Re: [Python-Dev] PEP 315 - do while
Hi Nick,
Yep, PEP 315. Sorry about that.
Now, about your suggestion
do:
setup code
while condition
loop body
else:
loop completion code
This is pythonic, but not logical. The 'do' will execute at least once, so
the else clause is not needed, nor is the loop completion code. The loop
body should go before the while terminator.
I'm bound to reiterate my proposal:
before loop code
do:
loop body
setup code
while condition
after loop code
Example (if you know there will be at least one val).
source.open()
do:
val = source.read(1)
process(val)
while val != lastitem
source.close()
The c syntax is:
do
{
block of code
} while (condition is satisfied);
The VB syntax is:
do
block
loop while cond
Cheers thanks for your reply,
Hans Polak.
-Original Message-
From: Nick Coghlan [mailto:[EMAIL PROTECTED]
Sent: domingo, 01 de octubre de 2006 6:18
To: Hans Polak
Cc: python-dev@python.org
Subject: Re: [Python-Dev] PEP 351 - do while
Hans Polak wrote:
Hi,
Just an opinion, but many uses of the 'while true loop' are instances of
a 'do loop'. I appreciate the language layout question, so I'll give you
an alternative:
do:
body
setup code
while condition
I believe you meant to write PEP 315 in the subject line :)
To fully account for loop else clauses, this suggestion would probably need
to
be modified to look something like this:
Basic while loop:
setup code
while condition:
loop body
setup code
else:
loop completion code
Using break to avoid code duplication:
while True:
setup code
if not condition:
loop completion code
break
loop body
Current version of PEP 315:
do:
setup code
while condition:
loop body
else:
loop completion code
This suggestion:
do:
setup code
while condition
loop body
else:
loop completion code
I personally like that style, and if the compiler can dig through a function
looking for yield statements to identify generators, it should be able to
dig
through a do-loop looking for the termination condition.
As I recall, the main objection to this style was that it could hide the
loop
termination condition, but that isn't actually mentioned in the PEP (and in
the typical do-while case, the loop condition will still be clearly visible
at
the end of the loop body).
Regards,
Nick.
--
Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia
---
http://www.boredomandlaziness.org
This message contains information that may be privileged or confidential and is
the property of the Capgemini Group. It is intended only for the person to whom
it is addressed. If you are not the intended recipient, you are not authorized
to read, print, retain, copy, disseminate, distribute, or use this message or
any part thereof. If you receive this message in error, please notify the
sender immediately and delete all copies of this message.
___
Python-Dev mailing list
Python-Dev@python.org
http://mail.python.org/mailman/listinfo/python-dev
Unsubscribe:
http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Ok, I see your point. Really, I've read more about Python than worked with it, so I'm out of my league here. Can I combine your suggestion with mine and come up with the following: do: setup code loop body while condition else: loop completion code Cheers, Hans. -Original Message- From: Nick Coghlan [mailto:[EMAIL PROTECTED] Sent: lunes, 02 de octubre de 2006 12:48 To: Hans Polak Cc: python-dev@python.org Subject: Re: [Python-Dev] PEP 315 - do while Hans Polak wrote: Hi Nick, Yep, PEP 315. Sorry about that. Now, about your suggestion do: setup code while condition loop body else: loop completion code This is pythonic, but not logical. The 'do' will execute at least once, so the else clause is not needed, nor is the loop completion code. The loop body should go before the while terminator. This objection is based on a misunderstanding of what the else clause is for in a Python loop. The else clause is only executed if the loop terminated naturally (the exit condition became false) rather than being explicitly terminated using a break statement. This behaviour is most commonly useful when using a for loop to search through an iterable (breaking when the object is found, and using the else clause to handle the 'not found' case), but it is also defined for while loops. Regards, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://www.boredomandlaziness.org This message contains information that may be privileged or confidential and is the property of the Capgemini Group. It is intended only for the person to whom it is addressed. If you are not the intended recipient, you are not authorized to read, print, retain, copy, disseminate, distribute, or use this message or any part thereof. If you receive this message in error, please notify the sender immediately and delete all copies of this message. