Contradictory Counter() calculations.

Table of Contents

1  The issue

1.1  Summary

2  END.

The issue

I just got caught by this issue with doing maths with collections.Counter classes.

In [5]:

from collections import Counter

a = Counter(x=1, y=2)
b = Counter(x=1, y=2, z = 0)

In [6]:

a == b

Out[6]:

False

Now, it is quite obvious that the above is False as they are given as two different values, but In the case of Counters the count of b['z'] is zero and when counting in two different ways, or doing addition or subtraction of counts then you can end up with zero counts on items and because their is a difference in zero counted items then two Counter instances with otherwise the same counts are not considered ==.

I found that the best way around this is to subtract and compare to the empty Counter, as it seems subtraction removes zero counted items.

In [7]:

a - b == Counter()

Out[7]:

True

The docs say the unary plus operator removes zero and negative counted items whichmay be what you want, but is subtly different:

In [8]:

+a == +b

Out[8]:

True

But

In [12]:

c = Counter(x=1, y=2, z = -1)

In [11]:

+a == +b == +c

Out[11]:

True

In [13]:

a - c

Out[13]:

Counter({'z': 1})

In [14]:

a - b

Out[14]:

Counter()

Summary

I guess you need to be aware of this when using the excellant collections.Counter.

END.