On Fri, Mar 31, 2017 at 06:58:21PM +1100, Chris Angelico wrote:
This keeps on coming up in one form or another - either someone multiplies a list of lists and ends up surprised that they're all the same, or is frustrated with the verbosity of the alternatives.
Can we use the matmul operator for this?
I like the idea of using * for repetition without copying, and @ for repetition with shallow copying. That does mean that now you have a built-in operator which relies on the copy module, since it has to work with arbitrary objects. Isn't copy written in Python? [...]
If this were supported by the built-in list type, it would be either of these:
x = [[0] * 4] @ 2 x = [[0] @ 4] @ 4
(identical functionality, as copying an integer has no effect).
I think that's an implementation detail: copying immutable objects *might* return a reference to the original immutable object, or it might return a new object. For ints, any sane implementation would surely behave as we say, but let's not specify that as part of the behaviour of @ itself.
The semantics could be either as shown above (copy.copy()), or something very simple and narrow like "lists get shallow-copied, other objects get referenced".
I prefer the distinction copy versus non-copy. That makes it simple to understand, and means that it works if somebody wants a list of dicts instead of a list of lists: data = [{'a': 1, 'b': 2}] @ 5 -- Steve