0025-lambda-functions-in-python.md

Lambda Functions in Python

A lambda function in Python is a small, anonymous function defined using the lambda keyword.

• It has no name
• It can take any number of arguments
• It can contain only one expression
• It returns the result automatically

Lambda functions are often used when a function is needed for a short time and simple logic.

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What is a Lambda Function?

Normal Function

def square(x):
    return x * x

Equivalent Lambda Function

square = lambda x: x * x

Both produce the same result.

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Syntax of Lambda Function

lambda arguments: expression

• lambda → keyword
• arguments → inputs (like parameters)
• expression → single expression whose result is returned

⚠️ No return keyword is used — it’s implicit.

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1. Simple Lambda Function

square = lambda x: x * x
print(square(5))

Output:

25

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2. Lambda with Multiple Arguments

add = lambda a, b: a + b
print(add(3, 4))

Output:

7

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3. Lambda with No Arguments

say_hello = lambda: "Hello!"
print(say_hello())

Output:

Hello!

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4. Lambda with Conditional Logic (Ternary)

Since lambdas allow only one expression, conditional logic must be written using a ternary expression.

is_even = lambda x: "Even" if x % 2 == 0 else "Odd"
print(is_even(10))
print(is_even(7))

Output:

Even
Odd

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5. Lambda vs Normal Function

Normal Function

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

Lambda Function

multiply = lambda a, b: a * b

✅ Use lambda when:

• Logic is simple
• Function is short-lived

❌ Avoid lambda when:

• Logic is complex
• Multiple statements are required

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6. Lambda with Built-in Functions

Lambda functions shine when used with higher-order functions.

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map() + Lambda

numbers = [1, 2, 3, 4]
squares = list(map(lambda x: x * x, numbers))
print(squares)

Output:

[1, 4, 9, 16]

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filter() + Lambda

numbers = [1, 2, 3, 4, 5, 6]
evens = list(filter(lambda x: x % 2 == 0, numbers))
print(evens)

Output:

[2, 4, 6]

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sorted() + Lambda

points = [(1, 2), (3, 1), (5, 0)]
sorted_points = sorted(points, key=lambda x: x[1])
print(sorted_points)

Output:

[(5, 0), (3, 1), (1, 2)]

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7. Lambda Inside Functions

def apply_function(func, value):
    return func(value)

print(apply_function(lambda x: x * 2, 10))

Output:

20

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##️ 8. Lambda and *args

sum_all = lambda *args: sum(args)
print(sum_all(1, 2, 3, 4))

Output:

10

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9. What Lambda Functions CANNOT Do

❌ Cannot contain multiple expressions
❌ Cannot have statements like print, for, while, try
❌ Cannot have assignments (=)
❌ Hard to debug when overused

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Common Mistakes & Errors

Mistake	Why It Happens
Writing complex logic	Lambda allows only one expression
Overusing lambdas	Reduces readability
Using for long functions	Lambdas are meant to be short
Expecting statements	Lambdas support expressions only
Debugging difficulty	No function name

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When to Use Lambda vs def

Scenario	Use
One-line simple logic	Lambda
Reusable logic	def
Passed as argument	Lambda
Complex logic	def
Debugging needed	def

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Summary

Feature	Lambda Function
Name	Anonymous
Lines	Single line
Return	Implicit
Statements	❌ Not allowed
Best use	Short-lived functions

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Practice Tasks

1. Create a lambda to check if a number is positive or negative.
2. Use map() with lambda to convert a list of temperatures from °C to °F.
3. Sort a list of dictionaries by a specific key using lambda.
4. Rewrite a small def function as a lambda.
5. Explain why lambda should not replace all functions.