heapq "key" arguments

Raymond Hettinger python at rcn.com
Mon Aug 3 19:36:28 CEST 2009


[Joshua Bronson]:
> According tohttp://docs.python.org/library/heapq.html, Python 2.5
> added an optional "key" argument to heapq.nsmallest and
> heapq.nlargest. I could never understand why they didn't also add a
> "key" argument to the other relevant functions (heapify, heappush,
> etc).

The problem is that heapq acts on regular lists, so it does not
have exclusive access to the structure.  So, there is no reliable
way for it to maintain a separate list of keys.  Since the keys
can't be saved in the structure (without possibly breaking other
code), the fine grained heapq functions (like heappop and heappush)
would need to call key functions every time they are invoked.
This is at odds with the implicit guarantee of the key function
that it will be called no more than once per key.

The overall problem is one of granularity.  A key function should
be applied once in an initial pass, not on every call to a push/pop
function.  The everyday solution that most people use is to operate
on a list of (key, record) tuples and let tuple comparison do the
work for you.  Another solution is to build a Heap class that does
have exclusive access to the structure, but the API sugar often
isn't worth the slightly weaker performance.

One other thought.  Heaps are a lazy evaluation structure, so their
fined-grained mutation functions only work well with just a single
ordering function, so there is not need to have (and every reason
to avoid) changing key functions in mid-stream.  IOW, the key
function needs to be constant across all accesses.  Contrast this
with other uses of key functions where it makes perfect sense
to run minage=min(data, key=attrgetter('age')) and then running
minsal=min(data, key=attrgetter('salary')).  The flexibility to
change key functions just doesn't make sense in the context of
the fine-grained heap functions.

Accordingly, this is why I put key functions in nlargest() and
nsmallest() but not in heappush() and friends.  The former can
guarantee no more than one key function call per entry and they
evaluate immediately instead of lazily.


Raymond







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