Default arguments in Python - Time & Space Complexity

Time Complexity: Default arguments

O(times)

Understanding Time Complexity

When using default arguments in functions, it's important to see how they affect the work done each time the function runs.

We want to know if having default values changes how long the function takes as input grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

def greet(name, times=1):
    for _ in range(times):
        print(f"Hello, {name}!")

# Calling the function
# greet("Alice")  # uses default times=1
# greet("Bob", 3) # times=3

This function prints a greeting message a number of times, using a default value if no number is given.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop that prints the greeting.
• How many times: It runs exactly times times, which can be the default or a given number.

How Execution Grows With Input

Each time the function runs, the number of print operations grows with the times value.

Input Size (times)	Approx. Operations
1	1 print
10	10 prints
100	100 prints

Pattern observation: The work grows directly with the number of times the loop runs.

Final Time Complexity

Time Complexity: O(times)

This means the time taken grows in a straight line with how many times we want to print the greeting.

Common Mistake

[X] Wrong: "Using default arguments makes the function run faster or slower depending on the default value."

[OK] Correct: The default value just sets a starting number; the function's time depends on the actual number used when called, not on whether it was default or given.

Interview Connect

Understanding how default arguments affect loops helps you explain how your code behaves with different inputs, a skill that shows clear thinking in interviews.

Self-Check

"What if the function called another function inside the loop? How would that change the time complexity?"

Practice

1. What is the main purpose of default arguments in Python functions?

easy

A. To create variables inside the function

B. To provide preset values for parameters if no argument is given

C. To make functions run faster

D. To force the user to always provide all arguments

Solution

1. Step 1: Understand default arguments

   Default arguments allow a function to have parameters with preset values if the caller does not provide them.

2. Step 2: Identify the purpose

   This makes the function easier to call without needing all arguments every time.

3. Final Answer:

   To provide preset values for parameters if no argument is given -> Option B

4. Quick Check:

   Default arguments = preset values [OK]

Hint: Default arguments fill in missing inputs automatically [OK]

Common Mistakes:

• Thinking default arguments speed up code
• Believing default arguments force all inputs
• Confusing default arguments with variable creation

2. Which of the following function definitions uses default arguments correctly?

easy

A. def greet(name='Friend', age):

B. def greet(name=, age=30):

C. def greet(name, age=30):

D. def greet(name, age=):

Solution

1. Step 1: Check default argument placement

   Parameters with default values must come after parameters without defaults.

2. Step 2: Analyze each option

   def greet(name='Friend', age): places a default before a non-default parameter, which is invalid syntax. Options C and D have syntax errors. def greet(name, age=30): correctly places the default argument after a required parameter.

3. Final Answer:

   def greet(name, age=30): -> Option C

4. Quick Check:

   Defaults after required params = correct syntax [OK]

Hint: Put default parameters after required ones [OK]

Common Mistakes:

• Placing default arguments before required ones
• Leaving default values empty
• Using invalid syntax for defaults

3. What is the output of this code?

def multiply(a, b=2):
    return a * b

print(multiply(4))
print(multiply(4, 3))

medium

A. Error

B. 6 and 12

C. 8 and 6

D. 8 and 12

Solution

1. Step 1: Understand default argument usage

   When multiply(4) is called, b uses its default value 2, so 4 * 2 = 8.

2. Step 2: Evaluate second call

   When multiply(4, 3) is called, b is 3, so 4 * 3 = 12.

3. Final Answer:

   8 and 12 -> Option D

4. Quick Check:

   Default used first call, explicit second call = 8 and 12 [OK]

Hint: Default used if argument missing, else explicit value [OK]

Common Mistakes:

• Assuming default is ignored when argument is given
• Confusing order of arguments
• Expecting error when default is used

4. Find the error in this function definition:

def add_numbers(x=5, y):
    return x + y

medium

A. Default argument before non-default argument causes SyntaxError

B. Missing return statement

C. Function name is invalid

D. No error, code runs fine

Solution

1. Step 1: Check parameter order

   Parameters with default values must come after parameters without defaults in Python.

2. Step 2: Identify error

   Here, x has a default but y does not, so this causes a SyntaxError.

3. Final Answer:

   Default argument before non-default argument causes SyntaxError -> Option A

4. Quick Check:

   Default before required param = SyntaxError [OK]

Hint: Non-default params must come before default ones [OK]

Common Mistakes:

• Ignoring parameter order rules
• Thinking missing return causes error here
• Assuming function name is invalid

5. You want a function that greets a user with their name and an optional greeting word (default is 'Hello'). Which function definition is correct and prints "Hi, Alice!" when called as greet('Alice', 'Hi') and "Hello, Bob!" when called as greet('Bob')?

hard

A. def greet(name, greeting='Hello'):
print(f"{greeting}, {name}!")

B. def greet(greeting='Hello', name):
print(f"{greeting}, {name}!")

C. def greet(name, greeting):
print(f"{greeting}, {name}!")

D. def greet(name='User', greeting='Hello'):
print(f"{name}, {greeting}!")

Solution

1. Step 1: Check parameter order and defaults

   Parameters with defaults must come after those without. def greet(name, greeting='Hello'): print(f"{greeting}, {name}!") has name first (no default), greeting second (default 'Hello').

2. Step 2: Verify behavior

   Calling greet('Alice', 'Hi') prints "Hi, Alice!". Calling greet('Bob') uses default greeting "Hello" and prints "Hello, Bob!".

3. Final Answer:

   def greet(name, greeting='Hello'): print(f"{greeting}, {name}!") -> Option A

4. Quick Check:

   Defaults after required params, works as expected [OK]

Hint: Put required params first, defaults after [OK]

Common Mistakes:

• Placing default param before required param
• Not providing default for optional greeting
• Using default for name causing unexpected calls