keeping a variable inside a range in python

Question

i am trying to write a code for the game pong but i am facing a problem when trying to control the range of the paddles positions , the question is that : is there a way in python to keep a variable inside a certain range (with a maximum value and minimum value )that when the variable changes (to be increasing ) it will stuck on the maximum value of that range , and when this variable decreases it will stuck on the minimum value ? .

i had written this code :

Range = range(HALF_PAD_HEIGHT, HEIGHT - HALF_PAD_HEIGHT) 
if (paddle1_pos[1] in Range) and (paddle2_pos[1] in Range):    
      paddle1_pos[1] += paddle1_vel[1]
      paddle2_pos[1] += paddle2_vel[1]  

when the values of the paddles positions (paddle1_pos[1] and paddle2_pos[1] ) are going out off the range i am not able to update its position any more using the keyboard ( through the variables (paddle1_vel[1] and paddle2_val[2]) so , i am thinking that maybe exist something in python that allow me to update paddle_pos and when i reach one side of the range it keeps me on that side till i reverse the direction of updating . hopefully the question is clear .

thanks

Answer 1

You could define your own "bounded" numeric type. For example if paddle1_pos[1] was an integer value you could create a class like the following and use it instead

class BoundedInt(int):
    def __new__(cls, *args, **kwargs):
        lower, upper = bounds = kwargs.pop('bounds')

        val = int.__new__(cls, *args, **kwargs)  # supports all int() args
        val = lower if val < lower else upper if val > upper else val

        val = super(BoundedInt, cls).__new__(cls, val)
        val._bounds = bounds
        return val

    def __add__(self, other):
        return BoundedInt(int(self)+other, bounds=self._bounds)
    __iadd__ = __add__

    def __sub__(self, other):
        return BoundedInt(int(self)-other, bounds=self._bounds)
    __isub__ = __sub__

    def __mul__(self, other):
        return BoundedInt(int(self)*other, bounds=self._bounds)
    __imul__ = __mul__

    # etc, etc...

if __name__ == '__main__':
    v = BoundedInt(100, bounds=(0, 100))
    print type(v), v
    v += 10
    print type(v), v
    w = v + 10
    print type(w), w
    x = v - 110
    print type(x), x

Output:

<class '__main__.BoundedInt'> 100
<class '__main__.BoundedInt'> 100
<class '__main__.BoundedInt'> 100
<class '__main__.BoundedInt'> 0

Answer 2

For completeness here's another answer which shows how to programmatically add all the arithmetic methods that integers have to the custom class using a metaclass. Note it's unclear whether it makes sense in every case to return a BoundedInt with the same bounds as the operand. The code is also compatible with both Python 2 & 3.

class MetaBoundedInt(type):
    # int arithmetic methods that return an int
    _specials = ('abs add and div floordiv invert lshift mod mul neg or pos '
                 'pow radd rand rdiv rfloordiv rlshift rmod rmul ror rpow '
                 'rrshift rshift rsub rtruediv rxor sub truediv xor').split()
    _ops = set('__%s__' % name for name in _specials)

    def __new__(cls, name, bases, attrs):
        classobj = type.__new__(cls, name, bases, attrs)
        # create wrappers for specified arithmetic operations
        for name, meth in ((n, m) for n, m in vars(int).items() if n in cls._ops):
            setattr(classobj, name, cls._WrappedMethod(cls, meth))
        return classobj

    class _WrappedMethod(object):
        def __init__(self, cls, func):
            self.cls, self.func = cls, func

        def __get__(self, obj, cls=None):
            def wrapper(*args, **kwargs):
                # convert result of calling self.func() to cls instance
                return cls(self.func(obj, *args, **kwargs), bounds=obj._bounds)
            for attr in '__module__', '__name__', '__doc__':
                setattr(wrapper, attr, getattr(self.func, attr, None))
            return wrapper

def with_metaclass(meta, *bases):
    """ Py 2 & 3 compatible way to specifiy a metaclass. """
    return meta("NewBase", bases, {})

class BoundedInt(with_metaclass(MetaBoundedInt, int)):
    def __new__(cls, *args, **kwargs):
        lower, upper = bounds = kwargs.pop('bounds')
        val = int.__new__(cls, *args, **kwargs)  # supports all int() args
        val = super(BoundedInt, cls).__new__(cls, min(max(lower, val), upper))
        val._bounds = bounds
        return val

if __name__ == '__main__':
    # all results should be BoundInt instances with values within bounds
    v = BoundedInt('64', 16, bounds=(0, 100))  # 0x64 == 100
    print('type(v)={}, value={}, bounds={}'.format(type(v).__name__, v, v._bounds))
    v += 10
    print('type(v)={}, value={}, bounds={}'.format(type(v).__name__, v, v._bounds))
    w = v + 10
    print('type(w)={}, value={}, bounds={}'.format(type(w).__name__, w, w._bounds))
    x = v - 110
    print('type(x)={}, value={}, bounds={}'.format(type(x).__name__, x, x._bounds))

Output:

type(v)=BoundedInt, value=100, bounds=(0, 100)
type(v)=BoundedInt, value=100, bounds=(0, 100)
type(w)=BoundedInt, value=100, bounds=(0, 100)
type(x)=BoundedInt, value=0, bounds=(0, 100)