Could Emacs be rewritten in Python?

[Robin Munn]

Alex Martelli <aleax at aleax.it> wrote:
> Robin Munn wrote:
>    ...
>> 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)
>>
[snip...]
>> 
>> 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?
>
[snip...]
> 
> Another difference between the two snippets, which I don't
> understand, comes when function_blah_blah does its own rebinding
> of sys.stdout.  In that case, the first snippet "ignores" the
> rebinding (calls do_something_with with the value you had just
> bound to sys.stdout BEFORE calling function_blahblah), the
> second snippet "accepts" the rebinding (calls do_etc with the
> value CURRENTLY bound to sys.stdout). Since I don't know what's
> SUPPOSED to be happening under such circumstances, I can't tell
> which of the two versions (if either) is wrong, but I do find
> the difference puzzling.  Assuming the second hypothetical snippet
> has the correct semantics, then, maybe the first one should be:
> 
>     old_stdout = sys.stdout
>     sys.stdout = cStringIO.StringIO()
>     try:
>         function_whose_output_I_want_to_capture()
>         do_something_with(sys.stdout)
>     finally:
>         sys.stdout = old_stdout
> 
> to achieve the same semantics -- specifically, what we want
> to put in the try clause is just what would go in the with
> clause's body if it existed, while the "saving the previous
> state" (implicit in the hypothetical 'with') and the actual
> initialization (the assignment that follows the with keyword
> itself in the hypothetical case) need to be BEFORE the try
> clause.

You're right; using the new_stdout variable name was unnecessary and
would have rejected any sys.stdout rebinding that function_etc() might
have done.

I suppose I just illustrated how a try ... finally block might be
considered "broken and fragile". I tried to use it, and missed some
subtleties. I'm still not convinced that a "with"-like statement is
necessary in Python, but I think I just went from -1 to -0 on it.

-- 
Robin Munn <rmunn at pobox.com>
http://www.rmunn.com/
PGP key ID: 0x6AFB6838    50FF 2478 CFFB 081A 8338  54F7 845D ACFD 6AFB 6838