What is the default __hash__ method in Sage?

Question

I have a Sage class that inherits from SageObject. According to the Python documentation,

User-defined classes have __cmp__()and __hash__() methods by default; with them, all objects compare unequal (except with themselves) and x.__hash__() returns id(x).

However, my class doesn't do this, despite the fact that it doesn't implement a __hash__ method of its own. Instead, it uses the hash value of its string representation (the one returned by its __str__ method). Is this part of the design of Sage classes, something different from normal Python classes? Is there a hierarchy of places that Sage might look in order to find an acceptable hash value?

Answer 1

Luckily, you practically answered the question yourself. Try the ?? trick to find the source code.

sage: SageObject.__hash__??
Type:       wrapper_descriptor
Base Class: <type 'wrapper_descriptor'>
String Form:    <slot wrapper '__hash__' of 'sage.structure.sage_object.SageObject' objects>
Namespace:  Interactive
Definition: SageObject.__hash__(self)
Source:
    def __hash__(self):
        return hash(self.__repr__())

So yes, it's intentional for most of these things. If you wanted to implement something different for hashes, I guess you could. It would be worth asking on one of the Sage lists if this was code you were interested in contributing and thought it might conflict with something.