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Loop-and-a-half (Re: Curious assignment behaviour)

Huaiyu Zhu huaiyu at gauss.almadan.ibm.com
Thu Oct 11 17:58:35 EDT 2001 
On Thu, 11 Oct 2001 13:21:57 +1300, Greg Ewing <greg at cosc.canterbury.ac.nz>
wrote: 

>I feel the solution to this is *not* to go in for
>any sort of assignment-in-expressions hackery, but
>to provide a decent loop-and-a-half control structure.

A general "loop-and-half" structure is

    A
    start loop
    B
    if C: break
    D
    end loop
    E
    
Which could be written in the following pattern

    A
    while B; C:
       D
    E

Example 1: 

>  while:
>    x = get_next()
>  gives x:
>    whatever(x)

could be written as

    while x = get_next(); x:
       whatever(x)

Example 2:

> What if you want to read until you get to a delimiter?
> 
>   while (line := readline()) != 'end': ...

could be written as

    while line = readline(); line != 'end': ...

Example 3:  

    for (start; do_other, end; incr) do_something;

could be written as

    start   
    while do_other; not end:
        do_something
        incr

Likewise, a general nested "if-else-and-a-half" structure is like

    A
    if B:
        C
    else:
        D
        if E:
            F
        else:
            G
            if H:
                I
            else:
                ...

which could be written as

    if A; B:
        C
    elif D; E:
        F
    elif G; H:
        I
    else:
        ...

The advantage of this syntax pattern is that it is flatter than existing
Python syntax, which is a Good Thing <tm>.  The disadvantage is that it
cannot handle additional statement after nested 'else', but the existing
'elif' pattern could not do it, either.


Huaiyu
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