Enforce precision in decimal python

Question

In some environments, exact decimals (numerics, numbers...) are defined with scale and precision , with scale being all significant numbers, and precision being those right of the decimal point. I want to use python's decimal implementation to raise an error, if the precision of the casted string is higher than the one defined by the implementation.

So for example, I have an environment, where scale = 4 and precision = 2 . How can I achieve these commands to raise an error, because their precision exceeds that of the implementation?

decimals.Decimal('1234.1')
decimals.Decimal('0.123')

Answer 1

The closest I could find in the decimal module is in the context.create_decimal_from_float example, using the Inexact context trap :

>>> context = Context(prec=5, rounding=ROUND_DOWN)
>>> context.create_decimal_from_float(math.pi)
Decimal('3.1415')
>>> context = Context(prec=5, traps=[Inexact])
>>> context.create_decimal_from_float(math.pi)
Traceback (most recent call last):
    ...
Inexact: None

The decimal module doesn't seem to have the concept of scale. It's precision is basically your scale + your precision.

Answer 2

You could always define a stricter class of Decimal and check the number of decimals in the setter and constructor functions. This should act exactly like the Decimal object, except that it'll raise a ValueError when its created or set with more than two decimals.

class StrictDecimal:
    def __init__(self, x):
        x = Decimal(x)
        if format(x, '.2f') != str(x):
            raise ValueError('Precision must by limited to 2 digits')
        self._x = x

    @property
    def x(self):
        return self._x

    @x.setter
    def x(self, x):
        x = Decimal(x)
        if format(x, '.2f') != str(x):
            raise ValueError('Precision must by limited to 2 digits')
        self._x = x