Python One Line Generator

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A generator function is a Pythonic way to create an iterable without explicitly storing it in memory. This reduces memory usage of your code without incurring any additional costs.

The following code shows a function get_numbers(n) that returns a list of n random numbers.

import random


# NOT A GENERATOR!
# Create and return a list of numbers
def get_numbers(n):
    numbers = []
    for i in range(n):
        numbers.append(random.random())

    # List of n elements exists in memory
    return numbers


# Sum up 1000 random numbers
s = 0
for x in get_numbers(1000):
    s += x
print(s)

However, this is not very efficient code because you create a list in advance without need. What if you had 1,000,000,000 numbers? Your memory would quickly fill up!

A better way is to use a generator function with the yield keyword that creates the random numbers dynamically as they are iterated over:

import random


# GENERATOR
# Generate numbers one by one
def generate_numbers(n):
    for i in range(n):
        yield random.random()


# Sum up 1000 random numbers
s = 0
for x in generate_numbers(1000):
    s += x
print(s)

There are two big advantages to using a generator:

  • (1) You don’t have to create a huge list first and store it in memory but generate the next element as you iterate over it.
  • (2) It’s shorter and more concise.

However, it may not be concise enough for you! 😉 So, here’s the problem addressed in this article:

Problem: Can we write a one-line generator?

Let’s dive into different methods to accomplish this!

Method 1: One-Liner Generator Function

print(sum(random.random() for i in range(1000)))

The code consists of the following parts:

  • The print() function prints the result of the expression to the shell.
  • The sum() function sums over all values in the following iterable.
  • The generator expression random.random() for i in range(1000) generates 1000 random numbers and feeds them into the outer sum() function without creating all of them at once.

This way, we still don’t store the whole list of 1000 numbers in memory but create them dynamically.

Method 2: Generator Expression in Parentheses

By passing a statement of the form (expression context) into the parentheses (), such as (x**2 for x in range(10)), you create a generator object that modifies each element of an iterator specified in the context. In our example, the generator would create the first 10 square numbers.

But it doesn’t actually instantiate these numbers right away to remain memory-efficient as you can see here:

numbers = (x**2 for x in range(10))
print(numbers)
# <generator object <genexpr> at 0x0000021FB07F3EB0>

The numbers are not yet created, only the “blueprint” of how the numbers will be created if you force Python to do that.

Here, you iterate over the generator and print all ten numbers:

for i in numbers:
    print(i)

Output:

0
1
4
9
16
25
36
49
64
81

👉 Recommended Tutorial: Generator Expressions in Python

Method 3: exec()

The following method is not pretty—but it solves the problem of creating a generator in a single line of code.

exec('def g(n):\n    for i in range(n):\n        yield random.random()')

The exec() function can be used to one-linerize every Python code snippet under the sun. How?

💡 Just pass the code you want to run as a string and replace all newlines with the newline character '\n'. This way, you can create a generator function g(n) that dynamically creates n random numbers.

You can now iterate them using the standard code snippet:

s = 0
for x in g(1000):
    s += x
print(s)
# 488.318368852096

Because the numbers are random, the output will be different for you. You can try it yourself in our interactive shell:

Exercise: What’s the output for you? Why is it different than ours?

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