Class methods/instance methods in Python

Question

I come from Ruby and I'm getting used to the "explicit over implicit" philosophy that Python follows, but I was confused earlier about how I actually make class methods. Now, I want verification that I am indeed correct in the following:

Every method in a class is essentially a class method in Python.

For example:

class Employee(object):
 employeeCount = 0
 def __init__(self, name):
 self.name = name
 employeeCount += 1
 def get_name(self):
 return self.name
 def get_employee_count():
 return employeeCount

If I understand this correct, for instance david = Employee("david") the following two are equivalent:

david.get_name() and Employee.get_name(david). Likewise, it makes sense that we could say get_employee_count is a class method too, but it doesn't rely on any instance, hence why we don't pass in an instance. This is why it doesn't make sense to type david.get_employee_count(), because this would be Employee.get_employee_count(david), but get_employee_count doesn't take in a parameter, namely the instance. This concludes that we would just type Employee.get_employee_count().

Am I correct in my thinking? Thank you.

• Claudiu: try actually running the code you pasted. it's not gonna work
• Ashwini Chaudhary: get_employee_count() is a not a class method, it's a normal function inside a class.(or static method in py2.x, if used with @staticmethod)
• David Robinson: More specifically, e = Employee("David"); print e.get_employee_count() won't work because get_employee_count doesn't know where employeeCount is
• Martijn Pieters: @hcwhsa: It is still a method when accessed, but is missing the required self parameter, so that'll raise an exception.
• Ashwini Chaudhary: @MartijnPieters Not in py3.x, if called as Employee.get_employee_count().

Answer 1

No, you're not quite right. For something to be a class method rather than an instance method, you have to use the classmethod decorator. And that method still takes a parameter, but that param is the class, not the instance. So:

@classmethod
def get_employee_count(cls):
 return cls.employeeCount

Classes don't actually define a block scope inside them in Python, so just referring to employeeCount in the method wouldn't work - that's why the cls param is used.

For clarification, a decorator is simply a wrapper around a function - it could also be written

def _get_employee_count(cls):
 return cls.employeeCount
get_employee_count = classmethod(_get_employee_count)

and classmethod is a built-in function.

Also note, in Python (as I thought in Ruby too) we wouldn't normally write accessors for simple variables like name, we'd just access them directly.

• David: Interesting. Then, in my example, what exactly would you call get_employee_count()?
• David: Furthermore, how would I create a class variable in Python? Evidently, employeeCount = 0 would generate an error.
• Daniel Roseman: Yes, that's exactly what it does.
• Daniel Roseman: You can access it through the instance, but if you modify it in any way (eg david.employeeCount += 1 you'll actually create an instance attribute that shadows the class one, and the changes won't reflect in the class variable. In __init__, you'll need to explicitly access the class: self.__class__.employeeCount += 1 should work.
• Bi Rico: You can also use type(self).employeeCount instead of self.__class__.employeeCount

Answer 2

True. Functions that deal with objects' instances should have self in its parameters. Otherwise, they are considered class methods.

EDIT

Class methods doesn't mean static methods! They're not static methods unless you add the decorator @staticmethod before the definition of the class function

• Martijn Pieters: No, they are not. You need to use the @classmethod decorator to make class methods.
• David: @MartijnPieters I have not heard of decorators in Python. Is it just used for labeling purposes?
• SonicARG: @David Decorators are snippets of code that sets special attributes for the code. EDIT More info is [here][1] [1]:wiki.python.org/moin/PythonDecorators