The nltk.probability.ConditionalFreqDist class is a container for FreqDist instances, with one FreqDist per condition. It is used to count frequencies that are dependent on another condition, such as another word or a class label. It is being used here to create an API-compatible class on top of Redis using the RedisHashFreqDist .
In the code given below, a RedisConditionalHashFreqDist class that extends nltk.probability.ConditionalFreqDist and overrides the __getitem__() method. Override __getitem__() so as to create an instance of RedisHashFreqDist instead of a FreqDist.
Code :
from nltk.probability import ConditionalFreqDist from rediscollections import encode_key   class RedisConditionalHashFreqDist(ConditionalFreqDist):     def __init__(self, r, name, cond_samples = None):         self._r = r         self._name = name         ConditionalFreqDist.__init__(self, cond_samples)                   for key in self._r.keys(encode_key('% s:*' % name)):             condition = key.split(':')[1]             # calls self.__getitem__(condition)             self[condition]                   def __getitem__(self, condition):         if condition not in self._fdists:             key = '% s:% s' % (self._name, condition)             val = RedisHashFreqDist(self._r, key)             super(RedisConditionalHashFreqDist, self).__setitem__(                     condition, val)         return super(                 RedisConditionalHashFreqDist, self).__getitem__(condition)     def clear(self):         for fdist in self.values():             fdist.clear() |
An instance of this class can be created by passing in a Redis connection and a base name. After that, it works just like a ConditionalFreqDist as shown in the code below :
Code :
from redis import Redis from redisprob import RedisConditionalHashFreqDist   r = Redis() rchfd = RedisConditionalHashFreqDist(r, 'condhash')   print (rchfd.N())   print (rchfd.conditions())   rchfd['cond1']['foo'] += 1  print (rchfd.N())   print (rchfd['cond1']['foo'])   print (rchfd.conditions())   rchfd.clear() |
Output :
0 [] 1 1 ['cond1']
How it works ?
- The RedisConditionalHashFreqDist uses name prefixes to reference RedisHashFreqDist instances.
- The name passed into the RedisConditionalHashFreqDist is a base name that is combined with each condition to create a unique name for each RedisHashFreqDist.
- For example, if the base name of the RedisConditionalHashFreqDist is ‘condhash’, and the condition is ‘cond1’, then the final name for the RedisHashFreqDist is ‘condhash:cond1’.
- This naming pattern is used at initialization to find all the existing hash maps using the keys command.
- By searching for all keys matching ‘condhash:*’, user can identify all the existing conditions and create an instance of RedisHashFreqDist for each.
- Combining strings with colons is a common naming convention for Redis keys as a way to define namespaces.
- Each RedisConditionalHashFreqDist instance defines a single namespace of hash maps.
RedisConditionalHashFreqDist also defines a clear() method. This is a helper method that calls clear() on all the internal RedisHashFreqDist instances. The clear() method is not defined in ConditionalFreqDist.
