Missing __hash__ method with Python 3.x about furl HOT 10 CLOSED

Odd. This behavior differs between Python 2

>>> sys.version_info[:2]
(2, 7)
>>> from furl import furl
>>> hash(furl())
1520225

and Python 3.

>>> sys.version_info[:2]
(3, 5)
>>> from furl import furl
>>> hash(furl())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'furl'

I'll look into this.

from furl.

On reflection, furl objects should not be used as dictionary keys. They're mutable.

From Python 3's datamodel documentation

If a class defines mutable objects and implements an __eq__() method, it should
not implement __hash__(), since the implementation of hashable collections
requires that a key’s hash value is immutable (if the object’s hash value
changes, it will be in the wrong hash bucket).

Example of potential confusion due to mutability.

>>> f = furl('http://google.com/')
>>> d = {f: f.host}
>>> f.url, d[f] 
('http://google.com/', 'google.com')
>>> f.set(host='yahoo.com')
>>> f.url, d[f]
('http://yahoo.com/', 'google.com')

from furl.

@bodgit Does that answer your question?

from furl.

Your explanation and example makes complete sense, however I guess my problem still remains that the column type doesn't work without my monkeypatch.

I'm not sure why there's even a requirement to use the furl object as a hash key, I think it comes from SQLAlchemy direct rather than the code implementing this URLType column type, specifically this line https://github.com/zzzeek/sqlalchemy/blob/master/lib/sqlalchemy/orm/identity.py#L149 triggers the exception.

The other option I've seen used elsewhere (possibly incorrectly) is to use the return value of id() which would mean it's independent of the value of the object but then I guess two furl objects containing the same URL would have different id() values when you would expect them to hash to the same value.

I guess it's a balancing between what's correct and what just makes the error go away 😉

from furl.

Is it your code or SQLAlchemy-Utils' code that uses furl objects as dictionary keys?

from furl.

Neither, I think it's from the base SQLAlchemy library itself, see the line I linked above that shows where it first tries to hash the value.

from furl.

What I mean is, whose code stores furl objects as dictionary keys? That is, in
whose code is SQLAlchemy.orm.identity.WeakInstanceDict.__setitem__() (or
similar) called with a furl object?

If furl objects are stored as dictionary keys in your code, a simple solution is
to wrap the dictionary with a thin shim that serializes and deserializes furl
objects to their URL strings on insertion and back on retrieval.

If furl objects are stored as dictionary keys in SQLAlchemy-Utils' code, I'll
contact them and let them know furl objects are mutable and shouldn't be
dictionary keys.

from furl.

I think it's SQLAlchemy itself; neither my code nor what I can see in the SQLAlchemy-Utils does anything with dictionaries.

I think it must be doing it for some sort of de-duplication or maybe testing for modification.

I did inquire on their IRC channel but didn't really get anything useful back, I suspect their mailling list is the next step.

from furl.

I've since found I only get this error when I try to use the URLType as (part of) a primary key, which is what I was trying to do initially.

However whilst this worked with SQLite as a storage backend, MySQL for example complains that the underlying column type (TEXT) cannot be indexed without specifying how many characters to use so I've reworked my code so that I don't use the URL directly as a primary key and then the error disappeared and no monkeypatching is required.

from furl.

Great to hear you found a solution. Closing this issue.

Thank you for bringing this to my attention, Matt. Don't hesitate to let me know
if there's anything else I can help with.

from furl.