subclassing collections.Counter

Tue Dec 15 10:49:57 EST 2015

Hi,
I need to store values for metrics and return the average for some
and the sum for the rest. Thus, I thought I could extend
collections.Counter class by returning averages for some keys.
My class modifies the update() to increment a counter and the
__getitem__ to perform the calculation. But, I get RuntimeError: maximum
recursion depth exceeded as I access an attribute inside
__getitem__.
Does anyone has an idea how I can achieve this?
Example::
In [7]: metrics = {'f':6, 'g':1}
In [8]: avg_metrics = ['f']
In [9]: a = CounterExt(avg_metrics=avg_metrics)
In [10]: a.update(metrics)
In [11]: a
Out[11]: CounterExt({'f': 6, 'g': 1})
In [12]: a.update(metrics)
In [13]: a
Out[13]: CounterExt({'f': 12, 'g': 2})
In [14]: a['f']
.........
RuntimeError: maximum recursion depth exceeded
Code::
from collections import Counter
from collections.abc import Mapping
class CounterExt(Counter):
 def __init__(self, iterable=None, avg_metrics=None, **kwds):
 self._counter = 1 # a zero value will raise ZeroDivisionError
 self.avg_metrics = avg_metrics
 super().__init__()
 self.update(iterable, **kwds)
 def _count_elements(self, iterable):
 """Tally elements from the iterable.
 This is copied from collections/__init__.py
 original code::
 def _count_elements(mapping, iterable):
 'Tally elements from the iterable.'
 mapping_get = mapping.get
 for elem in iterable:
 mapping[elem] = mapping_get(elem, 0) + 1
 """
 for elem in iterable:
 self[elem] = self.get(elem, 0) + 1
 def __getitem__(self, key):
 if (self.avg_metrics is not None and key in self.avg_metrics):
 return self[key] / self._counter
 else:
 return self[key]
 def update(self, iterable=None, **kwds):
 if iterable is not None:
 if isinstance(iterable, Mapping):
 if self:
 self_get = self.get
 self._counter +=1 # local modification
 for elem, count in iterable.items():
 self[elem] = count + self_get(elem, 0)
 else:
 super().update(iterable)
 else:
 self._count_elements(iterable)
 if kwds:
 self.update(kwds)
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