dict diff

Steven D'Aprano steve at REMOVE-THIS-cybersource.com.au
Sat Nov 20 13:00:16 CET 2010

On Sat, 20 Nov 2010 01:11:53 -0500, Steve Holden wrote:

> On 11/19/2010 8:58 PM, Jin Yi wrote:
>> so i came up with a diff method to compare 2 dicts. 
> A PEP *and* some explanation of why you would want such an obscure piece
> of code built in to the dict object, yes.

You've never wanted to see how two dicts differ, as well as the fact that 
they do? I frequently find myself wanting to see why two dicts that 
should be equal aren't, especially for testing and debugging.

I use this function:

def dict_diffs(a, b):
    """dict_diffs(adict, bdict) -> (amissing, bmissing, different)

    Returns sets (amissing, bmissing, different) such that:

    amissing = keys missing from adict compared to bdict
    bmissing = keys missing from bdict compared to adict
    different = keys in both adict and bdict but with different values

    >>> dict_diffs({1:0, 2:0, 3:0}, {1:1, 2:0, 4:0})
    (set([4]), set([3]), set([1]))

    from collections import Mapping
    if not isinstance(a, Mapping) and isinstance(b, Mapping):
        raise TypeError('arguments must both be mappings')
    amissing, bmissing, different = set(), set(), set()
    for key, value in a.items():
        if key not in b:
        elif value != b[key]:
    for key, value in b.items():
        if key not in a:
    return (amissing, bmissing, different)


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