hash and __eq__

Piet van Oostrum piet at cs.uu.nl
Sun May 31 00:48:38 CEST 2009

>>>>> Arnaud Delobelle <arnodel at googlemail.com> (AD) wrote:

>AD> Piet van Oostrum <piet at cs.uu.nl> writes:
>>>>>>>> Aaron Brady <castironpi at gmail.com> (AB) wrote:
>AB> I am writing a mapping object, and I want to ask about the details of
>AB> __hash__ and __eq__.  IIUC if I understand correctly, the Python
>AB> dict's keys' hash codes are looked up first in O( 1 ), then all the
>AB> matching hash entries are compared on equality in O( n ).  That is,
>AB> the hash code just really narrows down the search.  Am I correct?
>>> The O(1) part is correct. The O(n) is incorrect if n as usual is taken
>>> to be the number of keys in the dict. But it is true if n is the number
>>> of keys in the `bucket' that belongs to the hash value. The average
>>> complexity of looking up a key is O(1).

>AD> As all the keys could be in the same bucket, O(n) seems correct to me
>AD> (with n the total number of keys).

For worst case only (mainly with a badly designed hash function). 
Piet van Oostrum <piet at cs.uu.nl>
URL: http://pietvanoostrum.com [PGP 8DAE142BE17999C4]
Private email: piet at vanoostrum.org

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