[Tutor] Unexpected result from decimal

[Kent Johnson]

Jacob S. wrote:
> Okay, so how do I get decimal to set precision of *significant digits*?

That is exactly what it is doing. getcontext().prec sets how many digits are used to represent the 
mantissa of the number.

Have you taken any physics classes? What is 1000/7 to two significant digits? It is 140, not 142.85, 
which has *5* significant digits. Changing prec has just this effect:
  >>> from decimal import *
  >>> getcontext().prec = 2
  >>> one = Decimal(1)
  >>> seven = Decimal(7)
  >>> one/seven
Decimal("0.14")
  >>> 1000 * one/seven
Decimal("1.4E+2")
  >>> getcontext().prec = 20
  >>> one/seven
Decimal("0.14285714285714285714")
  >>> 1000 * one/seven
Decimal("142.85714285714285714")

> Why have a decimal.getcontext().prec if it doesn't provide a useful result?
> The number of digits in a number is irrelevant to that numbers value.
> It just doesn't make sense to me.

It's just like choosing between float and double in Java or C - it sets the precision of the 
underlying representation. It is a tradeoff between accuracy, speed, and memory use.

Kent