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From: Pavlos Parissis <pavlos.parissis@gmail.com>
Newsgroups: comp.lang.python
Subject: subclassing collections.Counter
Date: Tue, 15 Dec 2015 16:49:57 +0100
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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 =3D {'f':6, 'g':1}

In [8]: avg_metrics =3D ['f']

In [9]: a =3D CounterExt(avg_metrics=3Davg_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']
=2E........
RuntimeError: maximum recursion depth exceeded


Code::
from collections import Counter
from collections.abc import Mapping
class CounterExt(Counter):
     def __init__(self, iterable=3DNone, avg_metrics=3DNone, **kwds):
        self._counter =3D 1  # a zero value will raise ZeroDivisionError
        self.avg_metrics =3D 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 =3D mapping.get
            for elem in iterable:
                mapping[elem] =3D mapping_get(elem, 0) + 1
        """
        for elem in iterable:
            self[elem] =3D 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=3DNone, **kwds):
        if iterable is not None:
            if isinstance(iterable, Mapping):
                if self:
                    self_get =3D self.get
                    self._counter +=3D1  # local modification
                    for elem, count in iterable.items():
                        self[elem] =3D count + self_get(elem, 0)
                else:
                    super().update(iterable)
            else:
                self._count_elements(iterable)
        if kwds:
            self.update(kwds)




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