Python’s creator, Guido van Rossum, mentions this when he wrote about generator expressions that were uniformly built into Python 3: (emphasis mine) We also made another change in Python 3, to improve equivalence between list comprehensions and generator expressions. In Python 2, the list comprehension “leaks” the loop control variable into the surrounding scope: x … Read more
I used LIME Parser generator for PHP a couple of years ago, and it was already mature and stable. The parser generator itself is written in PHP, which doesn’t really matter in any technical sense – as we require only that the generated parser be in PHP – but I like this detail nonetheless. It … Read more
itertools.chain() should do it. It takes multiple iterables and yields from each one by one, roughly equivalent to: def chain(*iterables): for it in iterables: for element in it: yield element Usage example: from itertools import chain g = (c for c in ‘ABC’) # Dummy generator, just for example c = chain(g, ‘DEF’) # Chain … Read more
You can use itertools.tee(): walk, walk2 = itertools.tee(walk) Note that this might “need significant extra storage”, as the documentation points out.
What is yield? The yield keyword returns data from a generator function: The heart of a generator function is the yield keyword. In its simplest form, a yield statement looks much like a return statement, except that instead of stopping execution of the function and returning, yield instead provides a value to the code looping … Read more
To understand what yield does, you must understand what generators are. And before you can understand generators, you must understand iterables. Iterables When you create a list, you can read its items one by one. Reading its items one by one is called iteration: >>> mylist = [1, 2, 3] >>> for i in mylist: … Read more
This is a new feature in Python 3.3 (as a comment notes, it doesn’t even work in 3.2). Much like return in a generator has long been equivalent to raise StopIteration(), return <something> in a generator is now equivalent to raise StopIteration(<something>). For that reason, the exception you’re seeing should be printed as StopIteration: 3, … Read more
It’s used to send values into a generator that just yielded. Here is an artificial (non-useful) explanatory example: >>> def double_inputs(): … while True: … x = yield … yield x * 2 … >>> gen = double_inputs() >>> next(gen) # run up to the first yield >>> gen.send(10) # goes into ‘x’ variable 20 … Read more
By using yield, generators can be suspended at any point in the control flow of your function, saving your current state of execution (scope & stack). Without generators, this is more complicated: you need to explicitly keep track of the state branching and (especially) looping control structures need to be represented in a functional way, … Read more
In general, the answer is itertools.islice, but you should note that islice doesn’t, and can’t, actually skip values. It just grabs and throws away start values before it starts yield-ing values. So it’s usually best to avoid islice if possible when you need to skip a lot of values and/or the values being skipped are … Read more