元类的__getattr__没有被调用

Question

As the title says. It seems no matter what I do, __getattr__ will not be called. I also tried it for instance (absurd, I know), with predictably no response. As if __getattr__ was banned in meta classes.

I'd appreciate any pointer to documentation about this.

The code:

class PreinsertMeta(type):

    def resolvedField(self):
        if isinstance(self.field, basestring):
            tbl, fld = self.field.split(".")
            self.field = (tbl, fld)
        return self.field

    Field = property(resolvedField)

    def __getattr__(self, attrname):
        if attrname == "field":
            if isinstance(self.field, basestring):
                tbl, fld = self.field.split(".")
                self.field = (tbl, fld)
            return self.field
        else:
            return super(PreinsertMeta, self).__getattr__(attrname)

    def __setattr__(self, attrname, value):
        super(PreinsertMeta, self).__setattr__(attrname, value)


class Test(object):
    __metaclass__ = PreinsertMeta
    field = "test.field"

print Test.field  # Should already print the tuple
Test.field = "another.field"  # __setattr__ gets called nicely
print Test.field  # Again with the string?
print Test.Field  # note the capital 'F', this actually calls resolvedField() and prints the tuple

Thanks to BrenBarn, here's the final working implementation:

class PreinsertMeta(type):

    def __getattribute__(self, attrname):
        if attrname == "field" and isinstance(object.__getattribute__(self, attrname), basestring):
            tbl, fld = object.__getattribute__(self, attrname).split(".")
            self.field = (tbl, fld)
        return object.__getattribute__(self, attrname)

Answer 1

As documented , __getattr__ is only called if the attribute does not exist. Since your class has a field attribute, that blocks __getattr__ . You can use __getattribute__ if you really want to intercept all attribute access, although it's not clear from your example why you need to do this. Note that this has nothing to do with metaclasses; you would see the same behavior if you created an instance of an ordinary class and gave it some attribute.

Even assuming you used __getattribute__ , so it was called when the attribute exists, your implementation doesn't make much sense. Inside __getattr__ you try to get a value for self.field . But if __getattribute__ was called in the first place, it will be called again for this access, creating an infinite recursion: in order to get self.field , it has to call __getattribute__ , which again tries to get self.field , which again calls __getattribute__ , etc. See the documentation for __getattribute__ for how to get around this.