Python Basics — Lambda Expressions

Lambda Expressions

A lambda is a small anonymous function defined in a single expression. Lambdas are useful when you need a quick throwaway function.

Lambda Syntax

lambda parameters: expression

The lambda automatically returns the result of the expression — no return keyword needed.

# Regular function
def square(x):
    return x ** 2

# Equivalent lambda
square = lambda x: x ** 2

print(square(5))  # 25

Lambda vs def Comparison

Feature	def	lambda
Name	Required	Optional (anonymous)
Body	Multiple statements	Single expression only
Return	Explicit return	Implicit (returns expression)
Readability	Better for complex logic	Better for simple operations
Docstrings	Supported	Not supported

# def — multi-line logic
def classify(n):
    if n > 0:
        return "positive"
    elif n < 0:
        return "negative"
    else:
        return "zero"

# lambda — single expression only
classify = lambda n: "positive" if n > 0 else ("negative" if n < 0 else "zero")

print(classify(5))   # positive
print(classify(-3))  # negative
print(classify(0))   # zero

When to Use Lambda

Lambdas are best used inline — passed directly as arguments to other functions:

# ✅ Good use — inline with sorted()
students = [("Alice", 85), ("Bob", 92), ("Charlie", 78)]
sorted_students = sorted(students, key=lambda s: s[1])
print(sorted_students)  # [('Charlie', 78), ('Alice', 85), ('Bob', 92)]

# ✅ Good use — inline with map()
numbers = [1, 2, 3, 4, 5]
doubled = list(map(lambda x: x * 2, numbers))
print(doubled)  # [2, 4, 6, 8, 10]

# ❌ Bad use — assigning to a variable (use def instead)
# PEP 8 discourages: multiply = lambda x, y: x * y
# Use: def multiply(x, y): return x * y

Lambda with Multiple Parameters

add = lambda a, b: a + b
print(add(3, 4))  # 7

full_name = lambda first, last: f"{first} {last}"
print(full_name("Ravi", "Kumar"))  # Ravi Kumar

# With default parameters
greet = lambda name, greeting="Hello": f"{greeting}, {name}!"
print(greet("Alice"))            # Hello, Alice!
print(greet("Bob", "Namaste"))   # Namaste, Bob!

Lambda Limitations

Lambdas can only contain a single expression — no statements like =, for, if:, while:, print, etc. (though ternary if/else is an expression and IS allowed):

# ✅ Ternary expression — allowed
check = lambda x: "even" if x % 2 == 0 else "odd"
print(check(4))  # even

# ❌ Multiple statements — NOT allowed
# bad = lambda x: x = x + 1    # SyntaxError
# bad = lambda x: print(x); return x   # SyntaxError

IIFE — Immediately Invoked Lambda

You can define and call a lambda in one step:

result = (lambda x, y: x + y)(3, 7)
print(result)  # 10

# Useful for quick computations
area = (lambda l, w: l * w)(5, 3)
print(area)  # 15

This is rare in practice but appears in some code patterns.

Common Lambda Patterns

# Identity function
identity = lambda x: x

# Always return a constant
always_true = lambda: True

# Accessing object attributes
get_name = lambda obj: obj["name"]

# Chained ternary
grade = lambda score: "A" if score >= 90 else ("B" if score >= 80 else ("C" if score >= 70 else "F"))
print(grade(85))  # B

# Tuple operations
first = lambda pair: pair[0]
second = lambda pair: pair[1]

Summary

• Lambda syntax: lambda params: expression
• Returns the expression result automatically
• Best used inline as arguments to sorted(), map(), filter()
• Single expression only — no multi-line logic
• PEP 8: don’t assign lambdas to variables — use def instead
• Great for short, simple operations; use def for anything complex

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Next up: map() Function →