Have you tried just instantiating a Counter() instead ? All missing keys are considerd to be 0 in a fresh counter. So for example:

...

...

c = Counter() c["a"] += 1 c Counter({'a': 1}) c["b"] 0

works exactly this way. Which means there's no difference between what you're suggesting Counter.fromkeys(key-list-or-set, 0) should do and what Counter() actually does. (Resend because somehow gmail tried to reply to the wrong python-ideas) 2018-06-27 22:36 GMT+02:00 Abe Dillon <abedillon@gmail.com>:

Consider the following update function for conway's game of life:

from collections import Counter

def update(live: Set[Tuple[Integer, Integer]]): counts = Counter.fromkeys(live, 0) + Counter(itertools.chain(neighbors(*cell) for cell in live)) flip = {cell for cell, count in counts.items() if (cell in live and not 1<count<4) or (cell not in live and count==3)} live ^= flip

around = frozenset(filter(any, itertools.product(range(-1,2), range(-1,2)))) def neighbors(r: Integer, c: Integer): return (((r+dr)%height, (c+dc)%width) for dr, dc in around)

The problem is, Count.fromkeys isn't implemented. I propose that it work exactly as it does for dict, otherwise it's difficult to add items to a Counter when you want them to start off at zero or some other count.

The best solution I came up with is to, more confusingly, count live cells once extra and adjust the rules accordingly:

def update(live: Set[Tuple[Integer, Integer]]): counts = Counter(itertools.chain(live, *(neighbors(*cell) for cell in live))) flip = {cell for cell, count in counts.items() if (cell in live and not 2<count<5) or (cell not in live and count==3)} live ^= flip

around = frozenset(filter(any, itertools.product(range(-1,2), range(-1,2)))) def neighbors(r: Integer, c: Integer): return (((r+dr)%height, (c+dc)%width) for dr, dc in around)

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