问题
I\'ve been trying to make a dict
subclass inheriting from UserDict.DictMixin
that supports non-hashable keys. Performance isn\'t a concern. Unfortunately, Python implements some of the functions in DictMixin
by trying to create a dict object from the subclass. I can implement these myself, but I am stuck on __cmp__
.
I cannot find a succinct description of the logic used by the built-in __cmp__
for the dict class.
回答1:
If you are asking how comparing dictionaries works, it is this:
- To compare dicts A and B, first compare their lengths. If they are unequal, then return cmp(len(A), len(B)).
- Next, find the key adiff in A that is the smallest key for which
adiff not in B or A[adiff] != B[adiff]
. (If there is no such key, the dicts are equal.) - Also find the smallest key bdiff in B for which
bdiff not in A or A[bdiff] != B[bdiff]
. - If adiff != bdiff, then return cmp(adiff, bdiff). Else return cmp(A[adiff], B[bdiff]).
In pseudo-code:
def smallest_diff_key(A, B):
"""return the smallest key adiff in A such that adiff not in B or A[adiff] != B[bdiff]"""
diff_keys = [k for k in A if k not in B or A[k] != B[k]]
return min(diff_keys)
def dict_cmp(A, B):
if len(A) != len(B):
return cmp(len(A), len(B))
try:
adiff = smallest_diff_key(A, B)
except ValueError:
# No difference.
return 0
bdiff = smallest_diff_key(B, A)
if adiff != bdiff:
return cmp(adiff, bdiff)
return cmp(A[adiff], b[bdiff])
This is translated from the 2.6.3 implementation in dictobject.c.
回答2:
An alternative is to use the Mapping ABC from the collections package. It is available in 2.6 and up. You just inherit from collections.Mapping and implement the __getitem__
, __contains__
, and __iter__
methods. You get everything else for free.
回答3:
There is a description of __cmp__
here, but I think the important thing to note is that __cmp__
is only used if the “rich comparison” methods, such as __lt__
and __eq__
are not defined. Moreover, in Python3, __cmp__
is removed from the language. So perhaps eschew __cmp__
altogether and just define __lt__
and __eq__
.
来源:https://stackoverflow.com/questions/3484293/is-there-a-description-of-how-cmp-works-for-dict-objects-in-python-2