mutually exclusive arguments to a constructor

[Arnaud Delobelle]

On 30 December 2011 20:40, Adam Funk <a24061 at ducksburg.com> wrote:
> (Warning: this question obviously reflects the fact that I am more
> accustomed to using Java than Python.)
>
> Suppose I'm creating a class that represents a bearing or azimuth,
> created either from a string of traditional bearing notation
> ("N24d30mE") or from a number indicating the angle in degrees as
> usually measured in trigonometry (65.5, measured counter-clockwise
> from the x-axis).  The class will have methods to return the same
> bearing in various formats.
>
> In Java, I would write two constructors, one taking a single String
> argument and one taking a single Double argument.  But in Python, a
> class can have only one __init__ method, although it can have a lot of
> optional arguments with default values.  What's the correct way to
> deal with a situation like the one I've outlined above?

(Using Python 3 below)

Method 1
----------
Your __init__ method could take the angle as an argument (which seems
the most natural to me).  Then you could have a class method that
takes the string

i.e.

class Bearing:
    def __init__(self, angle):
        self.angle = angle
        # or whatever your internal reprsentation is
    @classmethod
    def fromstring(cls, string):
        # Here, work out the angle from the string
        return cls(angle)

So you can do:

b = Bearing(65.5)

or

b = Bearing.fromstring("N24d30mE")

Method 2
----------

You can test the type of the argument of the __init__ method

class Bearing:
    def __init__(self, arg):
        if isinstance(arg, str):
            # here calculate the value of angle
        else:
            angle = float(angle)
        self.angle = angle

Now you can do:

b = Bearing(65.5)

or

b = Bearing("N24d30mE")

Both methods are used for builtin types:

>>> int('12')
12
>>> int(12.5)
12
>>> dict([(1, 2), (3, 4)])
{1: 2, 3: 4}
>>> dict.fromkeys([1, 2])
{1: None, 2: None}

HTH

-- 
Arnaud