Two mappings inverse to each other: f, g = biject()

[Jonathan Fine]

Nick Vatamaniuc wrote:

> If you need to get a short name, given a long name or vice-verse _and_
> the set of short names and long names is distinct (it would be
> confusing if it wasn't!) then you can just have one dictionary, no
> need to complicate things too much:
> f[a]=b
> f[b]=a
> You won't know which is a short and which is a long based just on
> this, so you need to keep track of it. But it will give you the
> mapping.

Thank you for this suggestion, Nick.  It's not
something I thought of.  And I'm sure that in some
cases it might be just the right thing.  It would
hold something like 'other-name' values.  (Every
cat should have at least two names ...)

But for my application, I think it complicates the
code that uses the bijection.

For example, I want to say:
   # Write the font dictionary to a file
   for key in font_dict:
       # write the font

   # Does value already exist in the font dictionary?
   # If not, add it to the font dictionary.
   key = font_dict.inverse.get(value)
   if key is None:
      key = len(font_dict)
      font_dict[key] = value

Following your suggestion, ingenious though it is,
would make the above code much more complicated and
error prone.

Perhaps it helps to think of
   f, g = biject()
as establishing a database, that has a consistency
condition, and which has two views.

There we are:  biject() gives two views on a
mapping (of a particular type).  Thank you for
you suggestion - it has clarified my thinking.

-- 
Jonathan