Why decimal.Decimal doesn't support arithmetic operations with float-data type?

Question

I am working on some scripts that handle calculation with the need of maintaining high accuracy of the results so I started using decimal.Decimal instead of built-in float.

I know it doesn't work with float-type values and it returns TypeError: unsupported operand type(s) for +: 'decimal.Decimal' and 'float' What I need to know exactly is why it doesn't support this type of operations with float.

Sample:

from decimal import Decimal

expr_= 1.2+Decimal('1')

As this seems logic for me when I try to add str to float to get this type of error but not while trying to calculate two numeric values.

While I can easily do the same thing using SymPy Float data-type without facing any issues. Ex:

from decimal import Decimal
from sympy import Float
expr_=Decimal('1.6')+Float(1.2)

Answer 1

PEP 327 -- Decimal Data Type explains:

Aahz is strongly opposed to interact with float, suggesting an explicit conversion:

The problem is that Decimal is capable of greater precision, accuracy, and range than float.

The example of the valid python expression, 35 + 1.1, seems to suggest that Decimal(35) + 1.1 should also be valid. However, a closer look shows that it only demonstrates the feasibility of integer to floating point conversions. Hence, the correct analog for decimal floating point is 35 + Decimal(1.1). Both coercions, int-to-float and int-to-Decimal, can be done without incurring representation error.

The question of how to coerce between binary and decimal floating point is more complex. I proposed allowing the interaction with float, making an exact conversion and raising ValueError if exceeds the precision in the current context (this is maybe too tricky, because for example with a precision of 9, Decimal(35) + 1.2 is OK but Decimal(35) + 1.1 raises an error).

This resulted to be too tricky. So tricky, that c.l.p agreed to raise TypeError in this case: you could not mix Decimal and float.

This is the python developer's mailing list discussion on the subject. For the full thread, see the 2003-10 archive and search for "Decimal".

So in the end, the "Explicit is better than implicit" philosophy won out.