I am writing a class method that I would like to use class variables if no other values are provided
def transform_point(self, x=self.x, y=self.y):
BUT... that doesn't seems to work:
NameError: name 'self' is not defined
I am getting the feeling there is a more clever way of doing this. What would you do?
You need to use sentinel values, then replace those with the desired instance attributes. None is a good option:
def transform_point(self, x=None, y=None):
if x is None:
x = self.x
if y is None:
y = self.y
Note that the function signature is executed only once; you cannot use expressions for default values and expect those to change with each call to the function.
If you have to be able to set x or y to None then you need to use a different, unique singleton value as the default instead. Using an instance of object() is usually a great sentinel in that case:
_sentinel = object()
def transform_point(self, x=_sentinel, y=_sentinel):
if x is _sentinel:
x = self.x
if y is _sentinel:
y = self.y
and now you can call .transform_point(None, None) too.
def transform_point(self, x=None, y=None):
if x is None:
x = self.x
if y is None:
y = self.y
etc
transform_point(obj) will cause all subsequent calls to transform_point to use the default arg None for x and y.