[Python-Dev] Hashes in Python3.5 for tuples and frozensets (original) (raw)

Chris Angelico rosuav at gmail.com
Thu May 17 10:34:50 EDT 2018

On Fri, May 18, 2018 at 12:15 AM, Anthony Flury via Python-Dev <python-dev at python.org> wrote:

Chris, I entirely agree. The same questioner also asked about the fastest data type to use as a key in a dictionary; and which data structure is fastest. I get the impression the person is very into micro-optimization, without profiling their application. It seems every choice is made based on the speed of that operation; without consideration of how often that operation is used.

Sounds like we're on the same page here.

On 17/05/18 09:16, Chris Angelico wrote:

The hash values of Python objects are calculated by the hash method, so arbitrary objects can do what they like, including degenerate algorithms such as:

class X: def hash(self): return 7 Agreed - I should have said the default hash algorithm. Hashes for custom object are entirely application dependent.

There isn't a single "default hash algorithm"; in fact, I'm not sure that there's even a single algorithm used for all strings. Certainly the algorithm used for integers is completely different from the one(s) used for strings; we have a guarantee that ints and floats representing the same real number are going to have the same hash (even if that hash isn't equal to the number - hash(1e22)==hash(1022)!=1022 is True), since they compare equal. The algorithms used and the resulting hashes may change between Python versions, when you change interpreters (PyPy vs Jython vs CPython vs Brython...), or even when you change word sizes, I believe (32-bit vs 64-bit).

So, this is (a) a premature optimization, (b) depending on something that's not guaranteed, and (c) is a great way to paint yourself into a corner. Perfect! :)

ChrisA

More information about the Python-Dev mailing list