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Introduction to Generator Expressions in Python

Generator expressions, often referred to as genexps, are a high-performance, memory-efficient generalization of list comprehensions and generators. If you’re familiar with the Python language, you might have used or seen list comprehensions. Generator expressions are like a list comprehension, but instead of finding all the items you’re interested in and packing them into list, it waits, and yields each item out of the expression, one by one.

In this blog post, we will introduce generator expressions in Python, illustrate their use and showcase their advantages.

What is a Generator Expression?

A generator expression is a high performance, memory efficient tool that shares some syntax with list comprehensions but instead creates a generator object. They have a syntax quite similar to list comprehensions, but with parentheses () instead of brackets [].

Here’s a simple generator expression:

gen = (x**2 for x in range(10))

The above generator expression would generate each square of numbers from 0 to 9, one at a time.

Using a Generator Expression

To use a generator expression, you need to iterate over its elements. You can do this using a for loop.

gen = (x**2 for x in range(10))
for val in gen:

This will print the square of each number from 0 to 9, one at a time.

Difference Between List Comprehensions and Generator Expressions

While they might look similar, there’s a major difference. List comprehensions return a list with all the computed values, whereas generator expressions return a generator object. This means that genexps have a significant advantage when dealing with large sequences of data, as they don’t hold all the computed values in memory at once, they generate each value on-the-fly.

Let’s compare a list comprehension with a generator expression:

# List comprehension
list_comp = [x**2 for x in range(1000000)]

# Generator expression
gen_exp = (x**2 for x in range(1000000))

In the above examples, the list comprehension will output a list of a million squared numbers, and will consume significant memory. On the other hand, the generator expression will only return a generator object. This object doesn’t hold a million values in memory, instead it holds instructions on how to generate each item.

Consuming Generator Expressions

Since generator expressions create generator objects, they can be consumed by Python built-in functions that consume iterables. Here are some examples:

# Sum of squares
gen = (x**2 for x in range(10))
print(sum(gen))  # Outputs: 285

# Maximum square
gen = (x**2 for x in range(10))
print(max(gen))  # Outputs: 81

These examples showcase the power of generator expressions. They allow you to succinctly express computations without incurring the memory costs of storing the entire result set in memory.


Generator expressions are a powerful, high performance, memory-efficient tool in Python that allows you to generate values on-the-fly. Whether you’re dealing with a sequence of a few elements, or a sequence of a million elements, generator expressions can be an efficient way to handle these sequences. Happy coding!

Remember, the true power of generator expressions is revealed when dealing with large streams of data. They are particularly useful when the produced results are consumed right away, as you don’t need to wait for all the results to be generated.

So, next time when you think about creating a list and populating it in a loop, consider using a generator expression instead, you may just find it’s exactly what you need.