The use of :

[Chang LI]

Some statements use : in the tail such as while x > 0: and def func():
What is the meaning and the usage of : in Python?

[Erik Max Francis]

Chang LI wrote:

Some statements use : in the tail such as while x > 0: and def func():
What is the meaning and the usage of : in Python?

It means something else follows in an indented block as part of the same
statement.

--
Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 20 N 121 53 W && AIM erikmaxfrancis
To perceive is to suffer.
-- Aristotle

[Paul Robson]

On Thu, 25 Nov 2004 18:58:16 -0800, Chang LI wrote:

Some statements use : in the tail such as while x > 0: and def func():
What is the meaning and the usage of : in Python?

It, together with the indent, identifies a block of code. Most languages
use either a terminating keyword like "WEND" "ENDIF" "NEXT" to identify
the end of a group of instructions, or alternatively a bracketing syntax
such as {} begin..end or $( $)

So these are roughly equivalent. The difference with the Python version
(the last one) is without the indentation it won't work - the indentation
does the job of WEND, { } and begin .. end

WHILE I > 0
PRINT I
I = I-1
WEND

while (i > 0)
{
printf("%d\n",i);
i = i-1;
}

while i > 0 do
begin
writeln i;
i := i - 1;
end;

while i > 0:
print i
i = i - 1

this code, unlike most other languages is different to the one above.
Changing the indentation actually changes what the code does.

while i > 0:
print i
i = i - 1

it is equivalent to C

while (i > 0)
{
printf("%d\n",i);
}
i = i-1;

[Chang LI]

Paul Robson <au******@autismuk.muralichucks.freeserve.co.uk> wrote in message news:<pa****************************@autismuk.mura lichucks.freeserve.co.uk>...

while i > 0 do
begin
writeln i;
i := i - 1;
end;

while i > 0:
print i
i = i - 1

So the : is similar to "begin" and the last space line is similar to
"end", right? How about

while i > 0 :
print i
i = i-1

[Dave Anderson]

this is answered in the python faq:

http://www.python.org/doc/faq/genera...ass-statements

The colon is required primarily to enhance readability

Chang LI wrote: Some statements use : in the tail such as while x > 0: and def func():
What is the meaning and the usage of : in Python?

[Timo Virkkala]

Chang LI wrote:

Paul Robson <au******@autismuk.muralichucks.freeserve.co.uk> wrote in message news:<pa****************************@autismuk.mura lichucks.freeserve.co.uk>...
So the : is similar to "begin"
Correct
and the last space line is similar to "end", right?
Nope. You don't need a space line (you mean an empty line by that, right?),
you can just outdent one level and continue without any empty lines
(although in the interactive interpreter you need the empty line to end the
block on the first indentation level). So you could do:

i = 5
while i > 0:
print i
i = i-1
print "That's it"
How about
while i > 0 :
print i
i = i-1

Works just fine. Other than the indentation, spacing is pretty much
irrelevant in Python.

--
Timo Virkkala

[Nick Coghlan]

Dave Anderson wrote:

this is answered in the python faq:

http://www.python.org/doc/faq/genera...ass-statements

The colon is required primarily to enhance readability

It also allows 'suiteless' versions of some statements:

if x > 0: print "Greater that 0"
while x > 0: x -= 1
def f(): pass
class c(object): pass
for x in range(4): print x
Actually using that feature outside sample code is generally bad karma, though ;)

Cheers,
Nick.

[bruno modulix]

Timo Virkkala a écrit :

Chang LI wrote:

Paul Robson <au******@autismuk.muralichucks.freeserve.co.uk> wrote in
message
news:<pa****************************@autismuk.mura lichucks.freeserve.co.uk>...

So the : is similar to "begin"

Correct

Not correct. You need both the ':' and the extra level of indentation on
next line, ie:

Python 2.3.3 (#2, Feb 17 2004, 11:45:40)

while True: .... print "aha"
File "<stdin>", line 2
print "aha"
^
IndentationError: expected an indented block

and the last space line is similar to "end", right?

