Op 2004-12-16, Paul Rubin schreef <http>: > Antoon Pardon <[EMAIL PROTECTED]> writes: >> Two guidelines can make it easier for a programmer to do this. >> >> 1) Put a copy in the dictionary, so that mutating the original >> object won't affect what is in the dictonary. > > What's supposed to happen here? > > a = [1,2,3] > d[a] = 9 > a.append(4) > print d[a] > > It doesn't sound like good dictionary semantics to me.
That depends on whether the programmes wants value equality or identity equality. In the first case the programmer shouldn't mutate a after it was introduced as key in the dictionary; but should either introduce a copy or work on a copy later. As such your snippet of code would become. a = [1,2,3] d[a[:]] = 9 a.append(4) print d[a] And this would raise a KeyError, unless the list [1,2,3,4] would be in the dictionary. In the second case your code would produce 9. -- Antoon Pardon -- http://mail.python.org/mailman/listinfo/python-list
