[Tutor] getting started

[Magnus Lycka]

At 19:54 2002-11-15 +0100, Gregor Lingl wrote:
>alan.gauld@bt.com schrieb:
>
>>>If you are really desperate to run a few lines at once
>>>in interactive mode, you can do this with a little trick:
>>>
>>> >>> if 1 == 1:
>>>         print "aaa"
>>>         print "bbb"
>>>         print "ccc"
>>
>>I like it Magnus! I hadn't thought of that before...

:) Even if Python is easy to learn, we can always learn more
things. There's no end to the fun!

I've seen the "if 1:" or "if 0:" construct here and there in
code to make it easy to switch blocks in and out during testing
and debugging. An alternative to commenting out code I guess.

>Me too! But why not
>
> >>> if 1:
>               print "aaa"
>               print "bbb"

Actually, this is what I would do myself, but it seemed more
obvious that "if 1==1:" has to be true every time. "if 1:"
assumes that you understand Python's notion about true and
false, and I didn't want to explain too many concepts at the
same time. (I did mention it in the end of the mail though...)

While (heh?) we're at it, it's a common Python idiom to
code like this:

while 1:
     bla
     bla
     if some_condition:
         break
     maybe more bla bla bla

This corresponds more or less to

do {
    bla;
} while condition;

in C and derivates or

repeat
     bla;
     bla;
until condition;

in Pascal and its derivates.

It is sometimes argued that Python should have such a construct,
so that we could stop this "while 1:" lie. (We don't intend to run
the loop for ever, so why claim that?) I think the reason that
this hasn't happened is that it can't be done in a pretty way in
Python. The clear and consistent block structure by indentaion
would be broken if we did:

do:
     bla
     bla
while x

since that last line shouldn't be in the loop. And if we did

do:
     bla
     bla
     while x

we would also get confusing and ugly code. First of all, the
while statement doesn't stick out much. But secondly, you could
also want to do:

do:
     bla
     bla
     while x:
        foo()
        bar()
     while y

Now it starts to look wierd, and if you happen to forget the
colon after the x, you will probably get a very confusing
error message from the poor interpreter, which thinks that
the indentation is incorrect in the "foo()" line.

I bet that a proposal to introduce either of these syntaxes
would have a more or less equal following of people who would
prefer it the other way around. I also think that the majority
would agree that this very disagreement shows us that do/while
doesn't really fit smoothly into Python.

Conceptually, it's a typical feature of Python that block just
end, without any particular ta-da in the end. No "}" and no
"end;". The do/while or repeat/until idiom seems to go against
that.


-- 
Magnus Lycka, Thinkware AB
Alvans vag 99, SE-907 50 UMEA, SWEDEN
phone: int+46 70 582 80 65, fax: int+46 70 612 80 65
http://www.thinkware.se/  mailto:magnus@thinkware.se