What is an anonymous function (lambda)?

Lambda is an anonymous (unnamed) function defined in a single line. It can take any number of arguments but contains only one expression.

Syntax:

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

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

square(5)  # 25

Usage with sorted:

users = [
    {"name": "Anna", "age": 25},
    {"name": "Boris", "age": 30},
    {"name": "Vera", "age": 20},
]

# Sort by age
sorted_users = sorted(users, key=lambda u: u["age"])

Usage with map and filter:

numbers = [1, 2, 3, 4, 5]

squares = list(map(lambda x: x ** 2, numbers))
# [1, 4, 9, 16, 25]

evens = list(filter(lambda x: x % 2 == 0, numbers))
# [2, 4]

Limitations:

• Only one expression — you cannot use multi-line logic, loops, or assignments.
• Reduces readability for complex expressions — it's better to use a regular function.
• No name — makes debugging harder (appears as <lambda> in the traceback).