Python One Line Function Definition

A lambda function allows you to define a function in a single line. It starts with the keyword lambda, followed by a comma-separated list of zero or more arguments, followed by the colon and the return expression. For example, lambda x, y: x+y calculates the sum of the two argument values x+y in one line of Python code.

Problem: How to define a function in a single line of Python code?

Example: Say, you’ve got the following function in three lines. How to compress them into a single line of Python code?

def say_hi(*friends):
    for friend in friends:
        print('hi', friend)

friends = ['Alice', 'Bob', 'Ann']
say_hi(*friends)

The code defines a function say_hi that takes an iterable as input—the names of your friends—and prints 'hi x' for each element x in your iterable.

The output is:

'''
hi Alice
hi Bob
hi Ann
'''

Let’s dive into the different methods to accomplish this! First, here’s a quick interactive overview to test the waters:

Exercise: Run the code—is the output the same for all four methods?

Next, you’ll learn about each method in greater detail!

Method 1: Lambda Function

You can use a simple lambda function to accomplish this.

A lambda function is an anonymous function in Python. It starts with the keyword lambda, followed by a comma-separated list of zero or more arguments, followed by the colon and the return expression. For example, lambda x, y, z: x+y+z would calculate the sum of the three argument values x+y+z.

friends = ['Alice', 'Bob', 'Ann']

# Method 1: Lambda Function
hi = lambda lst: [print('hi', x) for x in lst]

In the example, you want to print a string for each element in an iterable—but the lambda function only returns an object. Thus, we return a dummy object: a list of None objects. The only purpose of creating this list is to execute the print() function repeatedly, for each element in the friends list.

You obtain the following output:

hi(friends)
'''
hi Alice
hi Bob
hi Ann
'''

Method 2: Function Definition

A similar idea is employed in this one-liner example—but instead of using a lambda function, we define a regular function and simply skip the newline. This is possible if the function body has only one expression:

friends = ['Alice', 'Bob', 'Ann']

# Method 2: Function Definition
def hi(lst): [print('hi', x) for x in lst]

The output is the same as before:

hi(friends)
'''
hi Alice
hi Bob
hi Ann
'''

This approach is more Pythonic than the first one because there’s no throw-away return value and it’s more concise.

Method 3: exec()

The third method uses the exec() function. This is the brute-force approach to one-linerize any multi-liner!

To make a Python one-liner out of any multi-line Python script, replace the new lines with a new line character '\n' and pass the result into the exec(...) function. You can run this script from the outside (command line, shell, terminal) by using the command python -c "exec(...)".

We can apply this technique to the first example code snippet (the multi-line function definition) and rename the variables to make it more concise:

friends = ['Alice', 'Bob', 'Ann']

# Method 3: exec()
exec("def hi(*lst):\n    for x in lst:\n        print('hi', x)\nhi(*friends)")

If you run the code, you’ll see the same output as before:

hi(friends)
'''
hi Alice
hi Bob
hi Ann
'''

This is very hard to read—our brain cannot grasp the whitespaces and newline characters easily. But I still wanted to include this method here because it shows how you or anyone else can compress complicated algorithms in a single line of Python code!

Watch the video if you want to learn more details about this technique:

[Don't Do This At Home] How To One-Linerize Every Multi-Line Python Script & Run It From The Shell

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