"Mike Meyer" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED]
PEP: XXX Title: A rational number module for Python Version: $Revision: 1.4 $ Last-Modified: $Date: 2003/09/22 04:51:50 $ Author: Mike Meyer <[EMAIL PROTECTED]> Status: Draft Type: Staqndards Content-Type: text/x-rst Created: 16-Dec-2004 Python-Version: 2.5 Post-History: 30-Aug-2002
Abstract ========
This PEP proposes a rational number module to add to the Python standard library.
[snip]
The ``Rational`` class shall define all the standard mathematical operations: addition, subtraction, multiplication, division, modulo and power. It will also provide the methods:
- max(*args): return the largest of a list of numbers and self. - min(*args): return the smallest of a list of numbers and self.
max() and min() are already part of the standard library. Providing them as instance methods is quite irregular.
- decimal(): return the decimal approximation to the rational in the
current context.
This ought to be the responsibility of the decimal() constructor. I can see including it here to avoid adding it to the decimal() constructor, but IMO it's bad design.
- inv(): Return the inverse of self.
I presume this means that if the rational is x/y, then it returns y/x?
Rationals will mix with all other numeric types. When combined with an integer type, that integer will be converted to a rational before the operation. When combined with a floating type - either complex or float - the rational will be converted to a floating approximation before the operation, and a float or complex will be returned. The reason for this is that floating point numbers - including complex - are already imprecise. To convert them to rational would give an incorrect impression that the results of the operation are precise. Decimals will be converted to rationals before the operation. [Open question: is this the right thing to do?]
I'd prefer to have rationals converted to decimals before the operation, for the same reason that they're converted to floats. Decimals also have limited precision.
Rationals can be converted to floats by float(rational), and to integers by int(rational).
If this is the case, then there is no reason to have a separate member operation to convert rationals to decimal. Consistency says to do it the same way for all similar operations.
...
John Roth
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