What's the pythonic way to use getters and setters?
Try this: Python Property
The sample code is:
class C(object): def __init__(self): self._x = None def x(self): """I'm the 'x' property.""" print("getter of x called") return self._x def x(self, value): print("setter of x called") self._x = value def x(self): print("deleter of x called") del self._xc = C()c.x = 'foo' # setter calledfoo = c.x # getter calleddel c.x # deleter called
What's the pythonic way to use getters and setters?
The "Pythonic" way is not to use "getters" and "setters", but to use plain attributes, like the question demonstrates, and
del for deleting (but the names are changed to protect the innocent... builtins):
value = 'something'obj.attribute = value value = obj.attributedel obj.attribute
If later, you want to modify the setting and getting, you can do so without having to alter user code, by using the
class Obj: """property demo""" ## first decorate the getter method def attribute(self): # This getter method name is *the* name return self._attribute ## the property decorates with `.setter` now def attribute(self, value): # name, e.g. "attribute", is the same self._attribute = value # the "value" name isn't special ## decorate with `.deleter` def attribute(self): # again, the method name is the same del self._attribute
(Each decorator usage copies and updates the prior property object, so note that you should use the same name for each set, get, and delete function/method.
After defining the above, the original setting, getting, and deleting code is the same:
obj = Obj()obj.attribute = value the_value = obj.attributedel obj.attribute
You should avoid this:
def set_property(property,value): def get_property(property):
Firstly, the above doesn't work, because you don't provide an argument for the instance that the property would be set to (usually
self), which would be:
class Obj: def set_property(self, property, value): # don't do this ... def get_property(self, property): # don't do this either ...
Secondly, this duplicates the purpose of two special methods,
Thirdly, we also have the
getattr builtin functions.
setattr(object, 'property_name', value)getattr(object, 'property_name', default_value) # default is optional
@property decorator is for creating getters and setters.
For example, we could modify the setting behavior to place restrictions the value being set:
class Protective(object): def protected_value(self): return self._protected_value def protected_value(self, value): if acceptable(value): # e.g. type or range check self._protected_value = value
In general, we want to avoid using
property and just use direct attributes.
This is what is expected by users of Python. Following the rule of least-surprise, you should try to give your users what they expect unless you have a very compelling reason to the contrary.
For example, say we needed our object's protected attribute to be an integer between 0 and 100 inclusive, and prevent its deletion, with appropriate messages to inform the user of its proper usage:
class Protective(object): """protected property demo""" # def __init__(self, start_protected_value=0): self.protected_value = start_protected_value # def protected_value(self): return self._protected_value # def protected_value(self, value): if value != int(value): raise TypeError("protected_value must be an integer") if 0 <= value <= 100: self._protected_value = int(value) else: raise ValueError("protected_value must be " + "between 0 and 100 inclusive") # def protected_value(self): raise AttributeError("do not delete, protected_value can be set to 0")
__init__ refers to
self.protected_value but the property methods refer to
self._protected_value. This is so that
__init__ uses the property through the public API, ensuring it is "protected".)
3) p1.protected_value3p1 = Protective(5.0) p1.protected_value5p2 = Protective(-5)Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 3, in __init__ File "<stdin>", line 15, in protected_valueValueError: protectected_value must be between 0 and 100 inclusive p1.protected_value = 7.3Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 17, in protected_valueTypeError: protected_value must be an integer p1.protected_value = 101Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 15, in protected_valueValueError: protectected_value must be between 0 and 100 inclusivedel p1.protected_valueTraceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 18, in protected_valueAttributeError: do not delete, protected_value can be set to 0p1 = Protective(
Do the names matter?
Yes they do.
.deleter make copies of the original property. This allows subclasses to properly modify behavior without altering the behavior in the parent.
class Obj: """property demo""" # def get_only(self): return self._attribute # def get_or_set(self, value): self._attribute = value # def get_set_or_delete(self): del self._attribute
Now for this to work, you have to use the respective names:
obj = Obj()# obj.get_only = 'value' # would errorobj.get_or_set = 'value' obj.get_set_or_delete = 'new value'the_value = obj.get_onlydel obj.get_set_or_delete# del obj.get_or_set # would error
I'm not sure where this would be useful, but the use-case is if you want a get, set, and/or delete-only property. Probably best to stick to semantically same property having the same name.
Start with simple attributes.
If you later need functionality around the setting, getting, and deleting, you can add it with the property decorator.
Avoid functions named
get_... - that's what properties are for.