Why can’t Python lambda expressions contain statements?

Yes, Python Lambda Expressions cannot contain statements. Before deep diving the reason, let us understand what is a Lambda, its expressions, and statements.

The Lambda expressions allow defining anonymous functions. A lambda function is an anonymous function i.e. a function without a name. Let us see the syntax −

lambda arguments: expressions

The keyword lambda defines a lambda function. A lambda expression contains one or more arguments, but it can have only one expression.

Lambda Example

Let us see an example −

myStr = "Thisisit!"
(lambda myStr : print(myStr))(myStr)

Output

Thisisit!

Sort a List by values from another list using Lambda

In this example, we will sort a list by values from another list i.e. the 2nd list will have the index in the order in which they are placed in sorted order −

# Two Lists
list1 = ['BMW', 'Toyota', 'Audi', 'Tesla', 'Hyundai']
list2 = [2, 5, 1, 4, 3]

print("List1 = \n",list1)
print("List2 (indexes) = \n",list2)

# Sorting the List1 based on List2
res = [val for (_, val) in sorted(zip(list2, list1), key=lambda x: x[0])]
print("\nSorted List = ",res)

Output

List1 = 
 ['BMW', 'Toyota', 'Audi', 'Tesla', 'Hyundai']
List2 (indexes) = 
 [2, 5, 1, 4, 3]

Sorted List =  ['Audi', 'BMW', 'Hyundai', 'Tesla', 'Toyota']

Lambda Expressions cannot contain statements

We have seen two examples above wherein we have used Lambda expressions. Python lambda expressions cannot contain statements because Python’s syntactic framework can’t handle statements nested inside expressions.

Functions are already first-class objects in Python, and can be declared in a local scope. Therefore, the only advantage of using a lambda instead of a locally defined function is that you don’t need to invent a name for the function i.e. anonymous, but that’s just a local variable to which the function object is assigned!