
Python lambda Functions – Anonymous Power in One Line
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Python’s lambda
keyword allows you to write anonymous, one-line functions. These are useful for short, throwaway functions where using def
might feel too heavy.
In this article, you’ll learn:
-
What
lambda
functions are -
Syntax and how they compare to regular functions
-
Practical use cases
-
Common pitfalls
-
Tips and best practices
-
A complete example at the end
✅ What Is a lambda
Function?
A lambda
function in Python is a small, anonymous function defined using the lambda
keyword. It can have any number of arguments, but only one expression.
It’s typically used for:
-
Inline function definitions
-
Short, simple operations
-
Passing functions as arguments
Syntax
lambda arguments: expression
This is equivalent to:
def function_name(arguments):
return expression
Basic Example
square = lambda x: x * x
print(square(5))
Output:
25
Equivalent using def
:
def square(x):
return x * x
Multiple Parameters
add = lambda a, b: a + b
print(add(3, 4)) # 7
Lambda with map()
, filter()
, reduce()
1️⃣ map()
: Apply a function to every item in an iterable
nums = [1, 2, 3, 4]
squared = list(map(lambda x: x ** 2, nums))
print(squared) # [1, 4, 9, 16]
2️⃣ filter()
: Filter items based on a condition
nums = [1, 2, 3, 4, 5]
evens = list(filter(lambda x: x % 2 == 0, nums))
print(evens) # [2, 4]
3️⃣ reduce()
: Cumulative reduction (needs functools
)
from functools import reduce
nums = [1, 2, 3, 4]
product = reduce(lambda x, y: x * y, nums)
print(product) # 24
Sorting with Lambda
pairs = [(1, 'b'), (2, 'a'), (3, 'c')]
sorted_pairs = sorted(pairs, key=lambda x: x[1])
print(sorted_pairs)
Output:
[(2, 'a'), (1, 'b'), (3, 'c')]
Lambda Inside List Comprehension
funcs = [lambda x: x + n for n in range(3)]
print([f(10) for f in funcs]) # Output?
Tricky Output:
[12, 12, 12]
Because lambda x: x + n
captures the reference to n
, not its value at each loop.
✅ Fix using default arguments:
funcs = [lambda x, n=n: x + n for n in range(3)]
print([f(10) for f in funcs]) # [10, 11, 12]
Full Practical Example: Sorting a List of Dictionaries
people = [
{"name": "Alice", "age": 30},
{"name": "Bob", "age": 25},
{"name": "Charlie", "age": 35}
]
# Sort by age
sorted_people = sorted(people, key=lambda person: person["age"])
for person in sorted_people:
print(person)
Output:
{'name': 'Bob', 'age': 25}
{'name': 'Alice', 'age': 30}
{'name': 'Charlie', 'age': 35}
⚠️ Common Pitfalls
Mistake | Why It Happens | Fix |
---|---|---|
Too much logic in a lambda |
lambda supports only one expression |
Use def for complex logic |
Confusing closures (e.g., in loops) | Late binding captures variable reference | Use default args in lambda (e.g., n=n ) |
Ignoring readability | Overusing lambda can make code confusing |
Use only for short, simple expressions |
Tips and Best Practices
-
✅ Use
lambda
only when defining short, simple functions. -
✅ Prefer
def
when:-
The function has multiple expressions
-
You need docstrings or annotations
-
You want to reuse the function across many places
-
-
✅ Combine with
sorted()
,map()
,filter()
for quick tasks
Summary Table
Concept | Example |
---|---|
One-arg lambda | lambda x: x + 1 |
Two-arg lambda | lambda x, y: x * y |
Use with map() |
map(lambda x: x+2, [1,2,3]) |
Use with filter() |
filter(lambda x: x>0, [-1,0,1]) |
Use with sorted() |
sorted(data, key=lambda x: x[1]) |
What’s Next?
After learning lambda
, you can explore:
-
map()
,filter()
, andreduce()
in detail -
List comprehensions vs
map()
withlambda
-
Closures and decorators (which may use
lambda
) -
Generator expressions for efficient iteration
Tips and Tricks
What is pass in Python?
Python | Pass Statement
The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.
def myfunction():
pass
How can you generate random numbers?
Python | Generate random numbers
Python provides a module called random using which we can generate random numbers. e.g: print(random.random())
We have to import a random module and call the random() method as shown below:
import random
print(random.random())
The random() method generates float values lying between 0 and 1 randomly.
To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)
import random
print(random.randrange(5,100,2))
What is lambda in Python?
Python | Lambda function
A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.
Syntex:
lambda arguments : expression
a = lambda x,y : x+y
print(a(5, 6))
It also provides a nice way to write closures. With that power, you can do things like this.
def adder(x):
return lambda y: x + y
add5 = adder(5)
add5(1) #6
As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.
What is swapcase() function in the Python?
Python | swapcase() Function
It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.
string = "IT IS IN LOWERCASE."
print(string.swapcase())
How to remove whitespaces from a string in Python?
Python | strip() Function | Remove whitespaces from a string
To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.
string = " Python "
print(string.strip())
What is the usage of enumerate() function in Python?
Python | enumerate() Function
The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.
lst = ["A","B","C"]
print (list(enumerate(lst)))
#[(0, 'A'), (1, 'B'), (2, 'C')]
Can you explain the filter(), map(), and reduce() functions?
Python | filter(), map(), and reduce() Functions
- filter() function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
>>> set(filter(lambda x:x>4, range(7))) # {5, 6}
-
map() function calls the specified function for each item of an iterable and returns a list of result
>>> set(map(lambda x:x**3, range(7))) # {0, 1, 64, 8, 216, 27, 125}
-
reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..
>>> reduce(lambda x,y:y-x, [1,2,3,4,5]) # 3
Let’s understand this:
2-1=1
3-1=2
4-2=2
5-2=3Hence, 3.
What is a namedtuple?
Python | namedtuple
A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.
>>> from collections import namedtuple
#format
>>> result=namedtuple('result','Physics Chemistry Maths')
#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80)
>>> Chris.Chemistry
# 92
Write a code to add the values of same keys in two different dictionaries and return a new dictionary.
We can use the Counter method from the collections module
from collections import Counter
dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}
new_dict = Counter(dict1) + Counter(dict2)
print(new_dict)
# Print: Counter({'a': 7, 'b': 7, 'c': 5})
Python In-place swapping of two numbers
Python | In-place swapping of two numbers
>>> a, b = 10, 20
>>> print(a, b)
10 20
>>> a, b = b, a
>>> print(a, b)
20 10
Reversing a String in Python
Python | Reversing a String
>>> x = 'PythonWorld'
>>> print(x[: : -1])
dlroWnohtyP
Python join all items of a list to convert into a single string
Python | Join all items of a list to convert into a single string
>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training
python return multiple values from functions
Python | Return multiple values from functions
>>> def A():
return 2, 3, 4
>>> a, b, c = A()
>>> print(a, b, c)
2 3 4
Python Print String N times
Python | Print String N times
>>> s = 'Python'
>>> n = 5
>>> print(s * n)
PythonPythonPythonPythonPython
Python check the memory usage of an object
Python | Check the memory usage of an object
>>> import sys
>>> x = 100
>>> print(sys.getsizeof(x))
28