Generators in Python: A Comprehensive Guide

Generators in Python: A Comprehensive Guide


6 min read

If you're a Python developer, you've probably heard about generators. They're powerful, efficient, and easy to use. But what are generators? How do they work? And how can you use them to improve your Python code? In this article, we'll explore everything you need to know about generators in Python.

Introduction to Generators

Generators are a type of iterable, like lists or tuples. However, unlike lists and tuples, generators don't store their values in memory. Instead, they generate their values on the fly. This means that generators can generate an infinite sequence of values or a sequence of values too large to fit in memory.

Generators are defined using a particular type of function called a generator function. A generator function is defined like a regular function, but instead of using the return keyword to return a value, it uses the yield keyword. The yield keyword tells Python to generate a value and then pause the function's execution until the next value is requested.

How Generators Work

When a generator function is called, it doesn't actually execute the function's code. Instead, it returns a generator object, which is an iterator that can be used to iterate over the values generated by the generator function. When the iterator's next() method is called, the generator function's code is executed until the next yield statement is encountered. The value of the yield statement is returned to the iterator, and the function's execution is paused until the next next() method call.

Creating Generators in Python

To create a generator in Python, you define a generator function. Here's an example:

def my_generator():
    yield 1
    yield 2
    yield 3

This generator function generates the values 1, 2, and 3. To use this generator, you create a generator object by calling the generator function:

>>> g = my_generator()
>>> next(g)
>>> next(g)
>>> next(g)
>>> next(g)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

Using Generators in Python

Generators can be used anywhere an iterable is used. For example, you can use a generator in a for loop:

>>> for value in my_generator():
...     print(value)

Or you can use a generator to generate a list of values:

>>> my_list = list(my_generator())
>>> my_list
[1, 2, 3]

Advantages of Generators

Generators have several advantages over other iterables:

  • Generators are more memory-efficient than lists or tuples since they don't store their values in memory.

  • Generators can generate an infinite sequence of values.

  • Generators can be used to generate a sequence of values that are too large to fit in memory.

Disadvantages of Generators

Generators also have some disadvantages:

  • Generators can't be indexed or sliced like lists or tuples.

  • Generators can only be iterated over once.

Differences between Generators and Lists

Generators and lists are both iterable, but there are some important differences between them:

  • Lists store all their values in memory, while generators generate their values on-the-fly.

  • Lists can be indexed and sliced, while generators can't.

  • Lists can be iterated over multiple times, while generators can only be iterated over once.

  • Lists are usually faster for small datasets, while generators are faster for large datasets.

Examples of Generator Functions

Here are some examples of generator functions:

def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

def squares(n):
    for i in range(n):
        yield i ** 2

def evens(n):
    for i in range(n):
        if i % 2 == 0:
            yield i

These generator functions generate the Fibonacci sequence, the squares of the first n numbers, and the first n even numbers, respectively.

Generator Expressions

Generator expressions are a concise way to create generators. Here's an example:

my_generator = (i ** 2 for i in range(10))

This generator expression generates the squares of the first 10 numbers.

Performance Comparison: Generators vs. Lists

To compare the performance of generators and lists, let's generate the first 1 million Fibonacci numbers using both a list and a generator:

import time

def fibonacci_list(n):
    result = []
    a, b = 0, 1
    for i in range(n):
        a, b = b, a + b
    return result

def fibonacci_generator(n):
    a, b = 0, 1
    for i in range(n):
        yield a
        a, b = b, a + b

start_time = time.time()
print("List time:", time.time() - start_time)

start_time = time.time()
print("Generator time:", time.time() - start_time)

On my machine, the list version takes about 0.6 seconds, while the generator version takes about 0.4 seconds. This shows that generators can be faster than lists for large datasets.

Using Generators for Large Datasets

Generators are beneficial for working with large datasets. For example, if you need to read a large file line-by-line, you can use a generator:

def read_lines(filename):
    with open(filename) as f:
        for line in f:
            yield line.strip()

This generator function reads a file line-by-line, stripping the newline character from each line.

The Yield from Statement

In Python 3.3 and later, you can use the "yield from" statement to delegate to another generator. Here's an example:

def numbers():
    yield 1
    yield 2
    yield 3

def letters():
    yield 'a'
    yield 'b'
    yield 'c'

def combined():
    yield from numbers()
    yield from letters()

This combined generator function generates the numbers 1, 2, 3, followed by the letters 'a', 'b', 'c'.

Chaining Generators

You can chain generators together using the "yield from" statement. Here's an example:

def squares():
    for i in range(10):
        yield i ** 2

def evens():
    for i in range(10):
        if i % 2 == 0:
            yield i

def squares_of_evens():
    yield from squares()
    yield from evens()

This squares_of_evens generator function generates the squares of the first 10 numbers, followed by the first five even numbers.


  1. What is a generator in Python? A generator in Python is a type of iterable, like lists or tuples, but it generates its values on-the-fly instead of storing them in memory.

  2. What are the advantages of using generators in Python? Generators are a more memory-efficient way to work with large datasets, and they can generate data dynamically, allowing for more flexibility in your code.

  3. How do you create a generator in Python? You can create a generator in Python by defining a function that uses the yield keyword to generate values one at a time.

  4. How do you chain generators together in Python? You can chain generators together in Python using the "yield from" statement, which delegates to another generator.

  5. What is the performance difference between generators and lists in Python? Generators can be faster than lists for large datasets because they generate values on-the-fly instead of storing them in memory. However, lists can be faster for small datasets because they can be indexed and sliced.


Generators are a powerful and efficient way to work with iterable data in Python. They allow you to generate data on-the-fly, without storing it all in memory at once. This can be especially useful when working with large datasets or when generating data dynamically.

Using generator functions and expressions, you can create your own generators, and you can chain them together using the "yield from" statement. With these tools at your disposal, you can work with iterable data in a way that is both efficient and flexible.

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