So, I found the answer directly in the source code of cpython/numbers.py
:
## Notes on Decimal
## ----------------
## Decimal has all of the methods specified by the Real abc, but it should
## not be registered as a Real because decimals do not interoperate with
## binary floats (i.e. Decimal('3.14') + 2.71828 is undefined). But,
## abstract reals are expected to interoperate (i.e. R1 + R2 should be
## expected to work if R1 and R2 are both Reals).
Indeed, adding Decimal
to float
would raise a TypeError
.
In my point of view, it violates the principle of least astonishment, but it does not matter much.
As a workaround, I use:
import numbers
import decimal
Real = (numbers.Real, decimal.Decimal)
print(isinstance(decimal.Decimal(1), Real))
# True
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