Dynamically adding a property to a class

Question

This has been previously asked on Stack Overflow, but none of the answers seem to address exactly what I need to do. In my case, I want these dynamically-added properties to be a shortcut to store and read values from a database, so unfortunately it's not as easy as in this answer (where a lambda function was used) or this one (where values where stored in a dictionary): I must call other methods of the class.

This is my attempt:

import operator class Foo(object): def get_value(self, name): # read and return value from database return -1 def set_value(self, name, value): # store value in database pass def add_metadata_property(name): getter = operator.methodcaller('get_value', name) setter = operator.methodcaller('set_value', name) # gets value at runtime setattr(Foo, name, property(getter, setter)) add_metadata_property('spam') f = Foo() f.spam # works! f.spam = 2

The last line, however, raises:

Traceback (most recent call last): File "<stdin>", line 27, in <module> TypeError: methodcaller expected 1 arguments, got 2

Any ideas on how to achieve this?

Have you tried using the lambda answer, providing function objects instead of lambdas? — loopbackbee
– loopbackbee, Commented Jul 1, 2014 at 12:15
You might be interested in this answer stackoverflow.com/questions/1224962/… — Germano
– Germano, Commented Jul 1, 2014 at 12:37

Stephen Lin · Accepted Answer · 2014-07-05 00:05:26Z

I don't know why you use operator.methodcaller here. When you call f.spam=2, it will invoke setter. setter = operator.methodcaller('set_value', name) means setter(r) = r.set_value(name). Make no sense in your case.

I suggest you write this way, using @classmethod:

class Foo(object): @classmethod def get_value(self, name): # read and return value from database return -1 @classmethod def set_value(self, name, value): # store value in database pass def add_metadata_property(name): setattr(Foo, name, property(Foo.get_value, Foo.set_value)) add_metadata_property('spam') f = Foo() f.spam # works! f.spam = 2

If this helped you, please confirm it as the answer. Thanks!

greatvovan · Accepted Answer · 2014-07-01 13:34:00Z

Modifying class template looks a bit odd for me. I would suggest to overload __getattr__() and __setattr__() methods in your case.

class Foo: def __getattr__(self, name): print('read and return value from database for ', name) return 123 def __setattr__(self, name, value): print('store value', value, 'for', name, 'in database')

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