What is a clean, pythonic way to have multiple constructors in Python?
I can't find a definitive answer for this. AFAIK, you can't have multiple __init__ functions in a Python class. So how do I solve this problem?
Suppose I have an class called Cheese with the number_of_holes property. How can I have two ways of creating cheese-objects...
parmesan = Cheese(num_holes = 15) number_of_holes property: gouda = Cheese() I can think of only one way to do this, but that seems kinda clunky:
class Cheese():
def __init__(self, num_holes = 0):
if (num_holes == 0):
# randomize number_of_holes
else:
number_of_holes = num_holes
What do you say? Is there another way?
Actually None is much better for "magic" values:
class Cheese():
def __init__(self, num_holes = None):
if num_holes is None:
...
Now if you want complete freedom of adding more parameters:
class Cheese():
def __init__(self, *args, **kwargs):
#args -- tuple of anonymous arguments
#kwargs -- dictionary of named arguments
self.num_holes = kwargs.get('num_holes',random_holes())
To better explain the concept of *args and **kwargs (you can actually change these names):
def f(*args, **kwargs):
print 'args: ', args, ' kwargs: ', kwargs
>>> f('a')
args: ('a',) kwargs: {}
>>> f(ar='a')
args: () kwargs: {'ar': 'a'}
>>> f(1,2,param=3)
args: (1, 2) kwargs: {'param': 3}
http://docs.python.org/reference/expressions.html#calls
Using num_holes=None as the default is fine if you are going to have just __init__ .
If you want multiple, independent "constructors", you can provide these as class methods. These are usually called factory methods. In this case you could have the default for num_holes be 0 .
class Cheese(object):
def __init__(self, num_holes=0):
"defaults to a solid cheese"
self.number_of_holes = num_holes
@classmethod
def random(cls):
return cls(randint(0, 100))
@classmethod
def slightly_holey(cls):
return cls(randint((0,33))
@classmethod
def very_holey(cls):
return cls(randint(66, 100))
Now create object like this:
gouda = Cheese()
emmentaler = Cheese.random()
leerdammer = Cheese.slightly_holey()
如果您想使用可选参数，所有这些答案都非常出色，但另一种Pythonic可能性是使用classmethod来生成工厂样式的伪构造函数：
def __init__(self, num_holes):
# do stuff with the number
@classmethod
def fromRandom(cls):
return cls( # some-random-number )