

A. The variable's type changes to the new value's type automatically.

B. The variable keeps its original type regardless of the new value.

C. Python raises an error if the new value's type is different.

D. The variable becomes a list containing both old and new types.

Solution

1. Step 1: Understand Python variable typing

   Python variables are dynamically typed, meaning their type depends on the current value assigned.

2. Step 2: Assigning a new value changes the type

   When you assign a new value of a different type, the variable's type updates to match the new value automatically.

3. Final Answer:

   The variable's type changes to the new value's type automatically. -> Option A

4. Quick Check:

   Python variables are dynamic = A [OK]

Hint: Remember: Python variables follow the value's type, not fixed type. [OK]

Common Mistakes:

• Thinking variables have fixed types like in some other languages.
• Assuming Python throws an error on type change.
• Believing variable types combine old and new types.

2. Which of the following is the correct way to assign a new value to a variable in Python so its type changes?

easy

A. x = 10

B. x := 10

C. int x = 10

D. var x = 10

Solution

1. Step 1: Identify Python assignment syntax

   Python uses the single equals sign (=) to assign values to variables.

2. Step 2: Check each option

   x = 10 uses correct syntax. x := 10 uses walrus operator which is for expressions, not simple assignment. int x = 10 and var x = 10 are invalid in Python.

3. Final Answer:

   x = 10 -> Option A

4. Quick Check:

   Assignment uses = in Python = A [OK]

Hint: Use single '=' for assignment in Python, not ':=' or types. [OK]

Common Mistakes:

• Confusing ':=' walrus operator with assignment.
• Using typed declarations like other languages.
• Trying to declare variable types explicitly.

3. What will be the output of this code?

var = 5
print(type(var))
var = 'hello'
print(type(var))

medium

A. \n

B. \n

C. \n

D. Error

Solution

1. Step 1: Check initial assignment and type

   var is first assigned 5, an integer, so type(var) is <class 'int'>.

2. Step 2: Reassign and check new type

   var is then assigned 'hello', a string, so type(var) is <class 'str'>.

3. Final Answer:

   <class 'int'>\n<class 'str'> -> Option C

4. Quick Check:

   Type changes with value = C [OK]

Hint: Track variable type after each assignment line by line. [OK]

Common Mistakes:

• Assuming type stays the same after reassignment.
• Confusing output order of print statements.
• Expecting a runtime error on type change.

4. Find the error in this code snippet:

value = 10
value = value + '5'
print(value)

medium

A. NameError because 'value' is undefined

B. SyntaxError due to invalid assignment

C. No error, output is 15

D. TypeError because int and str cannot be added

Solution

1. Step 1: Analyze the operation between int and str

   value is an int (10), then tries to add a string ('5'), which is not allowed.

2. Step 2: Identify the error type

   Python raises a TypeError when adding incompatible types like int and str.

3. Final Answer:

   TypeError because int and str cannot be added -> Option D

4. Quick Check:

   Adding int + str causes TypeError = D [OK]

Hint: Check types before adding; int + str causes error. [OK]

Common Mistakes:

• Thinking Python auto-converts types in addition.
• Expecting output '15' as string concatenation.
• Confusing SyntaxError with TypeError.

5. Given the code below, what will be the final type of data?

data = 100
if isinstance(data, int):
    data = str(data)
else:
    data = [data]

hard

A. int

B. str

C. list

D. bool

Solution

1. Step 1: Check initial type of data

   data starts as 100, which is an int.

2. Step 2: Evaluate the if condition

   Since data is int, the if block runs, converting data to str('100').

3. Final Answer:

   str -> Option B

4. Quick Check:

   int converted to str by condition = B [OK]

Hint: If int, convert to str; else make list. [OK]

Common Mistakes:

• Ignoring the if condition and assuming list.
• Thinking data remains int after reassignment.
• Confusing str and bool types.