Question

I am reading the Python Cookbook and they have this program there:

class Temperature(object):
    coefficients = {'c': (1.0, 0.0, -273.15), 'f': (1.8, -273.15, 32.0),
                    'r': (1.8, 0.0, 0.0)}


    def __init__(self, **kwargs):
        # default to absolute (Kelvin) 0, but allow one named argument,
        # with name being k, c, f or r, to use any of the scales
        try:
            name, value = kwargs.popitem( )
        except KeyError:
            # no arguments, so default to k=0
            name, value = 'k', 0
        # error if there are more arguments, or the arg's name is unknown
        if kwargs or name not in 'kcfr':
            kwargs[name] = value             # put it back for diagnosis
            raise TypeError, 'invalid arguments %r' % kwargs
        setattr(self, name, float(value))


    def __getattr__(self, name):
        # maps getting of c, f, r, to computation from k
        try:
            eq = self.coefficients[name]
        except KeyError:
            # unknown name, give error message
            raise AttributeError, name
        return (self.k + eq[1]) * eq[0] + eq[2]


    def __setattr__(self, name, value):
        # maps settings of k, c, f, r, to setting of k; forbids others
        if name in self.coefficients:
            # name is c, f or r -- compute and set k
            eq = self.coefficients[name]
            self.k = (value - eq[2]) / eq[0] - eq[1]
        elif name == 'k':
            # name is k, just set it
            object.__setattr__(self, name, value)
        else:
            # unknown name, give error message
            raise AttributeError, name

    def __str__(self):
        # readable, concise representation as string
        return "%s K" % self.k


    def __repr__(self):
        # detailed, precise representation as string
        return "Temperature(k=%r)" % self.k

I didn’t understand the following – could someone help me to do so?:

1. what does this function do name, value = kwargs.popitem( )
2. What does __getattr__ and __setattr__ do. He didn’t used these in the final calling of the program

This was the ouput:

>>> from te import Temperature
>>> t = Temperature(f=70)        # 70 F is...
>>> print t.c                    # ...a bit over 21 C
21.1111111111
>>> t.c = 23                     # 23 C is...
>>> print t.f                    # ...a bit over 73 F
73.4

Answer 1

• The kwargs.popitem() method removes one (arbitrary) item from the kwargs dictionary and returns that as a (key, value) tuple. This is then assigned to two variables, name and value.

  In this case it means the class takes one keyword argument (one of k, f, c or r) and if you specify more than one it’ll complain (throw a TypeError) after having looked at one of the keyword arguments you passed in.

• __getattr__ and __setattr__ are special methods used by python when looking up attributes.

  t.c translates to t.__getattr__('c'), and t.c = 23 translates to t.__setattr__('c', 23).

  So, setting one of t.c, t.f or t.r to an integer is routed to the __setattr__ method, which then uses the self.coefficients mapping to calculate and set self.k instead.

  Looking up one of t.c, t.f or t.r is routed to the __getattr__ method, which then returns a value based on the self.coefficients mapping together with the existing value of self.k to give you a temperature in the requested scale.