Python Lambda Functions: Syntax, Use Cases & Limitations

Lambda syntax

A lambda is a single-expression anonymous function. It can take any number of arguments but the body must be a single expression — no statements, no assignments.

# lambda arguments: expression
square = lambda x: x ** 2
print(square(5))   # 25

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

# Equivalent def form:
def add(a, b):
    return a + b

Using lambda with sorted()

`sorted()` accepts a `key=` function. Lambda is the most common way to provide a one-line sort key.

people = [('Alice', 30), ('Bob', 25), ('Carol', 35)]

# Sort by age (second element)
by_age = sorted(people, key=lambda p: p[1])
print(by_age)
# [('Bob', 25), ('Alice', 30), ('Carol', 35)]

# Sort strings by length, then alphabetically
words = ['banana', 'fig', 'apple', 'kiwi']
result = sorted(words, key=lambda w: (len(w), w))
print(result)  # ['fig', 'kiwi', 'apple', 'banana']

Using lambda with map() and filter()

`map()` applies a function to each item; `filter()` keeps items where the function returns True. Both return lazy iterators.

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

doubled = list(map(lambda x: x * 2, nums))
print(doubled)    # [2, 4, 6, 8, 10]

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

# Modern alternative — list comprehensions are usually clearer
doubled = [x * 2 for x in nums]
evens   = [x for x in nums if x % 2 == 0]

Conditional expression inside lambda

You can use the ternary `x if condition else y` expression inside a lambda — but not an `if` statement.

# Ternary expression IS an expression — allowed in lambda
clamp = lambda x, lo, hi: lo if x < lo else (hi if x > hi else x)
print(clamp(5, 0, 10))   # 5
print(clamp(-3, 0, 10))  # 0
print(clamp(15, 0, 10))  # 10

Limitations of lambda

Lambda is intentionally restricted. These limitations frequently appear on exams.

# These are ILLEGAL inside a lambda:
# f = lambda x: x = x + 1   # assignment — SyntaxError
# f = lambda x: return x     # return keyword — SyntaxError
# f = lambda x: if x > 0: x  # if statement — SyntaxError

# Multiple expressions are not possible:
# f = lambda x: x + 1; x * 2  # only x * 2 would be the body

# Use def instead when you need any of the above

Exam tip

The most-tested lambda fact: it is a single *expression* — no statements allowed. Exams also test the difference between `map(lambda x: x*2, lst)` (returns an iterator, not a list) and `[x*2 for x in lst]` (returns a list directly).