3 ms·

Would you expect that `str(some_dict) == str(some_dict)`?

StavrosK 9y ago
No, of course not. Some order has to be applied to serialize it to a string, but that order is not defined between calls.
- eesmith 9y ago
  Python long ago made the guarantee that: > Keys and values are iterated over in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions. If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond. That is, the order will be maintained between calls, so long as the dictionary had not been modified. Going back to "str(some_dict) == str(some_dict)". I would not expect to always be the same, but for entirely different reasons. Consider: class Strange(): def __repr__(self): import random return str(random.random()) some_dict = {1: Strange()} >>> str(some_dict) == str(some_dict) False
  - StavrosK 9y ago
    A section called "CPython implementation detail" looks like the very antithesis of a guarantee, to me.
    - eesmith 9y ago
      My apologies. I looked for the answer I knew was there, and quoted the wrong section because it matched what I was looking for. I should have quoted the next section: > If items(), keys(), values(), iteritems(), iterkeys(), and itervalues() are called with no intervening modifications to the dictionary, the lists will directly correspond. Curiously, the "Dictionary view objects" section at https://docs.python.org/2.7/library/stdtypes.html#dictionary-view-objects https://docs.python.org/2.7/library/stdtypes.html#dictionary... has the same "Keys and values are iterated over in an arbitrary order which is non-random ..." text, but without being inside of a "CPython implementation detail" box.
      - StavrosK 9y ago
        Huh, that's inconsistent. I guess they changed it to avoid the security problem and forgot to change the docs? Or maybe they didn't update 2.7 at all and it still behaves that way.
PeterisP 9y ago
For that exact line I might expect that the result is true but still consider that line as "smelly" and meaningless, since dict equality cannot be compared by comparing their string representation. I'd expect that any two ways of forming a dict with exactly same contents can result in different str(x) representation, and also that serializing and deserializing a dict can result in a different string representation.

No, of course not. Some order has to be applied to serialize it to a string, but that order is not defined between calls.

eesmith 9y ago
Python long ago made the guarantee that: > Keys and values are iterated over in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions. If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond. That is, the order will be maintained between calls, so long as the dictionary had not been modified. Going back to "str(some_dict) == str(some_dict)". I would not expect to always be the same, but for entirely different reasons. Consider: class Strange(): def __repr__(self): import random return str(random.random()) some_dict = {1: Strange()} >>> str(some_dict) == str(some_dict) False
- StavrosK 9y ago
  A section called "CPython implementation detail" looks like the very antithesis of a guarantee, to me.
  - eesmith 9y ago
    My apologies. I looked for the answer I knew was there, and quoted the wrong section because it matched what I was looking for. I should have quoted the next section: > If items(), keys(), values(), iteritems(), iterkeys(), and itervalues() are called with no intervening modifications to the dictionary, the lists will directly correspond. Curiously, the "Dictionary view objects" section at https://docs.python.org/2.7/library/stdtypes.html#dictionary-view-objects https://docs.python.org/2.7/library/stdtypes.html#dictionary... has the same "Keys and values are iterated over in an arbitrary order which is non-random ..." text, but without being inside of a "CPython implementation detail" box.
    - StavrosK 9y ago
      Huh, that's inconsistent. I guess they changed it to avoid the security problem and forgot to change the docs? Or maybe they didn't update 2.7 at all and it still behaves that way.

PeterisP 9y ago

For that exact line I might expect that the result is true but still consider that line as "smelly" and meaningless, since dict equality cannot be compared by comparing their string representation. I'd expect that any two ways of forming a dict with exactly same contents can result in different str(x) representation, and also that serializing and deserializing a dict can result in a different string representation.