On Sun, Aug 26, 2018 at 06:19:36PM +0100, Jonathan Fine wrote: > Finally, here's something that surprised me a little bit > > >>> x = [1, 2]; id(x) > 140161160364616 > >>> x += [3, 4]; id(x) > 140161160364616
> >>> x = (1, 2); id(x) > 140161159928520 > >>> x += (3, 4); id(x) > 140161225906440 > > Notice that '+=' creates uses the same object when the object is a > list, but creates a new object. This raises the question: Why and how > does Python behave in this way? Lists are mutable and can be modified in place. Tuples are immutable and cannot be. By the way: it's not reliable to compare ID numbers for objects which don't necessarily exist similtaneously. The Python interpreter is permitted to re-use ID numbers. For example: py> s = [1, 4] py> id(s) 3080212620 py> del s py> s = [-1, 3] py> id(s) 3080212620 The only reliable use of ID numbers is to compare the ID of objects which are known to still exist. -- Steve _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
