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POJoining multiple iteratorars by a key
 

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  1. POJoining multiple iteratorars by a key
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    <p>Given: n iterators, and a function to get a key for an item for each of them</p> <p>Assuming: </p> <ul> <li>The iterators provide the items sorted by the key</li> <li>The keys from any iterator are unique</li> </ul> <p>I want to iterate through them joined by the keys. Eg, given the following 2 lists:</p> <pre><code>[('a', {type:'x', mtime:Datetime()}), ('b', {type='y', mtime:Datetime()})] [('b', Datetime()), ('c', Datetime())] </code></pre> <p>Using the first item in each tuple as the key, I want to get:</p> <pre><code>(('a', {type:'x', mtime:Datetime()}), None) (('b', {type:'y', mtime:Datetime()}), ('b', Datetime()),) (None, ('c', Datetime()),) </code></pre> <p>So I hacked up this method:</p> <pre><code>def iter_join(*iterables_and_key_funcs): iterables_len = len(iterables_and_key_funcs) keys_funcs = tuple(key_func for iterable, key_func in iterables_and_key_funcs) iters = tuple(iterable.__iter__() for iterable, key_func in iterables_and_key_funcs) current_values = [None] * iterables_len current_keys= [None] * iterables_len iters_stoped = [False] * iterables_len def get_nexts(iters_needing_fetch): for i, fetch in enumerate(iters_needing_fetch): if fetch and not iters_stoped[i]: try: current_values[i] = iters[i].next() current_keys[i] = keys_funcs[i](current_values[i]) except StopIteration: iters_stoped[i] = True current_values[i] = None current_keys[i] = None get_nexts([True] * iterables_len) while not all(iters_stoped): min_key = min(key for key, iter_stoped in zip(current_keys, iters_stoped) if not iter_stoped) keys_equal_to_min = tuple(key == min_key for key in current_keys) yield tuple(value if key_eq_min else None for key_eq_min, value in zip(keys_equal_to_min, current_values)) get_nexts(keys_equal_to_min) </code></pre> <p>and test it:</p> <pre><code>key_is_value = lambda v: v a = ( 2, 3, 4, ) b = (1, ) c = ( 5,) d = (1, 3, 5,) l = list(iter_join( (a, key_is_value), (b, key_is_value), (c, key_is_value), (d, key_is_value), )) import pprint; pprint.pprint(l) </code></pre> <p>which outputs:</p> <pre><code>[(None, 1, None, 1), (2, None, None, None), (3, None, None, 3), (4, None, None, None), (None, None, 5, 5)] </code></pre> <p>Is there an existing method to do this? I checkout itertools, but could not find anything.</p> <p>Are there any ways to improve my method? Make it simpler, faster, etc..</p> <h2>Update: Solution used</h2> <p>I decided to simplify the contract for this function by requiring the iterators to yield tuple(key, value) or tuple(key, *values). Using agf's answer as a starting point, I came up with this :</p> <pre><code>def join_items(*iterables): iters = tuple(iter(iterable) for iterable in iterables) current_items = [next(itr, None) for itr in iters] while True: try: key = min(item[0] for item in current_items if item != None) except ValueError: break yield tuple(item if item != None and item[0]==key else None for item in current_items) for i, (item, itr) in enumerate(zip(current_items, iters)): if item != None and item[0] == key: current_items[i] = next(itr, None) a = ( (2,), (3,), (4,), ) b = ((1,), ) c = ( (5,),) d = ((1,), (3,), (5,),) e = ( ) import pprint; pprint.pprint(list(join_items(a, b, c, d, e))) [(None, (1,), None, (1,), None), ((2,), None, None, None, None), ((3,), None, None, (3,), None), ((4,), None, None, None, None), (None, None, (5,), (5,), None)] </code></pre>
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    1. USGary van der Merwe
    1. USGary van der Merwe
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    1. COare `{...}` in the first 2 code blocks supposed to denote functions? I don't seen the relationship between the first code block, and the `None`s that show up in the 2nd one.
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      1. USagscala
    2. COThe {...} are to denote dictionaries containing arbitrary data. I'll update the question to show this.
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      1. USGary van der Merwe
    3. COYou got the syntax for dictionaries wrong in your example. It should read `[('a', {'type':'x', 'mtime':Datetime()})...`.
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      1. USDaniel Hepper
 

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