On Wed, Feb 04, 2015 at 01:14:14PM -0800, Neil Girdhar wrote:
Hundreds of people want an orderedset (http://stackoverflow.com/questions/1653970/does-python-have-an-ordered-set) and yet the ordered sets that are in PyPI (oset, ordered-set) are incomplete. Please consider adding an ordered set that has *all* of the same functionality as set.
I think you have this backwards: more likely, first you find a complete implementation on PyPI, then you propose it for the standard library.
The only major design decision is (I think) whether to store the elements in a linked list or dynamic vector.
That's an implementation detail and therefore irrelevant (in the sense that it is subject to change without affecting code that uses ordered sets).
I think there are plenty of other major design decisions to be made. What's the union between a regular set and an ordered set? Is there a frozen ordered set? Do ordered sets compare unequal if they differ only in order (like lists)?
My personal preference is to specify the ordered set via a flag to the constructor, which can be intercepted by __new__ to return a different object type as necessary.
-1 on a flag to the constructor.
I don't know if this rule (well, guideline) has a formal name anywhere, so I'm going to call it Guido's Rule: no constant mode arguments. I think this is a powerful design principle.
Functions or methods (including constructors) should not take arguments which, when given, are invariably supplied as constants. This especially goes for "mode" arguments, usually a flag, where the implemention of the function then looks something like this:
def thing(arg, flag=True): if flag: return this(arg) else: return that(arg)
In cases like this, you should expose this and that as distinct functions.
(If somebody -- Guido? -- would like to propose a cleaner explanation for the rule, I suggest we put it somewhere in the documention. Perhaps in the FAQs.)
(1) We have list and tuple as separate functions, not sequences(*args, *, mutable=False).
(2) str and unicode (str and bytes), not str(obj, unicode=True).
(3) pairs of (sin, sin), (cos, acos), (tan, atan) functions, rather than sin(x, inverse=False). For that matter, same with sin/sinh etc.
Note that this is a "should not", not "must not": exceptions are allowed where the API clearly improved by it:
sorted(seq, reversed=True) rather than sorted/reverse_sorted.
(If anyone wants to add a complete ordered set to PyPI that would also be very useful for me!)
I'm sure it would be very useful to lots of people. Perhaps you could scratch your own itch and solve their problem at the same time?