Could Emacs be rewritten in Python?

Christian Tanzer tanzer at
Mon Apr 14 19:01:41 CEST 2003

Robin Munn <rmunn at> wrote:

> Paul Foley <see at below.invalid> wrote:
> > I.e., write broken and fragile code just to avoid a perfectly good
> > construct that you happen to dislike for religious reasons.
> > Brilliant!
> How is try ... finally "broken and fragile code"? How is this:
>     try:
>         old_stdout = sys.stdout
>         new_stdout = cStringIO.StringIO()
>         sys.stdout = new_stdout
>         function_whose_output_I_want_to_capture()
>         do_something_with(new_stdout)
>     finally:
>         sys.stdout = old_stdout
> in any way inferior to this (making up a semi-Pythonic syntax):
>     with sys.stdout = cStringIO.StringIO():
>         function_whose_output_I_want_to_capture()
>         do_something_with(sys.stdout)
> Yes, sample #2 is shorter. But it would require adding a new keyword to
> the language, which is a pretty heavy price to pay. And sample #2
> doesn't need a temporary variable -- but sample #1's temporary variable
> is a function-local variable, so speed and namespace-clutter costs are
> minimal.
> What I really want to know is why you would consider sample #1 "broken
> and fragile code". I can't think of any circumstances where sample #1
> would fail to achieve its desired effect. Can you?

Disclaimer: I'm not arguing for adding dynamic scopes or variable
declarations to Python <wink>.

I see three problems with try/finally for temporary rebindings of
multiple variables:

- It is fragile for multiple rebindings unless you save all the old
  values before the try.

- It is easy to forget/inadvertantly delete some restore operations in
  the finally clause.

- It destroys locality of reference for the human reader. Your toy
  example needs eight try/finally lines vs. three lines for the
  alternative solution. For multiple rebindings, the ratio gets worse.

Still-greatly-preferring-Python-over-Lisp-ly yr's,

Christian Tanzer                                         tanzer at
Glasauergasse 32                                       Tel: +43 1 876 62 36
A-1130 Vienna, Austria                                 Fax: +43 1 877 66 92

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