New submission from Radosław Szalski:
Python 2.7.11 (default, May 9 2016, 19:53:39)
[GCC 4.2.1 Compatible Apple LLVM 7.3.0 (clang-703.0.31)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from decimal import Decimal
>>> hash(Decimal('0.05')) == hash(Decimal('0.005'))
True
>>> hash(Decimal(0.05)) == hash(Decimal(0.005))
False
Interactive example: https://repl.it/CZUJ/6
Those values are numerically different and IMO should not have equal hashes. I
am aware of the quirk with hash(-1) == hash(-2) which is at play here.
This only applies to Decimals created from strings as they are hashed
differently than float-based ones.
What happens? The following equation is True:
>>> hash(Decimal('0.005')) == hash(Decimal('0.05'))
What I expected to happen? The following equation to be False:
>>> hash(Decimal('0.005')) == hash(Decimal('0.05'))
Platform: MacBook Pro Retina, 13", early 2015, OSX 10.11.5
Tested on Python Versions: 2.7.3, 2.7.10, 2.7.11 (installed via pyenv), all
exhibit the same behavior
Relevant (not duplicate) issues I've found:
http://bugs.python.org/issue10356 - decimal.py: hash of -1
http://bugs.python.org/issue8188 - Unified hash for numeric types.
---
Transcript of the interactive example: https://repl.it/CZUJ/6:
from decimal import Decimal
# These two different decimals have equal hashes
print(hash(Decimal('0.005')) == hash(Decimal('0.05')))
# Internally, Decimal's hash is computed like this (sign, exp + len(int), int)
print(hash((0, -2+len('5'), '5')) == hash((0, -3+len('5'), '5')))
# Removing constants, this becomes
print(hash(-2+len('5')) == hash(-3+len('5')))
# Which can be further simplified to:
print(hash(-1) == hash(-2))
# They both return -2, because at the CPython level, -1 is reserved for errors
# Whereas:
print(hash(Decimal(0.005)) == hash(Decimal(0.05)))
# And this is because Decimals created from floats are hashed as floats
----------
components: Library (Lib)
messages: 267802
nosy: Radosław Szalski, mark.dickinson
priority: normal
severity: normal
status: open
title: Hash of different, specific Decimals created from str is the same
type: behavior
versions: Python 2.7
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue27265>
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