Josh Rosenberg added the comment:
Well, I could see a "friendly" repr being avoided to:
1. Save work (iterating safely involves iteration guards and explicitly
acquiring strong references to every key)
2. Avoid pretending the state is stable (the repr at time X could be different
at time X+epsilon)
3. Avoiding the possibility of __repr__ implementations for contained items
doing terrible things (e.g. mutating the WeakKeyDictionary, or deleting the
last external reference to a key). In the case where a __repr__ deletes the
last external reference to a key that WeakKeyDictionary's __repr__ already
iterated past, it means the repr is wrong before it even returns.
4. The repr wouldn't match the normal repr behavior; ideally, you can copy and
paste a repr and get back an equivalent object. But of course, copy and pasting
WeakKeyDictionary({Spam(1): 1, Spam(2): 2}) would actually get you back
WeakKeyDictionary({}), because the objects you passed in don't share the
identity of the ones in the original WeakKeyDictionary (which have existing
references somewhere), and the instant the constructor returns, they'd lose
their last reference and be deleted from the newly created WeakKeyDictionary
5. The above problems get even worse if the repr ends up being reentrant (which
is more likely in this case; weakrefs are often used to break cycles); any
stage of the recursive repr could end up mutating
None of these mean that the feature would be impossible/undesirable. #4 *might*
be a reason to keep __repr__ as is though, and have __str__ display the more
friendly version (e.g. `return '{}({!r})'.format(type(self).__name__,
dict(self))` ); __str__ is supposed to be friendly without being a means of
reproducing the object, so it wouldn't be quite as misleading.
----------
nosy: +josh.r
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