How can I return a default value for an attribute? [duplicate]

Answer 1

a=myobect.id if myobject else None

Answer 2

Help on built-in function getattr in module builtins:

getattr(...)
    getattr(object, name[, default]) -> value

Get a named attribute from an object; getattr(x, 'y') is equivalent to x.y. When a default argument is given, it is returned when the attribute doesn't exist; without it, an exception is raised in that case.

Following should work:

a = getattr(myobject, 'id', None)

Answer 3

try:
    a = myobject.id
except AttributeError:
    a = None

Will also work and is clearer, IMO

Answer 4

The simplest way is to use the ternary operator:

a = myobject.id if myobject is not None else None

The ternary operator returns the first expression if the middle value is true, otherwise it returns the latter expression.

Note that you could also do this in another way, using exceptions:

try:
    a = myobject.id
except AttributeError:
    a = None

This fits the Pythonic ideal that it's easier to ask for forgiveness than permission - what is best will depend on the situation.

Answer 5

in my object class you can put override

class Foo(object):
   def __getattribute__(self, name):
      if not name in self;
        return None;
      else:
        # Default behaviour
        return object.__getattribute__(self, name)

Answer 6

If you want to solve the problem in the definition of the class of myobject (like in Black Diamond's answer) you can simply define __getattr__ to return None:

class Myobject:
    def __getattr__(self, name):
        return None

This works because __getattr__ is only called when trying to access an attribute that does not exist, whereas __getattribute__ is always called first no matter the name of the attribute. (See also this SO post.)

To try out:

myobject = Myobject()
print myobject.id 
myobject.id = 7
print myobject.id