On Wed, Dec 20, 2017 at 03:23:16PM -0800, Chris Barker wrote:

On Wed, Dec 20, 2017 at 2:31 AM, Steven D'Aprano <steve@pearwood.info> wrote:

...

Even when it works, the guarantee is quite weak. For instance, even the object type is not preserved:

py> class MyDict(dict): ... pass ... py> d = MyDict() py> x = eval(repr(d)) py> assert d == x py> assert type(d) == type(x) Traceback (most recent call last): File "<stdin>", line 1, in <module> AssertionError

Oh come on! If you subclass, and don't override __repr__ -- you're written a (very slightly) broken class (OK, a class with a broken __repr__).

Why is it broken? Is it documented somewhere that every subclass MUST override __repr__? If there's a bug here, and I'm not sure that there is, the bug is in dict itself, for having a repr which isn't friendly to subclasses. But in practice, why would I care? Obviously sometimes I do care, and for debugging it is often good to have a custom repr for subclasses, but it isn't mandatory or even always useful. Especially since in practice, it isn't that common to round-trip repr though eval (apart from the REPL itself, of course). -- Steve