I can’t find a definitive answer for this. As far as I know, you can’t have multiple __init__ functions in a Python class. So how do I solve this problem?
Suppose I have a class called Cheese with the number_of_holes property. How can I have two ways of creating cheese objects…
- One that takes a number of holes like this:
parmesan = Cheese(num_holes = 15). - And one that takes no arguments and just randomizes the
number_of_holesproperty:gouda = Cheese().
I can think of only one way to do this, but this seems 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?
Answers:
Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.
Method 1
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
Method 2
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()
Method 3
One should definitely prefer the solutions already posted, but since no one mentioned this solution yet, I think it is worth mentioning for completeness.
The @classmethod approach can be modified to provide an alternative constructor which does not invoke the default constructor (__init__). Instead, an instance is created using __new__.
This could be used if the type of initialization cannot be selected based on the type of the constructor argument, and the constructors do not share code.
Example:
class MyClass(set):
def __init__(self, filename):
self._value = load_from_file(filename)
@classmethod
def from_somewhere(cls, somename):
obj = cls.__new__(cls) # Does not call __init__
super(MyClass, obj).__init__() # Don't forget to call any polymorphic base class initializers
obj._value = load_from_somewhere(somename)
return obj
Method 4
All of these answers are excellent if you want to use optional parameters, but another Pythonic possibility is to use a classmethod to generate a factory-style pseudo-constructor:
def __init__(self, num_holes): # do stuff with the number @classmethod def fromRandom(cls): return cls( # some-random-number )
Method 5
Why do you think your solution is “clunky”? Personally I would prefer one constructor with default values over multiple overloaded constructors in situations like yours (Python does not support method overloading anyway):
def __init__(self, num_holes=None):
if num_holes is None:
# Construct a gouda
else:
# custom cheese
# common initialization
For really complex cases with lots of different constructors, it might be cleaner to use different factory functions instead:
@classmethod
def create_gouda(cls):
c = Cheese()
# ...
return c
@classmethod
def create_cheddar(cls):
# ...
In your cheese example you might want to use a Gouda subclass of Cheese though…
Method 6
Those are good ideas for your implementation, but if you are presenting a cheese making interface to a user. They don’t care how many holes the cheese has or what internals go into making cheese. The user of your code just wants “gouda” or “parmesean” right?
So why not do this:
# cheese_user.py from cheeses import make_gouda, make_parmesean gouda = make_gouda() paremesean = make_parmesean()
And then you can use any of the methods above to actually implement the functions:
# cheeses.py
class Cheese(object):
def __init__(self, *args, **kwargs):
#args -- tuple of anonymous arguments
#kwargs -- dictionary of named arguments
self.num_holes = kwargs.get('num_holes',random_holes())
def make_gouda():
return Cheese()
def make_paremesean():
return Cheese(num_holes=15)
This is a good encapsulation technique, and I think it is more Pythonic. To me this way of doing things fits more in line more with duck typing. You are simply asking for a gouda object and you don’t really care what class it is.
Method 7
Use num_holes=None as a default, instead. Then check for whether num_holes is None, and if so, randomize. That’s what I generally see, anyway.
More radically different construction methods may warrant a classmethod that returns an instance of cls.
Method 8
Overview
For the specific cheese example, I agree with many of the other answers about using default values to signal random initialization or to use a static factory method. However, there may also be related scenarios that you had in mind where there is value in having alternative, concise ways of calling the constructor without hurting the quality of parameter names or type information.
Since Python 3.8 and functools.singledispatchmethod can help accomplish this in many cases (and the more flexible multimethod can apply in even more scenarios). (This related post describes how one could accomplish the same in Python 3.4 without a library.) I haven’t seen examples in the documentation for either of these that specifically shows overloading __init__ as you ask about, but it appears that the same principles for overloading any member method apply (as shown below).
“Single dispatch” (available in the standard library) requires that there be at least one positional parameter and that the type of the first argument be sufficient to distinguish among the possible overloaded options. For the specific Cheese example, this doesn’t hold since you wanted random holes when no parameters were given, but multidispatch does support the very same syntax and can be used as long as each method version can be distinguish based on the number and type of all arguments together.
Example
Here is an example of how to use either method (some of the details are in order to please mypy which was my goal when I first put this together):
from functools import singledispatchmethod as overload
# or the following more flexible method after `pip install multimethod`
# from multimethod import multidispatch as overload
class MyClass:
@overload # type: ignore[misc]
def __init__(self, a: int = 0, b: str = 'default'):
self.a = a
self.b = b
@__init__.register
def _from_str(self, b: str, a: int = 0):
self.__init__(a, b) # type: ignore[misc]
def __repr__(self) -> str:
return f"({self.a}, {self.b})"
print([
MyClass(1, "test"),
MyClass("test", 1),
MyClass("test"),
MyClass(1, b="test"),
MyClass("test", a=1),
MyClass("test"),
MyClass(1),
# MyClass(), # `multidispatch` version handles these 3, too.
# MyClass(a=1, b="test"),
# MyClass(b="test", a=1),
])
Output:
[(1, test), (1, test), (0, test), (1, test), (1, test), (0, test), (1, default)]
Notes:
- I wouldn’t usually make the alias called
overload, but it helped make the diff between using the two methods just a matter of which import you use. - The
# type: ignore[misc]comments are not necessary to run, but I put them in there to pleasemypywhich doesn’t like decorating__init__nor calling__init__directly. - If you are new to the decorator syntax, realize that putting
@overloadbefore the definition of__init__is just sugar for__init__ = overload(the original definition of __init__). In this case,overloadis a class so the resulting__init__is an object that has a__call__method so that it looks like a function but that also has a.registermethod which is being called later to add another overloaded version of__init__. This is a bit messy, but it please mypy becuase there are no method names being defined twice. If you don’t care about mypy and are planning to use the external library anyway,multimethodalso has simpler alternative ways of specifying overloaded versions. - Defining
__repr__is simply there to make the printed output meaningful (you don’t need it in general). - Notice that
multidispatchis able to handle three additional input combinations that don’t have any positional parameters.
