itertools

For a small project I'm working on I need to cycle through a list. For each element of this cycle I have to start another cycle through the same list, with the former element as first element of the new cycle. For example I'd like to be able to produce something like this: 1, 2, 3, 4, 1, 2, 3, 4, 1, ... 2, 3, 4, 1, 2, 3, 4, 1, 2, ... 3, 4, 1, 2, 3, 4, 1, 2, 3, ... 4, 1, 2, 3, 4, 1, 2, 3, 4, ... 1, 2, 3, 4, 1, 2, 3, 4, 1, ... ... I thought that copying a itertools.cycle after each .next() would conserve the current state, so that I can begin the new cycle with the element from the "outer" cycle

I need a Python function iterate(f, x) that creates an iterator returning the values x, f(x), f(f(x)), f(f(f(x))), etc (like, e.g., Clojure's iterate ). First of all, I was wondering: Does this already exist somewhere in the standard library and I'm only missing it? Of course it's easy enough to implement with a generator: def iterate(f, x): while True: yield x x = f(x) Just out of curiosity: Is there a more functional way to do this in Python, e.g. with some itertools or functools magic? In Python 3.3 this would work def iterate(f, x): return accumulate(repeat(x), lambda acc, _ : f(acc)) but

I've been reading about itertools , which seems to be a very powerful module. I am particularly interested in itertools.product() which appears to give me all of the combinations of the iterable inputs. However, I would like to know which of the input iterables each of the outputs are coming from. For example, a simple standard example is: itertools.product([1, 2, 3], [1, 2]) If the user provided the inputs of [1,2,3], [1, 2] I won't know which order they came in, so getting a result of (1, 2) isn't much help, as I don't know which way round they will be. Is there some way of providing input

My function creates a chain of generators: def bar(num): import itertools some_sequence = (x*1.5 for x in range(num)) some_other_sequence = (x*2.6 for x in range(num)) chained = itertools.chain(some_sequence, some_other_sequence) return chained My function sometimes needs to return chained in reversed order. Conceptually, the following is what I would like to be able to do: if num < 0: return reversed(chained) return chained Unfortunately: >>> reversed(chained) TypeError: argument to reversed() must be a sequence What are my options? This is in some realtime graphic rendering code so I don't

Given a list of pairs xys , the Python idiom to unzip it into two lists is: xs, ys = zip(*xys) If xys is an iterator, how can I unzip it into two iterators, without storing everything in memory? Ami Tavory Suppose you have some iterable of pairs: a = zip(range(10), range(10)) If I'm correctly interpreting what you are asking for, you could generate independent iterators for the firsts and seconds using itertools.tee : xs, ys = itertools.tee(a) xs, ys = (x[0] for x in xs), (y[1] for y in ys) Note this will keep in memory the "difference" between how much you iterate one of them vs. the other.

>>> itertools.izip('ABCD', 'xy') Traceback (most recent call last): File "<pyshell#55>", line 1, in <module> itertools.izip('ABCD', 'xy') AttributeError: 'module' object has no attribute 'izip' In Python 3, there is no izip function in the itertools module because the builtin zip function (which doesn't require any imports to access) now behaves like itertools.izip did in Python 2. So, to make your code work, just use zip instead of itertools.izip . You also mentioned an issue with string.maketrans . That's another function that is no longer in a module in Python 3. It's now a method of the

The result created by Python's itertools.combinations() is the combinations of numbers. For example: a = [7, 5, 5, 4] b = list(itertools.combinations(a, 2)) # b = [(7, 5), (7, 5), (7, 4), (5, 5), (5, 4), (5, 4)] But I would like to also obtain the indices of the combinations such as: index = [(0, 1), (0, 2), (0, 3), (1, 2), (1, 3), (2, 3)] How can I do it? You can use enumerate: >>> a = [7, 5, 5, 4] >>> list(itertools.combinations(enumerate(a), 2)) [((0, 7), (1, 5)), ((0, 7), (2, 5)), ((0, 7), (3, 4)), ((1, 5), (2, 5)), ((1, 5), (3, 4)), ((2, 5), (3, 4))] >>> b = list((i,j) for ((i,_),(j,_))