Why the copy?  Python is pass by value so even if `start` is a decimal object it won't be modified:

```python
from decimal import Decimal
the_decimal_start = Decimal("1.0")
the_integer_start = 1
decimal = the_decimal_start
decimal += 1
integer = the_integer_start
integer += 1
print(the_decimal_start, decimal)  # 1.0 2.0
print(the_integer_start, integer)  # 1 2
```