[Python-ideas] [Python-Dev] hello, new dict addition for new eve ? (original) (raw)

Robert Kern robert.kern at gmail.com
Tue Jan 3 19:45:56 CET 2012

On 1/3/12 5:39 PM, Nathan Rice wrote:

On Tue, Jan 3, 2012 at 12:10 PM, Robert Kern<robert.kern at gmail.com> wrote:

On 1/3/12 4:59 PM, Nathan Rice wrote:

This is slightly tangential, but I've always wondered... Why aren't set operations implemented on dicts? It is fairly natural to view a dictionary as a set of (key, value) pairs. Things like subset/superset checking (with concomitant operator support) make sense. I have written stuff like set(dict1.items())< set(dict2.items()) many times. The values are unrestricted Python objects. They do not have to be hashable or sortable. The set operations you describe would have to be require one or both (or else do something algorithmically horrendous). I haven't had any problems with using set(somedict.items()). I will admit that I primarily do this in simple contexts. This brings me to another curiosity... Why do mutable items not implement hash using id()?

Usually because they do not implement eq using id(). The invariant that needs to be maintained is that if two objects compare equal, then they need to hash equal. Many mutable objects compare by value, and thus two non-identical objects can compare equal.

Further, you cannot treat dicts as sets of (key, value) pairs because dicts have unique keys, not unique (key, value) pairs. I'm confused. Because keys are unique, (key, value) pairs are unique as well. I realize the values are not unique but the tuple is guaranteed to be.

Yes, the dict.items() would be a valid, unique set, but set operations on those sets may not give valid dicts because the same key could point to different values in the two dict operands. For example:

[~] |7> a = {'one': 1}

[~] |8> b = {'one': '1'}

[~] |9> set(a.items()) | set(b.items()) set([('one', '1'), ('one', 1)])

[~] |10> dict(set(a.items()) | set(b.items())) {'one': 1}

You've lost a value there.

-- Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco

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