A realistic generator: clean lines from a file
Counters are a toy. Real programs use generators to read big files one line at a time, cleaning each line as it goes. io.StringIO acts like an open file here, so no real file is needed. Notice that nothing is read until next() asks for it.
A generator function uses yield to hand back values one at a time and freezes in place between them. It writes an iterator in a few lines, keeps its own state, and can produce infinite streams without using memory.
Change the code and press Run (Ctrl+Enter). Predict what will happen first, then break it on purpose and read what changes. Your edits are saved and match the lesson page.
Ready for more? This phase has 6 hands-on exercises with automatic checks.
Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Write a generator squares(n) that yields the squares of 1 up to n. Print list(squares(5)).
Write a generator countdown(n) that yields n, n-1, ... down to 1, then the text "liftoff". Print each value on its own line with a for loop.
Write a generator chunks_of_text(text, size) that yields the text in pieces of at most size characters, then print the pieces for "streaming" with size 4.
Explain it without notes
What happens when you call a generator function, and when does its body actually run?
How does a generator keep its state between values, and how is that different from an iterator class?
What is yield from for, and what does it do that a loop with yield does not?
Why can a generator describe an infinite sequence, and how do you use one safely?
What do return, raise StopIteration, close() and throw() each do inside or to a generator?
Expected output
nothing read yet: generator
alpha
beta
the rest: ['gamma']