Tony the Tiger <tony@tiger.invalid> writes: > radius='10', mass='1' > > if radius == '10' ... > > if mass == '1' ...
This ignores the problem as stated: The OP wants to distinguish between a value that was explicitly set by the caller, versus a value that was set by default because the caller did not specify the parameter. The general solution to this is to choose a value outside the domain, a <URL:https://en.wikipedia.org/wiki/Sentinel_value> which is different from any value the caller would choose for that parameter. Often, the Python singleton ‘None’ is good for the purpose. I think in this case, where the domain is numeric, a default of ‘None’ would do fine. class Foo: def __init__(centre=(0, 0), radius=None, mass=None): if radius is None: # Caller didn't specify. Set a default value. radius = 10 if mass is None: # Caller didn't specify. Set a default value. mass = 1 self.centre = centre self.radius = radius self.mass = mass In some other cases, ‘None’ is a valid value for the caller to set, and so cannot be used as a sentinel to distinguish the default. For those cases, creating a specific object instance to serve as the sentinel is simple and clear: class Foo: _SENTINEL = object() def __init__(centre=(0, 0), radius=_SENTINEL, mass=_SENTINEL): if radius is _SENTINEL: # Caller didn't specify. Set a default value. radius = 10 if mass is _SENTINEL: # Caller didn't specify. Set a default value. mass = 1 self.centre = centre self.radius = radius self.mass = mass Creating a new object specifically for the sentinel means there is no chance some other value will compare identical. Choosing a leading-underscore name indicates to the reader that this is not part of the public API. -- \ “Odious ideas are not entitled to hide from criticism behind | `\ the human shield of their believers' feelings.” —Richard M. | _o__) Stallman | Ben Finney -- https://mail.python.org/mailman/listinfo/python-list
