Question

Im trying to create a class with some formatting options. But i can’t figure out how to do it properly…

The code produced the following error:

AttributeError: ‘NoneType’ object has no attribute ‘valuesonly’

class Testings(object):

    def format_as_values_only(self,somedata):
        buildstring=somedata.values()
        return buildstring

    def format_as_keys_only(self):
        pass

    def format_as_numbers(self):
        pass


    def get_data_method(self):
        self.data= {'2_testkey':'2_testvalue',"2_testkey2":"2_testvalue2"}

        @property
        def valuesonly(self):
            return format_as_values_only(self.data)

test=Testings()
print test.get_data_method().valuesonly 

The important thing for me is to be able to get the formatters like: class.method.formatter or so…

Thanks a lot for any hints!

Answer 1

You can’t do things this way. Methods are just functions defined directly inside a class block. Your function is inside another function, so it’s not a method. The property decorator is useless except in a class block.

But, more fundamentally, function definitions just create local names, the same as variable assignments or anything else. Your valuesonly function is not accessible at all from outside the get_data_method function, because nothing from within a function is accessible except its return value. What you have done is no different than:

def get_data_method(self):
    a = 2

. . . and then expecting to be able to access the local variable a from outside the function. It won’t work. When you call get_data_method(), you get the value None, because get_data_method doesn’t return anything. Anything you subsequently do with the result of get_data_method() is just operating on that same None value.

If you want to access things using the syntax you describe, you will need to make get_data_method return an object that has properties like valuesonly. In other words, write another class that provides a valuesonly property, and have get_data_method return an instance of that class. A rough outline (untested):

class DataMethodGetter(object):
    def __init__(self, parent):
        self.parent = parent

    @property
    def valuesonly(self):
        return format_as_values_only(self.parent.data)

class Testings(object):
    # rest of class def here
    def get_data_method(self):
        self.data = {'blah': 'blah'}
        return DataMethodGetter(self)

However, you should think about why you want to do this. It’s likely to be simpler to set it up to just call valuesonly directly on the Testing object, or to pass a flag to get_data_method, doing something like get_data_method(valuesonly=True).