PEP: statements in control structures (Re: Conditional Expressions don't solve the problem)

Christopher A. Craig com-nospam at ccraig.org
Fri Oct 19 15:45:36 CEST 2001


huaiyu at gauss.almadan.ibm.com (Huaiyu Zhu) writes:

> Carl Banks gave some well-thought out objection, which I'd like to summarize
> them as:
> 
> (1) There may be many statements before the condition, resulting in long
>     while-lines. 
> (2) The else-clause for while is not necessary for while 1.
> 
> But then I was reminded of the examples I collected by searching for break
> statements in the source distribution.  Most usages are similar to
[example with a single assignment followed by a test]

I disagree.  I used zsh to do 

grep 'while 1' **/*.py 

in the standard library and then looked at the loops in the first few
files that came back and I found that

8 of those files had forms like 

# asyncore.py line 468
    while 1:
        tbinfo.append ((
            tb.tb_frame.f_code.co_filename,
            tb.tb_frame.f_code.co_name,
            str(tb.tb_lineno)
            ))
        tb = tb.tb_next
        if not tb:
            break

or

# code.py line 229
        while 1:
            try:
                if more:
                    prompt = sys.ps2
                else:
                    prompt = sys.ps1
                try:
                    line = self.raw_input(prompt)
                except EOFError:
                    self.write("\n")
                    break
                else:
                    more = self.push(line)
            except KeyboardInterrupt:
                self.write("\nKeyboardInterrupt\n")
                self.resetbuffer()
                more = 0

which do not translate well to your model, 3 have actual infinite
loops or loops that terminate by some means other than a break
(exception or return), 6 have loops that have a single statement
before the test, and 2 have a combination of these.  If this were
statistically significant (which it isn't as I am way too lazy to
write a parser to try a significant sample), it would imply that you
will fix somewhere around half of the instances of 'while 1:'
structures, at the cost of adding a structure to Python that requires
the use of semicolon delimited statements.

I will go ahead and state that I think the current 'while 1: if not'
loops are more readable than those your proposal, and that
iterators/generators solve the vast majority of problems that you have
raised, but I really only wanted to present this to make sure that the
Carl's first point does not get understated in your PEP (which I
believe you should finish, if for no other reason than hopefully
putting this argument to rest).


-- 
Christopher A. Craig <com-nospam at ccraig.org>




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