Static methods in Python?

Question

Is it possible to have static methods in Python so I can call them without initializing a class, like:

ClassName.StaticMethod ( )

Answer 1

Aside from the particularities of how static method objects behave, there is a certain kind of beauty you can strike with them when it comes to organizing your module-level code.

# garden.py
def trim(a):
    pass

def strip(a):
    pass

def bunch(a, b):
    pass

def _foo(foo):
    pass

class powertools(object):
    """
    Provides much regarded gardening power tools.
    """
    @staticmethod
    def answer_to_the_ultimate_question_of_life_the_universe_and_everything():
        return 42

    @staticmethod
    def random():
        return 13

    @staticmethod
    def promise():
        return True

def _bar(baz, quux):
    pass

class _Dice(object):
    pass

class _6d(_Dice):
    pass

class _12d(_Dice):
    pass

class _Smarter:
    pass

class _MagicalPonies:
    pass

class _Samurai:
    pass

class Foo(_6d, _Samurai):
    pass

class Bar(_12d, _Smarter, _MagicalPonies):
    pass

...

# tests.py
import unittest
import garden

class GardenTests(unittest.TestCase):
    pass

class PowertoolsTests(unittest.TestCase):
    pass

class FooTests(unittest.TestCase):
    pass

class BarTests(unittest.TestCase):
    pass

...

# interactive.py
from garden import trim, bunch, Foo

f = trim(Foo())
bunch(f, Foo())

...

# my_garden.py
import garden
from garden import powertools

class _Cowboy(garden._Samurai):
    def hit():
        return powertools.promise() and powertools.random() or 0

class Foo(_Cowboy, garden.Foo):
    pass

It now becomes a bit more intuitive and self-documenting in which context certain components are meant to be used and it pans out ideally for naming distinct test cases as well as having a straightforward approach to how test modules map to actual modules under tests for purists.

I frequently find it viable to apply this approach to organizing a project's utility code. Quite often, people immediately rush and create a utils package and end up with 9 modules of which one has 120 LOC and the rest are two dozen LOC at best. I prefer to start with this and convert it to a package and create modules only for the beasts that truly deserve them:

# utils.py
class socket(object):
    @staticmethod
    def check_if_port_available(port):
        pass

    @staticmethod
    def get_free_port(port)
        pass

class image(object):
    @staticmethod
    def to_rgb(image):
        pass

    @staticmethod
    def to_cmyk(image):
        pass

Answer 2

In Python 3:

Here is another good to use non-static methods as they were static methods.

class Account:
# here is the main class

# a non-static method
def login(self, url, email, password):
    # as a convention self is passed as a 
    # placeholder for the class Object itself

    self.login_url = url
    self.user_email = email
    self.user_password = password

    print(self.login_url, self.user_email, self.user_password)

Call a non-static method as static

"""
* Referencing the Account.login(self, url, email, password)
* Just call the `Account` object as the self 
"""
Account.login(Account, "https://example.com/login", "[e-mail]", "password_example")

:$ https://example.com/login [e-mail] password_example

Answer 3

I think that Steven is actually right. To answer the original question, then, in order to set up a class method, simply assume that the first argument is not going to be a calling instance, and then make sure that you only call the method from the class.

(Note that this answer refers to Python 3.x. In Python 2.x you'll get a TypeError for calling the method on the class itself.)

For example:

class Dog:
    count = 0 # this is a class variable
    dogs = [] # this is a class variable

    def __init__(self, name):
        self.name = name #self.name is an instance variable
        Dog.count += 1
        Dog.dogs.append(name)

    def bark(self, n): # this is an instance method
        print("{} says: {}".format(self.name, "woof! " * n))

    def rollCall(n): #this is implicitly a class method (see comments below)
        print("There are {} dogs.".format(Dog.count))
        if n >= len(Dog.dogs) or n < 0:
            print("They are:")
            for dog in Dog.dogs:
                print("  {}".format(dog))
        else:
            print("The dog indexed at {} is {}.".format(n, Dog.dogs[n]))

fido = Dog("Fido")
fido.bark(3)
Dog.rollCall(-1)
rex = Dog("Rex")
Dog.rollCall(0)

In this code, the "rollCall" method assumes that the first argument is not an instance (as it would be if it were called by an instance instead of a class). As long as "rollCall" is called from the class rather than an instance, the code will work fine. If we try to call "rollCall" from an instance, e.g.:

rex.rollCall(-1)

however, it would cause an exception to be raised because it would send two arguments: itself and -1, and "rollCall" is only defined to accept one argument.

Incidentally, rex.rollCall() would send the correct number of arguments, but would also cause an exception to be raised because now n would be representing a Dog instance (i.e., rex) when the function expects n to be numerical.

This is where the decoration comes in: If we precede the "rollCall" method with

@staticmethod

then, by explicitly stating that the method is static, we can even call it from an instance. Now,

rex.rollCall(-1)

would work. The insertion of @staticmethod before a method definition, then, stops an instance from sending itself as an argument.

You can verify this by trying the following code with and without the @staticmethod line commented out.

class Dog:
    count = 0 # this is a class variable
    dogs = [] # this is a class variable

    def __init__(self, name):
        self.name = name #self.name is an instance variable
        Dog.count += 1
        Dog.dogs.append(name)

    def bark(self, n): # this is an instance method
        print("{} says: {}".format(self.name, "woof! " * n))

    @staticmethod
    def rollCall(n):
        print("There are {} dogs.".format(Dog.count))
        if n >= len(Dog.dogs) or n < 0:
            print("They are:")
            for dog in Dog.dogs:
                print("  {}".format(dog))
        else:
            print("The dog indexed at {} is {}.".format(n, Dog.dogs[n]))


fido = Dog("Fido")
fido.bark(3)
Dog.rollCall(-1)
rex = Dog("Rex")
Dog.rollCall(0)
rex.rollCall(-1)

Answer 4

You don't really need to use the @staticmethod decorator. Just declaring a method (that doesn't expect the self parameter) and call it from the class. The decorator is only there in case you want to be able to call it from an instance as well (which was not what you wanted to do)

Mostly, you just use functions though...