python 带有排名功能的python字典类,来自python cookbook 2 Ch5.14
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#!/usr/bin/env python
''' An enriched dictionary that holds a mapping from keys to scores '''
from bisect import bisect_left, insort_left
import UserDict
class Ratings(UserDict.DictMixin, dict):
""" class Ratings is mostly like a dictionary, with extra features: the
value corresponding to each key is the 'score' for that key, and all
keys are ranked in terms their scores. Values must be comparable; keys,
as well as being hashable, must be comparable if any two keys may ever
have the same corresponding value (i.e., may be "tied" on score).
All mapping-like behavior is just as you would expect, such as:
>>> r = Ratings({"bob": 30, "john": 30})
>>> len(r)
2
>>> r.has_key("paul"), "paul" in r
(False, False)
>>> r["john"] = 20
>>> r.update({"paul": 20, "tom": 10})
>>> len(r)
4
>>> r.has_key("paul"), "paul" in r
(True, True)
>>> [r[key] for key in ["bob", "paul", "john", "tom"]]
[30, 20, 20, 10]
>>> r.get("nobody"), r.get("nobody", 0)
(None, 0)
In addition to the mapping interface, we offer rating-specific
methods. r.rating(key) returns the ranking of a key in the
ratings, with a ranking of 0 meaning the lowest score (when two
keys have equal scores, the keys themselves are compared, to
"break the tie", and the lesser key gets a lower ranking):
>>> [r.rating(key) for key in ["bob", "paul", "john", "tom"]]
[3, 2, 1, 0]
getValueByRating(ranking) and getKeyByRating(ranking) return the
score and key, respectively, for a given ranking index:
>>> [r.getValueByRating(rating) for rating in range(4)]
[10, 20, 20, 30]
>>> [r.getKeyByRating(rating) for rating in range(4)]
['tom', 'john', 'paul', 'bob']
An important feature is that the keys() method returns keys in
ascending order of ranking, and all other related methods return
lists or iterators fully consistent with this ordering:
>>> r.keys()
['tom', 'john', 'paul', 'bob']
>>> [key for key in r]
['tom', 'john', 'paul', 'bob']
>>> [key for key in r.iterkeys()]
['tom', 'john', 'paul', 'bob']
>>> r.values()
[10, 20, 20, 30]
>>> [value for value in r.itervalues()]
[10, 20, 20, 30]
>>> r.items()
[('tom', 10), ('john', 20), ('paul', 20), ('bob', 30)]
>>> [item for item in r.iteritems()]
[('tom', 10), ('john', 20), ('paul', 20), ('bob', 30)]
An instance can be modified (adding, changing and deleting
key-score correspondences), and every method of that instance
reflects the instance's current state at all times:
>>> r["tom"] = 100
>>> r.items()
[('john', 20), ('paul', 20), ('bob', 30), ('tom', 100)]
>>> del r["paul"]
>>> r.items()
[('john', 20), ('bob', 30), ('tom', 100)]
>>> r["paul"] = 25
>>> r.items()
[('john', 20), ('paul', 25), ('bob', 30), ('tom', 100)]
>>> r.clear()
>>> r.items()
[]
"""
''' the implementation carefully mixes inheritance and delegation
to achieve reasonable performance while minimizing boilerplate,
and, of course, to ensure semantic correctness as above. All
mappings' methods not implemented below get inherited, mostly
from DictMixin, but, crucially!, __getitem__ from dict. '''
def __init__(self, *args, **kwds):
''' This class gets instantiated just like 'dict' '''
dict.__init__(self, *args, **kwds)
# self._rating is the crucial auxiliary data structure: a list
# of all (value, key) pairs, kept in naturally-sorted order
self._rating = [ (v, k) for k, v in dict.iteritems(self) ]
self._rating.sort()
def copy(self):
''' Provide an identical but independent copy '''
return Ratings(self)
def __setitem__(self, k, v):
''' besides delegating to dict, we maintain self._rating '''
if k in self:
del self._rating[self.rating(k)]
dict.__setitem__(self, k, v)
insort_left(self._rating, (v, k))
def __delitem__(self, k):
''' besides delegating to dict, we maintain self._rating '''
del self._rating[self.rating(k)]
dict.__delitem__(self, k)
''' delegate some methods to dict explicitly to avoid getting
DictMixin's slower (though correct) implementations instead '''
__len__ = dict.__len__
__contains__ = dict.__contains__
has_key = __contains__
''' the key semantic connection between self._rating and the order
of self.keys() -- DictMixin gives us all other methods 'for
free', although we could implement them directly for slightly
better performance. '''
def __iter__(self):
for v, k in self._rating:
yield k
iterkeys = __iter__
def keys(self):
return list(self)
''' the three ratings-related methods '''
def rating(self, key):
item = self[key], key
i = bisect_left(self._rating, item)
if item == self._rating[i]:
return i
raise LookupError, "item not found in rating"
def getValueByRating(self, rating):
return self._rating[rating][0]
def getKeyByRating(self, rating):
return self._rating[rating][1]
def _test():
''' we use doctest to test this module, which must be named
rating.py, by validating all the examples in docstrings. '''
import doctest, rating
doctest.testmod(rating)
if __name__ == "__main__":
_test()
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