Nope. You don't need a space line (you mean an empty line by that,
right?), you can just outdent one level and continue without any empty
lines (although in the interactive interpreter you need the empty line
to end the block on the first indentation level). So you could do:

i = 5
while i > 0:
print i
i = i-1
print "That's it"

<meta> Is this a pb when typing of copying ? It's just plain wonrg code
anyway : while i > 0: .... print i
.... i -= 1
File "<stdin>", line 3
i = i-1
^
SyntaxError: invalid syntax while i > 0: .... print i
.... i -= 1
....
5
4
3
2
1

The level of indentation has to be the same within a block, and it has
to be *1* (*one*) extra-or-less level of indentation than the previous
block.

<please-someone-correct-me-if-i-am-wrong>
In fact, from a purely technical POV, the ':' could have been omitted
from the Python syntax, since indentation does the whole job of defining
blocks. It's only here for readability AFAIK.
</please-someone-correct-me-if-i-am-wrong>

Bruno

[Reinhold Birkenfeld]

bruno modulix wrote:

<please-someone-correct-me-if-i-am-wrong>
In fact, from a purely technical POV, the ':' could have been omitted
from the Python syntax, since indentation does the whole job of defining
blocks. It's only here for readability AFAIK.
</please-someone-correct-me-if-i-am-wrong>

Without the ':', single-line suites are impossible. If you allow only
multi-line suites, you're right.

Reinhold

--
[Windows ist wie] die Bahn: Man muss sich um nichts kuemmern, zahlt fuer
jede Kleinigkeit einen Aufpreis, der Service ist mies, Fremde koennen
jederzeit einsteigen, es ist unflexibel und zu allen anderen Verkehrs-
mitteln inkompatibel. -- Florian Diesch in dcoulm

[gabriele renzi]

Reinhold Birkenfeld ha scritto:

Without the ':', single-line suites are impossible. If you allow only
multi-line suites, you're right.

are they really?
if <expression> <expressions>
and the likes would invho parse just fine

[Jeremy Bowers]

On Sun, 28 Nov 2004 19:17:52 +0000, gabriele renzi wrote:

are they really?
if <expression> <expressions>
and the likes would invho parse just fine

if "" "a" print "Hi"

Does that print Hi or not?

Dig deeper into Python grammar; make sure you know that statements and
expressions are different, and the counter-example above is based on
Python's string concatenation rules:

Python 2.3.4 (#1, Oct 26 2004, 20:13:42)
[GCC 3.4.2 (Gentoo Linux 3.4.2-r2, ssp-3.4.1-1, pie-8.7.6.5)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

"a" "b"

'ab'
The question is, what is the "<expression>", "" or "" "a"? The first is
false, the second true.

(At this point the natural response of many people is to start adding
rules and exceptions and "well, obviously"s... but none of them will beat
if <expression>:, or justify the removal of a colon if you compare back
to the original; that's the *real* competition. This is just a pre-emptive
point 'cause I've seen people do this sort of thing too often, it isn't
targetted directly at you, gabriele.)

A completely new grammar could certainly do away with it and there are
languages that can do that, but you almost certainly won't be able to get
there from here via incremental changes. (Forth is close but it is spelled
differently. Lisp natually encapsulates all expressions; I don't think it
quite captures the spirit of what I think you're getting at, but for the
Lisp definition of "expression", it works.)

[BJörn Lindqvist]

In the faq, http://www.python.org/doc/faq/genera...ass-statements,
it is stated that the colon is there "primarily to enhance readability
(one of the results of the experimental ABC language)." But can that
statement really be backed up? Has someone made a study or something?
I always thought the rule was "the less useless symbols, the higher
the readability." I.e:

if (a == b)
{
print a
}

is less readable than:

if a == b:
print a

Because it contains more non-significant symbols (, ), { and } that
"steal" the programmers attention. But consider

def f(x, y, z)
print x, y, z

to

def f(x, y, z):
print x, y, z

IMHO, the colon-less variant is more readable than the one with the colon.

