python create class methods on the fly

Question

I have a rather complex base class for some sqlalchemy models and I want to create a rails style setters but, since i'm fairly new to python, I'm stuck with a problem that I cant seem to bypass. I create the setters on the new method so I can have the triggered both on new instances and on queries but no matter what setter I define and execute it always picks the last setter to execute. An example serves better:

class Test(object):

    columns = ['email', 'username']

    def __new__( cls, *args, **kwargs ):
        for column in cls.columns:
            setattr( cls, "set%s" % column.capitalize(), lambda cls, v: cls.setAttribute( cls, column, v ) )
        return super( Test, cls ).__new__( cls, *args, **kwargs )

    @staticmethod
    def setAttribute(cls, attribute, value):
        print "Setting attribute %s with value %s" % ( attribute, value )
        setattr( cls, attribute, value )

test = Test()
test.setEmail('test@test.com')

As you can see I'm setting the email but, when executed, the code tries to set the username which is the last column. Any idea why is that?

You need to learn about [closures](http://stackoverflow.com/questions/233673/lexical-closures-in-python) — JBernardo, Jun 10 '12 at 17:26
My code was perfectly valid. I just forgot to add the column as a parameter. Will take a look at closures now. — Romeo M., Jun 10 '12 at 17:43
Not related to the problem, but instead of overriding `__new__`, you should use a metaclass or a class decorator. — ecatmur, Jun 10 '12 at 19:37

Andrew Clark · Accepted Answer · 2012-06-10T17:34:50.850

This happens because your lambda function references column but doesn't pass it in as an argument:

lambda cls, v: cls.setAttribute( cls, column, v )

When this function is executed, it will look for the name column in a containing or global scope, and always find the value 'username' because that is what column was set to last.

Here is a straightforward way to fix this using a default argument value:

    def __new__( cls, *args, **kwargs ):
        for column in cls.columns:
            setattr( cls, "set%s" % column.capitalize(), lambda cls, v, column=column: cls.setAttribute( cls, column, v ) )
        return super( Test, cls ).__new__( cls, *args, **kwargs )

Another alternative would be to use a closure (in a way, the mutable default argument is a type of closure):

    def __new__( cls, *args, **kwargs ):
        def make_setter(column):
            return lambda cls, v: cls.setAttribute( cls, column, v )
        for column in cls.columns:
            setattr( cls, "set%s" % column.capitalize(), make_setter(column))
        return super( Test, cls ).__new__( cls, *args, **kwargs )

I forgot to add the column as a parameter dam. Thanks. – Romeo M. Jun 10 '12 at 17:43 — Romeo M., Jun 10 '12 at 17:43

python create class methods on the fly

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