Proposal: Magic Constants

[Raymond Hettinger]

[Chris Gonnerman]
> The gist is this:  Constants would be allowed to
> be trailed by any single alphabetic character.
> When compiled, a constant with a nonstandard
> letter code would be stored in the compiled code
> in the form of a text literal, appropriately
> marked.
>
> When the code is executed, and the special
> literal is processed, the interpreter would look
> up a conversion function in an internal list.  If
> no conversion is found, a runtime error would be
> raised.
>
> Modules to handle alternate numeric formats, such
> as decimal, fixed-point, rational, etc. would
> include a call to a builtin registration function
> which would allow them to "take over" a specific
> alphabetic specifier.

-1

This should not be done for several reasons:

* Constructors are more explicit:   Rational(12, 3)

* The appearance unpleasantly reminds me of VB

* There are potential conflicts between alphabetic specifiers which
   results in hard to spot bugs.  Doest "1.25D" represent the
   decimal class, Dewey decimals, or US dollars?

* Previous discussions on special syntaxes initially found them
  to be enticing and then someone would realize that real programs
  rarely have more than a handful of constants that would benefit
  from the syntax.  For instance, an accounting program is not
  filled with specific values like $1.83.  Instead, it constructs
  nearly all of its data from user input or files.  The program itself
  likely checks for zero and amounts being under $1.00.   Those
  constants are easily and explicitly codeable with a normal
  constructor:  Decimal("1.00", 2) or some such.

* The registration process by-passes Python's elegant namespaces
  and makes global changes resulting in hard to diagnose effects:

         import bookaccounting   # Hmm, did this just set D to DeweyDecimal?
         import Decimal               # Did this just reset the value of D or
was it F?
         . . .
         if foreign_currency:
               import Francs
         val = 15F                         # Was this the original F (for Fixed)
or the
                                                # new F (for Francs) ?
                                                 # Does it automatically convert
from Euros?

* Guido is already uncomfortable with the number of kinds of string literals:
   'abc', u'abc', ur'abc', r'abc', etc.   He is unlikely to adopt any related
ideas
   for numbers.

Other than that, it is a great idea ;-)


Raymond Hettinger