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17886546
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2013-07-26T16:33:47.100
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2013-07-26T16:33:47.100
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2017-05-23T12:21:37.550
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OK, everybody noticed the extremely obvious workaround (that took my some days to come up with), just put an attribute on <code>File</code> that tells you which folder it is in. (Don't worry, that is also what I did.) But, it turns out that I was working under wrong assumptions. You are not supposed to use mutable objects as keys, but that doesn't mean you can't (diabolic laughter)! The default implementation of <code>__hash__</code> returns a unique value, probably derived from the object's address, that remains constant in time. And the default <code>__eq__</code> follows the same notion of object identity. So you can put mutable objects in a dict, and they work as expected (if you expect equality based on instance, not on value). See also: <a href="https://stackoverflow.com/questions/4418741/im-able-to-use-a-mutable-object-as-a-dictionary-key-in-python-is-this-not-disa">I'm able to use a mutable object as a dictionary key in python. Is this not disallowed?</a> I was having problems because I was pickling/unpickling the objects, which of course changed the hashes. One could generate a unique ID in the constructor, and use that for equality and deriving a hash to overcome this. (For the curious, as to why such a "lookup based on instance identity" dict might be neccessary: I've been experimenting with a kind of <a href="https://stackoverflow.com/questions/5126776/query-language-for-python-objects">"object database"</a>. You have pure python objects, put them in lists/containers, and can define indexes on attributes for faster lookup, complex queries and so on. For foreign keys (1:n relationships) I can just use containers, but for the backlink I have to come up with something clever if I don't want to modify the objects on the n side.)
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1. POUse python dict to lookup mutable objects
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