使用双星号（**）实现属性

Question

Following http://www.programiz.com/python-programming/property , I'm familiar with the following method of implementing a property in Python:

class Celsius(object):
    def __init__(self, temperature = 0):
        self._temperature = temperature

    def get_temperature(self):
        print("Getting value")
        return self._temperature

    def set_temperature(self, value):
        if value < -273:
            raise ValueError("Temperature below -273 is not possible")
        print("Setting value")
        self._temperature = value

    temperature = property(get_temperature, set_temperature)

I'm now reading some code in which the double asterisk ( ** ) is used in defining the property. If I adapt it to my own example, it seems to be like this:

class Celsius(object):
    def __init__(self, temperature = 0):
        self._temperature = temperature

    def temperature():
        def fget(self):
            print("Getting value")
            return self._temperature
        def fset(self, value):
            if value < -273:
                raise ValueError("Temperature below -273 is not possible")
            print("Setting value")
            self._temperature = value

    temperature = property(**temperature())

However, if I try to run this code and instantiate a class through c = Celsius() , I get

TypeError: type object argument after ** must be a mapping, not NoneType

I understand that fget and fset are keyword arguments of property , so I would expect what comes out of **temperature() to be something like fget=fget, fset=fset , but I am not sure what is going wrong here or how to dissect it further. Any ideas?

Answer 1

You should return the nested functions and then unpack them according from the function call:

class Celsius(object):
    def __init__(self, temperature = 0):
        self._temperature = temperature

    def temperature():
        ...
        return fget, fset

    temperature = property(*temperature())

If you insist on using ** , then you'll be returning a mapping/dictionary from your function which will then be unpacked as keyword arguments.

def temperature():
    ...
    return {'fget': fget, 'fset': fset}

temperature = property(**temperature())

Answer 2

You are getting that error because the temperature() call returns None , which you then attempt to unpack as if it were a dictionary, using ** . So to get that code working you need to return an appropriate dictionary containing the setter & getter methods.

I prefer Moses' solution, but you could pass the methods with a dict instead of a tuple if you really want. Eg,

class Celsius(object):
    def __init__(self, temperature = 0):
        self._temperature = temperature

    def temperature():
        def fget(self):
            print("Getting value")
            return self._temperature
        def fset(self, value):
            if value < -273:
                raise ValueError("Temperature below -273 is not possible")
            print("Setting value")
            self._temperature = value
        return {'fget': fget, 'fset': fset}

    temperature = property(**temperature())

# Test
a = Celsius()
a.temperature = 20
print(a.temperature)

output

Setting value
Getting value
20

Answer 3

Indeed I had missed the return statement in the function definition of temperature . The original code actually contains the line

return locals()

Adding this line makes the property work as intended.

Answer 4

The temperature method you are defining does not have a return statement, so it implicitly returns None , this the ...,not NoneType in your error message. The method does have two sub-methods, but that does not cause it to return them in any form, I think you are missing a line such as ( untested ):

return { "fget": fget, "fset": fset }

You could also use a list and a single * .

That being said, it's a best practice to avoid * and ** magic, there's also a pylint rule to discourage it.