What happens when a generator function is called in Python?

When a generator function is called in Python, it returns an iterator without executing the function body immediately. This behavior is a fundamental characteristic of generator functions, which are defined using the def keyword and include one or more yield statements.

Unlike regular functions that execute the entirety of their code and return a single value, generator functions utilize yield to produce a sequence of values over time, pausing their state between each yield. When the generator function is called, it does not perform any computations but instead provides a generator object that can be iterated over. Each time the next() function is called on the generator object or a loop iterates over it, execution resumes from where it last yielded and continues until it hits another yield or the function returns.

This design allows for efficient memory usage, as large sequences can be generated on-the-fly without needing to store them in memory all at once. The ability to return an iterator without executing the function allows for deferred execution, which is a powerful feature in scenarios where the full dataset isn't needed upfront or would be prohibitively large.