Stop Saying Python Iterators Are Eager

As a backend developer, I sometimes help companies evaluate candidates by reviewing their recorded technical interviews. However, over time, I’ve noticed a deeply ingrained misconception. When discussing memory management or data streaming, many developers explicitly state: "Iterators in Python are inherently eager. If you want true lazy loading or lazy evaluation, you have to use generators and the yield keyword." This misconception is common. Many popular bootcamps and online courses introduce lazy evaluation exclusively through generators . Custom class-based iterators are usually skipped or dismissed as boilerplate-heavy OOP theory rarely used in production Python. This confusion is further reinforced by two common educational simplifications : The List vs. Generator Expression Analogy: Beginners are taught that square brackets [...] (list comprehensions) are eager and take up memory, while parentheses (...) (generator expressions) are lazy. This often creates a false binary mental model: "generators = lazy, everything else = eager." Standard "Textbook" Examples: When courses demonstrate a custom iterator, they usually write a basic class that accepts an already fully loaded list in its __init__ and simply increments an index in __next__ . While this is valid for in-memory data, it leads developers to assume that custom iterators inherently require loading all data upfront. In reality, generators are a specialized language feature designed to implement the iterator protocol automatically . They comply with the exact same interface ( __iter__ and __next__ ). A generator is lazy not because of some magical property of the yield keyword, but simply because it adheres to this underlying contract. To show that custom iterators can be lazy without using any generators or yield keywords, I’ve put together a lightweight and reproducible benchmark. 🧪 The Experiment: Proving Lazy Loading with Custom Iterators Suppose we need to read a database export file ( test_users_db.