Michael McLay wrote:
PEP: 2XX Title: Adding a Decimal type to Python Version: $Revision:$ Author: mclay@nist.gov <mclay@nist.gov> Status: Draft Type: ?? Created: 25-Jul-2001 Python-Version: 2.2
Introduction
This PEP describes the addition of a decimal number type to Python.
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Implementation
The tokenizer will be modified to recognized number literals with a 'd' suffix and a decimal() function will be added to __builtins__.
How will you be able to define the precision of decimals ? Implicit by providing a decimal string with enough 0s to let the parser deduce the precision ? Explicit like so: decimal(12, 5) ? Also, what happens to the precision of the decimal object resulting from numeric operations ?
A decimal number can be used to represent integers and floating point numbers and decimal numbers can also be displayed using scientific notation. Examples of decimal numbers include:
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This proposal will also add an optional 'b' suffix to the representation of binary float type literals and binary int type literals.
Hmm, I don't quite grasp the need for the 'b'... numbers without any modifier will work the same way as they do now, right ?
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Expressions that mix binary floats with decimals introduce the possibility of unexpected results because the two number types use different internal representations for the same numerical value.
I'd rather have this explicit in the sense that you define which assumptions will be made and what issues arise (rounding, truncation, loss of precision, etc.).
The severity of this problem is dependent on the application domain. For applications that normally use binary numbers the error may not be important and the conversion should be done silently. For newbie programmers a warning should be issued so the newbie will be able to locate the source of a discrepancy between the expected results and the results that were achieved. For financial applications the mixing of floating point with binary numbers should raise an exception.
To accommodate the three possible usage models the python interpreter command line options will be used to set the level for warning and error messages. The three levels are:
promiscuous mode, -f or --promiscuous safe mode -s or --save pedantic mode -p or --pedantic
How about a generic option: --numerics:[loose|safe|pedantic] or -n:[l|s|p]
The default setting will be set to the safe setting. In safe mode mixing decimal and binary floats in a calculation will trigger a warning message.
>>> type(12.3d + 12.2b) Warning: the calculation mixes decimal numbers with binary floats <type 'decimal'>
In promiscuous mode warnings will be turned off.
>>> type(12.3d + 12.2b) <type 'decimal'>
In pedantic mode warning from safe mode will be turned into exceptions.
>>> type(12.3d + 12.2b) Traceback (innermost last): File "<stdin>", line 1, in ? TypeError: the calculation mixes decimal numbers with binary floats
Semantics of Decimal Numbers
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