Re: problem with recursive container printing

Mark Hahn <[email protected]>:

>     class K:
>           def __str__(self):
>                 return 'hello world'
> 
>     k = K()
>     print k         # hello world
>     print [k]      # [<__main__.K instance at 0x008F9968>]
> 
> Prothon would print ['hello world'] of course.  Does anyone know why they do
> this?  Do you think it is to solve the recursive printing problem that I am
> chasing right now?

The reason this happens is that the str() implementations for
the built-in containers use repr(), not str(), on their
contents. Change __str__ to __repr__ above and you'll get the
recursive behaviour you want.

The reason it's done that way is that, otherwise, doing str() on a
list such as

  ["one, two", "buckle", "my shoe"]

would produce

  "[one, two, buckle, my shoe]"

which is highly misleading as to the number of items the original list
contained.

None of this has anything to do with handling self-containing
containers. To see what Python does in that situation, try this:

  a = [1, 2, 3]
  a[1] = a
  print a

As for exactly *how* it accomplishes that, you'll have to read the
code -- it's not something I've studied in detail.

Greg Ewing, Computer Science Dept, +--------------------------------------+
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