Method 9
The best answer is the one above about default arguments, but I had fun writing this, and it certainly does fit the bill for “multiple constructors”. Use at your own risk.
What about the new method.
“Typical implementations create a new instance of the class by invoking the superclass’s new() method using super(currentclass, cls).new(cls[, …]) with appropriate arguments and then modifying the newly-created instance as necessary before returning it.”
So you can have the new method modify your class definition by attaching the appropriate constructor method.
class Cheese(object):
def __new__(cls, *args, **kwargs):
obj = super(Cheese, cls).__new__(cls)
num_holes = kwargs.get('num_holes', random_holes())
if num_holes == 0:
cls.__init__ = cls.foomethod
else:
cls.__init__ = cls.barmethod
return obj
def foomethod(self, *args, **kwargs):
print "foomethod called as __init__ for Cheese"
def barmethod(self, *args, **kwargs):
print "barmethod called as __init__ for Cheese"
if __name__ == "__main__":
parm = Cheese(num_holes=5)
Method 10
I’d use inheritance. Especially if there are going to be more differences than number of holes. Especially if Gouda will need to have different set of members then Parmesan.
class Gouda(Cheese):
def __init__(self):
super(Gouda).__init__(num_holes=10)
class Parmesan(Cheese):
def __init__(self):
super(Parmesan).__init__(num_holes=15)
Method 11
Since my initial answer was criticised on the basis that my special-purpose constructors did not call the (unique) default constructor, I post here a modified version that honours the wishes that all constructors shall call the default one:
class Cheese:
def __init__(self, *args, _initialiser="_default_init", **kwargs):
"""A multi-initialiser.
"""
getattr(self, _initialiser)(*args, **kwargs)
def _default_init(self, ...):
"""A user-friendly smart or general-purpose initialiser.
"""
...
def _init_parmesan(self, ...):
"""A special initialiser for Parmesan cheese.
"""
...
def _init_gouda(self, ...):
"""A special initialiser for Gouda cheese.
"""
...
@classmethod
def make_parmesan(cls, *args, **kwargs):
return cls(*args, **kwargs, _initialiser="_init_parmesan")
@classmethod
def make_gouda(cls, *args, **kwargs):
return cls(*args, **kwargs, _initialiser="_init_gouda")
Method 12
This is how I solved it for a YearQuarter class I had to create. I created an __init__ which is very tolerant to a wide variety of input.
You use it like this:
>>> from datetime import date >>> temp1 = YearQuarter(year=2017, month=12) >>> print temp1 2017-Q4 >>> temp2 = YearQuarter(temp1) >>> print temp2 2017-Q4 >>> temp3 = YearQuarter((2017, 6)) >>> print temp3 2017-Q2 >>> temp4 = YearQuarter(date(2017, 1, 18)) >>> print temp4 2017-Q1 >>> temp5 = YearQuarter(year=2017, quarter = 3) >>> print temp5 2017-Q3
And this is how the __init__ and the rest of the class looks like:
import datetime
class YearQuarter:
def __init__(self, *args, **kwargs):
if len(args) == 1:
[x] = args
if isinstance(x, datetime.date):
self._year = int(x.year)
self._quarter = (int(x.month) + 2) / 3
elif isinstance(x, tuple):
year, month = x
self._year = int(year)
month = int(month)
if 1 <= month <= 12:
self._quarter = (month + 2) / 3
else:
raise ValueError
elif isinstance(x, YearQuarter):
self._year = x._year
self._quarter = x._quarter
elif len(args) == 2:
year, month = args
self._year = int(year)
month = int(month)
if 1 <= month <= 12:
self._quarter = (month + 2) / 3
else:
raise ValueError
elif kwargs:
self._year = int(kwargs["year"])
if "quarter" in kwargs:
quarter = int(kwargs["quarter"])
if 1 <= quarter <= 4:
self._quarter = quarter
else:
raise ValueError
elif "month" in kwargs:
month = int(kwargs["month"])
if 1 <= month <= 12:
self._quarter = (month + 2) / 3
else:
raise ValueError
def __str__(self):
return '{0}-Q{1}'.format(self._year, self._quarter)
Method 13
class Cheese:
def __init__(self, *args, **kwargs):
"""A user-friendly initialiser for the general-purpose constructor.
"""
...
def _init_parmesan(self, *args, **kwargs):
"""A special initialiser for Parmesan cheese.
"""
...
def _init_gauda(self, *args, **kwargs):
"""A special initialiser for Gauda cheese.
"""
...
@classmethod
def make_parmesan(cls, *args, **kwargs):
new = cls.__new__(cls)
new._init_parmesan(*args, **kwargs)
return new
@classmethod
def make_gauda(cls, *args, **kwargs):
new = cls.__new__(cls)
new._init_gauda(*args, **kwargs)
return new
Method 14
I do not see a straightforward answer with an example yet. The idea is simple:
- use
__init__as the “basic” constructor as python only allows one__init__method - use
@classmethodto create any other constructors and call the basic constructor
Here is a new try.
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
@classmethod
def fromBirthYear(cls, name, birthYear):
return cls(name, date.today().year - birthYear)
Usage:
p = Person('tim', age=18)
p = Person.fromBirthYear('tim', birthYear=2004)
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