Python Global Constant

[Peter Hansen]

Christoph Becker-Freyseng wrote:
> 
> >>*** module any ***
> >>import dirs;
> >>def CheckDir(Dir):
> >>  if Dir=dirs.Const_Up: xxx
> >
> >
> > This code should work fine, although you might want to follow
> > the Python conventions ........
> 
> What about using __builtins__ wouldn't make this the constant really global?

A module is already global, in the sense that if you do "import xxx"
you will get a reference to a singleton module object (generally from
xxx.py), which provides a nice clean namespace for a set of names such
as constants.  

This makes it easy for someone reading the code to see where a constant
is defined, and it provides a necessary level of organization.

Putting something in __builtins__ is not only stylistically a bad
idea for most things, but it also hides the source of the name and
anyone reading the file will be at a loss to figure out where it 
came from unless every reference to it is preceded by a big comment::

   # WARNING!  The author of this code chose the silly approach
   # of storing a reference to this constant in the __builtins__
   # namespace, which is why you can't find where it is defined.
   # This reference was stored in the initialization code that
   # is in init.py, and instead of just leaving the name there
   # so you could call it "init.CONSTANT", it was slipped in
   # through the back door to let the author save five keystrokes,
   # at the expense of readability, but with this comment everything
   # will be okay.
   x = CONSTANT

How is that better than using the proper approach of storing constants
in a module, such as "constant.py"?  ;-)

-Peter