On Sep 18, 7:39 pm, sapsi <[EMAIL PROTECTED]> wrote: > Hello, > I recently tried using the set function in Python and was surprised to > find that > > a=[ 1, 2,3, [1,2] ] > > doesn't work with 'set', throwing TyperError (unhashable exception). I > found out that this is because lists can't be hashed. > > So,this implies 'a' cannot be a set in python which i think is quite > unfortunate, after all 'a' does look like a mathematical set.
It is not the variable *a* itself that's a problem when constructing a set (ie. set(a)); it is the content. set() goes through each of the items and adds that item to the set. 1, 2, and 3 are valid because they can be hashed. The next item in the list, however, is [1,2], and cannot be hashed because it is a mutable list. The solution is as Raymond Hettinger said: a = set([1, 2, 3, frozenset([1, 2])]) > My question is, > 1) Why can't lists be hashed? They're mutable. > and > 2) This is not related, but is there i neat way (without pop and list > comprehension) to convert a set into a list? I say neat because i'm > guessing using list comprehension might turn out be slow and there > might be other methods which are faster. list(a_set) > Thank you for your time You're welcome. -- http://mail.python.org/mailman/listinfo/python-list
