What is a Python generator?

A **Python generator** is a special type of iterable that allows you to iterate over a sequence of values lazily, meaning it generates each value on-the-fly as you loop through it, rather than computing and storing the entire sequence in memory upfront. Generators are defined using functions and the `yield` keyword instead of `return`. When a generator function is called, it returns a generator object without actually running the function’s code immediately. The function’s code runs each time you request the next value, pausing at each `yield` and resuming from there on subsequent calls. Here’s a simple example of a generator function: ```python def count_up_to(n): count = 1 while count <= n: yield count count += 1 ``` When you iterate over `count_up_to(5)`, it yields numbers from 1 to 5 one at a time, without creating a list of all those numbers in memory. ### How generators work - When you call a generator function, it returns a generator object. - Each call to `next()` on the generator runs the function until it hits `yield`, which produces a value and pauses the function’s state. - The function’s local variables and execution state are saved between yields. - When the function runs out of values to yield or finishes, it raises `StopIteration`, signaling the end of iteration. ### When to use generators - **Memory efficiency:** Use generators when working with large datasets or streams of data where holding the entire dataset in memory would be impractical or impossible. - **Lazy evaluation:** When you want to compute values only as needed, which can improve performance and responsiveness, especially in pipelines or when processing data from files or network streams. - **Infinite sequences:** Generators can represent infinite sequences because they generate each value on demand instead of precomputing the whole sequence. In summary, generators provide a powerful and memory-efficient way to work with sequences and streams of data, making your programs more scalable and often easier to write.