March 2019, Reading time: 3 minutes
In this post I will explain lambda expressions in python. Simply put, lambda expressions is another way to create a function in python. However this function has some restrictions. It does not have a name (that’s why they are also called anonymous), they can not have assignments they do not have a ‘return’ keyword and they can only be one line. They usually used when a function is needed to be passed as an argument to another function. So they are used with higher order functions like map, filter, reduce. The syntax is:
lambda [parameters] : expression
lambda x : x returns x lambda x : x**2 returns x**2 lambda x,y : x+y returns x+y lambda: True returns True
Lets see an example first with a normal function and then with a lambda function. We will use the map function that accepts as arguments a function and an iterable and executes the function for each of the iterables. It returns a map object and we will use the list() to convert the output to list. In python 2 it is not needed, because it returns a list.
l = [1,2,3,4] def sq(x): return x**2 print(list(map(sq,l))) the above code prints [1, 4, 9, 16] Now with lambda l = [1,2,3,4] print(list(map(lambda x:x**2,l))) [1, 4, 9, 16]
Lets see an example with reduce. Lets assume we want to find the product of all elements in a list.
reduce(lambda x,y: x*y,[1,2,3,4]) 24
Another common example is to use lambda expression with sorted function. The sorted fucntion accepts as key a function that can customize the sort order
l=['a','z','c','A','D','b'] sorted(l) ['A', 'D', 'a', 'b', 'c', 'z']
as we can see above, because the list contains upper and lower case characters, the sorted function does not provide the output we want. Lets remedy this, by providing as key a lambda function that converts all characters at uppercase. Now the sorted function will have no problem sorting them.
sorted(l, key=lambda x: x.upper()) ['a', 'A', 'b', 'c', 'D', 'z']
Last example, lets assume I have a list of names and I want to sort them on the last name. By default it will use the first character to sort them
l =["George Smith","Adam Fry","John Black"] sorted(l) ['Adam Fry', 'George Smith', 'John Black']
Lets see how we can fix it
l =["George Smith","Adam Fry","John Black"] sorted(l, key=lambda x: x.split()) ['John Black', 'Adam Fry', 'George Smith']
What I did is split the full name in a list and use the second (1) element of the list for sorting. If I wanted to sort on the first name I would do this
sorted(l, key=lambda x: x.split()) ['Adam Fry', 'George Smith', 'John Black']
In addition in another post I have shown how to sort IP addresses. I will repeat the code here
ip_list = ["126.96.36.199","10.5.2.1","10.3.4.1","188.8.131.52","184.108.40.206"] sorted(ip_list, key = lambda ip: [int(ip) for ip in ip.split(".")] ) ['10.3.4.1', '10.5.2.1', '220.127.116.11', '18.104.22.168', '22.214.171.124']
You can find the whole script on the post how to sort ip addresses with python