Python Language Classes Class methods: alternate initializers
Example
Class methods present alternate ways to build instances of classes. To illustrate, let's look at an example.
Let's suppose we have a relatively simple Person class:
class Person(object):
def __init__(self, first_name, last_name, age):
self.first_name = first_name
self.last_name = last_name
self.age = age
self.full_name = first_name + " " + last_name
def greet(self):
print("Hello, my name is " + self.full_name + ".")
It might be handy to have a way to build instances of this class specifying a full name instead of first and last name separately. One way to do this would be to have last_name be an optional parameter, and assuming that if it isn't given, we passed the full name in:
class Person(object):
def __init__(self, first_name, age, last_name=None):
if last_name is None:
self.first_name, self.last_name = first_name.split(" ", 2)
else:
self.first_name = first_name
self.last_name = last_name
self.full_name = self.first_name + " " + self.last_name
self.age = age
def greet(self):
print("Hello, my name is " + self.full_name + ".")
However, there are two main problems with this bit of code:
-
The parameters
first_nameandlast_nameare now misleading, since you can enter a full name forfirst_name. Also, if there are more cases and/or more parameters that have this kind of flexibility, the if/elif/else branching can get annoying fast. -
Not quite as important, but still worth pointing out: what if
last_nameisNone, butfirst_namedoesn't split into two or more things via spaces? We have yet another layer of input validation and/or exception handling...
Enter class methods. Rather than having a single initializer, we will create a separate initializer, called from_full_name, and decorate it with the (built-in) classmethod decorator.
class Person(object):
def __init__(self, first_name, last_name, age):
self.first_name = first_name
self.last_name = last_name
self.age = age
self.full_name = first_name + " " + last_name
@classmethod
def from_full_name(cls, name, age):
if " " not in name:
raise ValueError
first_name, last_name = name.split(" ", 2)
return cls(first_name, last_name, age)
def greet(self):
print("Hello, my name is " + self.full_name + ".")
Notice cls instead of self as the first argument to from_full_name. Class methods are applied to the overall class, not an instance of a given class (which is what self usually denotes). So, if cls is our Person class, then the returned value from the from_full_name class method is Person(first_name, last_name, age), which uses Person's __init__ to create an instance of the Person class. In particular, if we were to make a subclass Employee of Person, then from_full_name would work in the Employee class as well.
To show that this works as expected, let's create instances of Person in more than one way without the branching in __init__:
In [2]: bob = Person("Bob", "Bobberson", 42)
In [3]: alice = Person.from_full_name("Alice Henderson", 31)
In [4]: bob.greet()
Hello, my name is Bob Bobberson.
In [5]: alice.greet()
Hello, my name is Alice Henderson.
Other references:
-
https://docs.python.org/2/library/functions.html#classmethod
-
https://docs.python.org/3.5/library/functions.html#classmethod
