[issue27265] Hash of different, specific Decimals created from str is the same

[Mark Dickinson]

Mark Dickinson added the comment:

For what it's worth, here are timings on my machine showing the overhead of the extra equality check when a hash collision occurs.

Python 2.7.11 (default, Mar  1 2016, 18:08:21) 
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In [1]: from decimal import Decimal

In [2]: set1 = set([Decimal(str(n/1000.0)) for n in range(1, 10)] + [Decimal(str(n/100.0)) for n in range(1, 10)])

In [3]: set2 = set([Decimal(str(n/1000.0)) for n in range(2, 20)])

In [4]: print len(set1), len(set2)  # Both sets have the same length
18 18

In [5]: print len(set(map(hash, set1))), len(set(map(hash, set2)))  # But set1 has hash collisions
9 18

In [6]: %timeit Decimal('0.005') in set1  # checking elt in the set, first match is the right one
The slowest run took 5.98 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 17.4 µs per loop

In [7]: %timeit Decimal('0.05') in set1  # checking elt in the set, collision resolution needed
The slowest run took 5.72 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 19.6 µs per loop

In [8]: %timeit Decimal('0.005') in set2  # should be similar to the first set1 result
The slowest run took 5.99 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 17.3 µs per loop

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status: pending -> open

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27265>
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