Request for useful functions on dicts

Ian Kelly ian.g.kelly at
Sun Jul 15 19:38:06 CEST 2012

On Sat, Jul 14, 2012 at 5:56 PM, Leif <leif.poorman at> wrote:
> Hi, everybody. I am trying to collect all the functions I've found useful for working with dicts into a library:
> If you have a favorite dict-related func / class, or know of similar projects, please let me know (or open an issue on github). Bear in mind that the functions don't even have to come from python; if you have a favorite PHP / APL / COBOL / etc associative array function, that's fair game.

Nothing in particular comes to mind, but I'll be happy to critique
what you've already got.

One thing that bothers me a bit is that get_in will accept any
iterable, but set_in and update_in can only accept a sequence in order
to do the slicing.  Here's an alternate implementation for set_in that
takes any iterable:

def set_in(d, ks, v):
    tmp = d
    i = None
    for i, next_key in enumerate(ks):
        if i > 0:
            tmp = tmp.setdefault(current_key, {})
        current_key = next_key
    if i is None:
        raise KeyError("Empty keys iterable")
    tmp[current_key] = v

update_in could be rewritten similarly.  You might also notice that
I'm not returning d here.  That's because the Pythonic way is to
either create a new object and return it, or mutate the existing
object and return None.  If you mutate the existing object and return
it, that can lead to confusion about which style the method takes.
This is why list.append (for example) always returns None.

In Python 2.7+, intersection and difference could be written using
dictviews, which act like sets.

partition_on_value, partition_on_key:  The only difference between
these is whether it passes the key or the value to the predicate.  You
could just pass both and let the predicate decide which one (or both)
to use, and then you only need a single function.  Also, why a
predicate specifically?  This could be generalized to partition any
equivalence relation, not just those with only two equivalence

def partition(f, d):
    """Partition the dict according to an equivalence relation.

    Calls f(key, value) for all (key, value) pairs in the dict d.  The return
    value of f must be hashable.
    Returns a new dict where the keys are distinct return values of f, and the
    values are dicts containing the equivalence classes distinguished by those
    return values.
    partition = defaultdict(dict)
    for k, v in d.iteritems():
        partition[f(k, v)][k] = v
    return partition

If you still wanted the predicate semantics, you could then define
that as a wrapper:

def partition_pred(f, d):
    p = partition(lambda k,v: bool(f(k,v)), d)
    return p[True], p[False]

issubdict could be implemented as a subset operation.  I haven't timed
it so it may not really be any faster, but this way the looping is in
C instead of in Python:

def issubdict(d1, d2):
    return set(d1.items()).issubset(d2.items())

Finally, isempty(d) is basically equivalent to "not d", which is both
shorter and faster.


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