Default arguments in Python - Step-by-Step Execution

Concept Flow - Default arguments

Function defined with default argument

↓

Function called

↓

Argument given

↓

Use given value

↓

Function body runs with chosen argument

↓

Return result

The function is defined with a default value for an argument. When called, if an argument is given, it uses that; otherwise, it uses the default.

Execution Sample

Python

def greet(name="Friend"):
    print(f"Hello, {name}!")

greet("Alice")
greet()

Defines a function with a default argument and calls it once with and once without an argument.

Execution Table

Step	Function Call	Argument Passed?	Value of 'name'	Action	Output
1	greet("Alice")	Yes	"Alice"	Use passed argument	Hello, Alice!
2	greet()	No	"Friend"	Use default argument	Hello, Friend!
3	End	-	-	No more calls	-

💡 All function calls completed, program ends.

Variable Tracker

Variable	Start	After Call 1	After Call 2	Final
name	-	"Alice"	"Friend"	-

Key Moments - 2 Insights

Why does the function print "Hello, Friend!" when called without an argument?

Because the function has a default argument 'name="Friend"', so when no argument is passed (see execution_table step 2), it uses the default value.

What happens if you pass an argument to the function?

The passed argument is used instead of the default (see execution_table step 1), so the function prints the greeting with the given name.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'name' during the second function call?

A "Alice"

B "Friend"

C None

D Error

💡 Hint

Check execution_table row 2 under 'Value of name'

At which step does the function use the default argument?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'Argument Passed?' column in execution_table

If you call greet("Bob"), what will be printed?

A Hello, Bob!

B Hello, Friend!

C Hello, Alice!

D Error

💡 Hint

See how the function uses passed arguments in execution_table step 1

Concept Snapshot

def function_name(arg=default):
    # function body

- If argument is passed, use it.
- Otherwise, use default value.
- Default arguments allow flexible function calls.

Full Transcript

This visual execution shows how Python functions use default arguments. The function greet is defined with a default argument name="Friend". When greet is called with an argument like "Alice", it uses that value. When called without an argument, it uses the default "Friend". The execution table tracks each call, showing the argument passed, the value used, and the output printed. The variable tracker shows how the variable 'name' changes with each call. Key moments clarify why default arguments work and when they are used. The quiz tests understanding by asking about values and behavior from the execution steps.

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