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

Devin Jeanpierre jeanpierreda at gmail.com
Tue Jan 3 21:50:51 CET 2012

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).

He only describes <, which can be implemented in linear time as:

def __lt__(self, d2):
    if not isinstance(d2, dict):
        return NotImplemented

    return all(key in d2 and d2[key] == value for key, value in

self.items())

Which set operations are you thinking of?

-- Devin

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). Further, you cannot treat dicts as sets of (key, value) pairs because dicts have unique keys, not unique (key, value) pairs. I don't even know what rich comparison operators on dictionaries do now, it isn't intuitive at all. The rich comparison operators are only defined for == and !=. In Python 2.x, there is a legacy implementation of cmp that does something more complicated. I recommend ignoring it. -- 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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