calling private methods for class method: python

Question

I am trying to implement multiple constructors in python and one of the suggestions (through online searching) was to use the classmethod. However, using this, I am having issues with code reuse and modularity. Here is an example where I can create an object based on a supplied file or through some other means:

class Image:

    def __init__(self, filename):
         self.image = lib.load(filename)
         self.init_others()

    @classmethod
    def from_data(cls, data, header):
        cls.image = lib.from_data(data, header)
        cls.init_others()
        return cos

    def init_others(self):
        # initialise some other variables
        self.something = numpy.matrix(4,4)

Now it seems that I cannot do that. The cls.init_others() call fails by saying that I have not provided the object to call it on. I guess I can initialise things in the from_data function itself but then I repeat the code in the init method and the other "constructors". Does anyone know how I can call these other initialiser methods from these @classmethod marked functions? Or perhaps someone knows a better way to initialise these variables.

I come from a C++ background. So still trying to find my way around the python constructs.

Your alternate conductor isn't creating an instance, but modifying the class. — kindall
– kindall, Commented Jul 25, 2014 at 0:12
No, because Image.init_others is an instance method, not a class method. You have decorated the Image.from_data method as a class method; this means that the function can be called on both instances of Image and the class Image itself, but the first argument will always be the class. — Brett Lempereur
– Brett Lempereur, Commented Jul 25, 2014 at 0:14

jonrsharpe · Accepted Answer · 2014-07-25 00:32:25Z

2

Your class method should create and return a new instance of the class, not assign class attributes and return the class itself. As an alternative to the keyword arguments, you could do something like:

class Image:

    def __init__(self, image):
        self.image = image
        self.init_others()

    @classmethod
    def from_data(cls, data, header):
        return cls(lib.from_data(data, header))

    @classmethod
    def from_filename(cls, filename):
        return cls(lib.load(filename))

    def init_others(self):
        # initialise some other variables
        self.something = numpy.matrix(4, 4)

This adds the ability to create an instance if you already have the image, too.

edited Jul 25, 2014 at 0:32

answered Jul 25, 2014 at 0:21

jonrsharpe

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dano · Accepted Answer · 2014-07-25 00:14:36Z

2

I would recommend not trying to create multiple constructors, and use keyword arguments instead:

class Image(object):
    def __init__(self, filename=None, data=None, header=None):
         if filename is not None:
             self.image = lib.load(filename)
         elif data is not None and header is not None:
             self.image = lib.from_data(data, header)
         else:
             raise ValueError("You must provide filename or both data and header")
         self.init_others()

    def init_others(self):
        # initialise some other variables
        self.something = numpy.matrix(4,4)

This is a more Pythonic way to handle this scenario.

answered Jul 25, 2014 at 0:14

dano

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TheSoundDefense · Accepted Answer · 2014-07-25 00:11:05Z

1

You should always pass in self as the first argument to any method that will act on a class instance. Python will not automatically determine the instance you're trying to call the method for unless you do that. So if you want to use a class function like

the_image = Image("file.txt")
the_image.interpolate(foo,bar)

You need to define the method within Image as

def interpolate(self,foo,bar):
  # Your code

answered Jul 25, 2014 at 0:11

TheSoundDefense

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1 Comment

Luca Over a year ago

Sorry that was a type. I do pass self. Edited it now.

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