[TIP] testing and hash values

[Chris Jerdonek]

On Sun, Sep 29, 2013 at 12:20 PM, Ned Batchelder <ned at nedbatchelder.com> wrote:
> On 9/29/13 12:56 PM, Chris Jerdonek wrote:
>>
>> I have a question about the behavior of hashing prior to Python 3.3
>> (when hash randomization was turned on by default [1]).
>>
>> I know that in earlier versions Python never made any guarantees about
>> hash values and their effect on dictionary key ordering, etc [2].  But
>> for testing purposes, in practice, to what extent does hashing behave
>> the same across systems and Python versions prior to Python 3.3?  For
>> example, the note at [2] says that "it typically varies between 32-bit
>> and 64-bit builds."
>>
>> I'm asking because I'm curious about the extent to which tests that
>> unknowingly depend on hash values are reproducible across systems and
>> versions.
>
>
> Tests like that are not reproducible across systems and versions. They may
> not be reproducible as the product code changes.  Two equal dicts may not
> iterate in the same order, even within a single process:

What explains the following then?  For quite a while, the unit tests
for a project I maintained always passed for all versions, but only
when I added 3.3 (when hash randomization was enabled) did I get
intermittent failures on such a test.  Do some such tests tend to
behave the same *in practice* -- even to a limited extent?  Otherwise,
I would have expected to see test failures in the earlier versions, at
least sometime.

--Chris


>
>>>> d1 = dict.fromkeys(str(i) for i in range(10))
>>>> d2 = dict.fromkeys(str(i) for i in range(1000000))
>>>> for i in range(10, 1000000):
> ...   del d2[str(i)]
> ...
>>>> d1 == d2
> True
>>>> d1
> {'1': None, '0': None, '3': None, '2': None, '5': None, '4': None, '7':
> None, '6': None, '9': None, '8': None}
>>>> d2
> {'9': None, '1': None, '5': None, '2': None, '0': None, '3': None, '4':
> None, '6': None, '7': None, '8': None}
>
> Be careful out there...
>
> --Ned.
>>
>> --Chris
>>
>> [1] http://docs.python.org/3/whatsnew/3.3.html#builtin-functions-and-types
>> [2] http://docs.python.org/2/using/cmdline.html#cmdoption-R
>>
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>
>