Functions & Lambdas

Functions & Lambdas in Python

1. Functions in Python (def)

A function in Python is a reusable block of code that performs a specific task.
It is defined using the def keyword and can accept parameters and return a value.

General Syntax

def function_name(parameters):
    # logic
    return result

Functions help in:

• Reusability
• Better readability
• Avoiding duplicate code
• Encapsulating logic

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2. Example Function Explained

def greet(name, exclaim=False):
    ending = "!" if exclaim else "."
    return f"Hello {name}{ending}"

Breakdown

Function Definition

def greet(name, exclaim=False):

• greet → function name
• name → required parameter
• exclaim=False → optional parameter with a default value

This is equivalent to a default parameter in C#:

void Greet(string name, bool exclaim = false)

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3. What is exclaim?

exclaim is a boolean flag that:

• Controls the behavior of the function
• Acts like an ON/OFF switch

Value	Meaning
False (default)	Normal greeting
True	Excited greeting

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4. Conditional (Ternary) Expression

ending = "!" if exclaim else "."

Equivalent long form:

if exclaim:
    ending = "!"
else:
    ending = "."

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5. Return Statement & f-Strings

return f"Hello {name}{ending}"

• return sends the result back to the caller
• f"..." is an f-string (string interpolation)
• {name} and {ending} insert variable values

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6. Function Usage Examples

greet("Banky")

Output:

Hello Banky.

greet("Banky", True)

Output:

Hello Banky!

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7. Lambda Functions (Anonymous Functions)

A lambda function is:

• A small, one-line function
• Anonymous (no name)
• Used for simple logic
• Automatically returns a value

Syntax

lambda arguments: expression

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8. Lambda Example Explained

adder = lambda a, b: a + b

Equivalent regular function:

def adder(a, b):
    return a + b

Equivalent C# lambda:

Func<int, int, int> adder = (a, b) => a + b;

Equivalent JavaScript arrow function:

const add = (a, b) => a + b;

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9. Lambda Usage

adder(3, 5)

Output:

8

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10. def vs lambda

Scenario	def	lambda
Multi-line logic	✅	❌
Complex logic	✅	❌
Readability	✅	⚠️
One-line logic	⚠️	✅
map / filter usage	⚠️	✅

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11. Python vs C# Default Parameters

Aspect	Python	C#
Default parameters	Yes	Yes
Evaluation time	Function definition	Compile time
Mutable defaults	⚠️ Avoid	Safe
Boolean flags	Common	Common

Note: Default parameters are safe for immutable types like bool, int, and string.

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12. Practice Questions

1. How do you give a parameter a default value in Python?

   Answer

   Assign it in the function definition: `def f(x=0):`

2. Name the JavaScript feature similar to Python’s lambda.

   Answer

   Arrow functions.

3. What is the difference between def and lambda?

   Answer

   `def` is used for multi-line, named functions with complex logic, while `lambda` is for short, anonymous, one-line expressions.

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13. Final Summary

• def defines reusable functions in Python
• Default parameters work the same as C# optional parameters
• exclaim is a boolean flag controlling behavior
• lambda creates small, one-line anonymous functions
• Use def for clarity and complex logic
• Use lambda for short, simple expressions