--
mvh Björn

[Jeremy Bowers]

On Sun, 28 Nov 2004 22:10:31 +0100, BJörn Lindqvist wrote:

I always thought the rule was "the less useless symbols, the higher the
readability." I.e:

There's a bit of circularity there, in that a symbol's use can *be* to
enhance readability. I'd certainly agree that instances where this holds
true is an exception rather than the rule, but it seems plausible to me
that Python is symbol-free enough for this to be a potential
justification.

The only punctuation you *need* is whitespace. See Forth (I don't know
if this is perfect but I'd bet the transform is simple), the HP RPN
calculator programming languages (IIRC, the only other punctuation is a
block delimiter, which would also be easy to transform away), or some
hacks I've cooked up for my own use where I minimized the token count
because I had to write the parser by hand and I was willing to trade a
little extra work to write the parsed stuff for an easier translation.
(Usually it is data, not programs, so while saying that's a "compilation"
is technically true it isn't what most people think of.) Take it to those
extremes and you'd see what I mean; extra symbols can help.

(Or, God help you, see the whitespace language for a demonstration that
all you need is whitespace, period.

http://compsoc.dur.ac.uk/whitespace/ )

[Greg Ewing]

Jeremy Bowers wrote:

The only punctuation you *need* is whitespace. See Forth

You don't even need that... see FORTRAN. :-)

DOI=1TO10-ly,

--
Greg Ewing, Computer Science Dept,
University of Canterbury,
Christchurch, New Zealand
http://www.cosc.canterbury.ac.nz/~greg

[Isaac To]

>>>>> "Greg" == Greg Ewing <gr**@cosc.canterbury.ac.nz> writes:

The only punctuation you *need* is whitespace. See Forth

Greg> You don't even need that... see FORTRAN. :-)

And you don't need everything else either... see this.

http://compsoc.dur.ac.uk/whitespace/

:-)

Regards,
Isaac.

[Jeremy Bowers]

A two-fer.

On Mon, 29 Nov 2004 10:28:42 +0800, Isaac To wrote:

>> "Greg" == Greg Ewing <gr**@cosc.canterbury.ac.nz> writes: >> The only punctuation you *need* is whitespace. See Forth

Greg> You don't even need that... see FORTRAN. :-)

Well, I for one don't like reading large programs with no line feeds :-)
And you don't need everything else either... see this.

http://compsoc.dur.ac.uk/whitespace/

I'm pretty sure if you remove all the whitespace and you remove all the
non-whitespace that you have indeed gotten a little too minimal :-)

[Paul Robson]

On Mon, 29 Nov 2004 01:12:16 -0500, Jeremy Bowers wrote:

The only punctuation you *need* is whitespace. See Forth (I don't know
if this is perfect but I'd bet the transform is simple),

: Announce ." Forth has a fair bit of punctuation" ;

[Marc 'BlackJack' Rintsch]

In <41***********************@news.free.fr>, bruno modulix wrote:

<please-someone-correct-me-if-i-am-wrong>
In fact, from a purely technical POV, the ':' could have been omitted
from the Python syntax, since indentation does the whole job of defining
blocks. It's only here for readability AFAIK.
</please-someone-correct-me-if-i-am-wrong>

The ':' serves as a very good hint to "python aware" text editors to
automagically indent the next line after hitting return.

Ciao,
Marc 'BlackJack' Rintsch

[Nick Coghlan]

BJörn Lindqvist wrote:

Because it contains more non-significant symbols (, ), { and } that
"steal" the programmers attention. But consider

def f(x, y, z)
print x, y, z

to

def f(x, y, z):
print x, y, z

IMHO, the colon-less variant is more readable than the one with the colon.

Except that it is quite acceptable to do the following:

def f(x, y, z,
long_func_arg_name):
long_func_arg_name(x, y, z)
def f(x, y, z,
long_func_arg_name)
long_func_arg_name(x, y, z)

The colons do a decent job of flagging the beginning of suites, mainly because
of Python general lack of *other* punctuation (e.g. the colon would be entirely
ineffective at improving readability if every line ended with a semi-colon).

Cheers,
Nick.