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Thanks for your reply Nick, and your support Michael. I'll leave the PEP talk to you guys :) Cheers, Hans -Original Message- From: Michael Foord [mailto:[EMAIL PROTECTED] On Behalf Of Fuzzyman Sent: martes, 03 de octubre de 2006 12:00 To: Nick Coghlan Cc: Hans Polak; python-dev@python.org Subject: Re: [Python-Dev] PEP 315 - do while Nick Coghlan wrote: Hans Polak wrote: Ok, I see your point. Really, I've read more about Python than worked with it, so I'm out of my league here. Can I combine your suggestion with mine and come up with the following: do: setup code loop body while condition else: loop completion code In my example, the 3 sections (setup code, loop body and loop completion code are all optional. A basic do-while loop would look like this: do: setup code while condition (That is, setup code is still repeated each time around the loop - it's called that because it is run before the loop evaluated condition is evaluated) +1 This looks good. The current idiom works fine, but looks unnatural : while True: if condition: break Would a 'while' outside of a 'do' block (but without the colon) then be a syntax error ? 'do:' would just be syntactic sugar for 'while True:' I guess. Michael Foord http://www.voidspace.org.uk Cheers, Nick. This message contains information that may be privileged or confidential and is the property of the Capgemini Group. It is intended only for the person to whom it is addressed. If you are not the intended recipient, you are not authorized to read, print, retain, copy, disseminate, distribute, or use this message or any part thereof. If you receive this message in error, please notify the sender immediately and delete all copies of this message. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
I'm against infinite loops -something religious :), which explains the call for the do loop. The issue about the parser is over my head, but the thought had occurred to me. Now, it would not affect while loops inside do loops, wouldn't it? Cheers, Hans. -Original Message- From: Nick Coghlan [mailto:[EMAIL PROTECTED] Sent: martes, 03 de octubre de 2006 15:51 To: Fuzzyman Cc: Hans Polak; python-dev@python.org Subject: Re: [Python-Dev] PEP 315 - do while Fuzzyman wrote: Nick Coghlan wrote: In my example, the 3 sections (setup code, loop body and loop completion code are all optional. A basic do-while loop would look like this: do: setup code while condition (That is, setup code is still repeated each time around the loop - it's called that because it is run before the loop evaluated condition is evaluated) +1 This looks good. I'm pretty sure it was proposed by someone else a long time ago - I was surprised to find it wasn't mentioned in PEP 315. That said, Guido's observation on PEP 315 from earlier this year holds for me too: I kind of like it but it doesn't strike me as super important [1] The current idiom works fine, but looks unnatural : while True: if condition: break There's the rationale for the PEP in a whole 5 lines counting whitespace ;) Would a 'while' outside of a 'do' block (but without the colon) then be a syntax error ? 'do:' would just be syntactic sugar for 'while True:' I guess. That's the slight issue I still have with the idea - you could end up with multiple ways of spelling some of the basic loop forms, such as these 3 flavours of infinite loop: do: pass # Is there an implicit 'while True' at the end of the loop body? do: while True while True: pass The other issue I have is that I'm not yet 100% certain it's implementable with Python's parser and grammar. I *think* changing the definition of the while statement from: while_stmt ::= while expression : suite [else : suite] to while_stmt ::= while expression [: suite [else : suite]] And adding a new AST node and a new type of compiler frame block DO_LOOP would do the trick (the compilation of a while statement without a trailing colon would then check that it was in a DO_LOOP block and raise an error if not). Cheers, Nick. [1] http://mail.python.org/pipermail/python-dev/2006-February/060711.html -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://www.boredomandlaziness.org This message contains information that may be privileged or confidential and is the property of the Capgemini Group. It is intended only for the person to whom it is addressed. If you are not the intended recipient, you are not authorized to read, print, retain, copy, disseminate, distribute, or use this message or any part thereof. If you receive this message in error, please notify the sender immediately and delete all copies of this message. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Please note that until == while not. do: code until count 10 do: code while count = 10 Cheers, Hans. -Original Message- From: Michael Foord [mailto:[EMAIL PROTECTED] On Behalf Of Fuzzyman Sent: martes, 03 de octubre de 2006 16:29 To: Nick Coghlan Cc: Hans Polak; python-dev@python.org Subject: Re: [Python-Dev] PEP 315 - do while Nick Coghlan wrote: [snip..] The current idiom works fine, but looks unnatural : while True: if condition: break There's the rationale for the PEP in a whole 5 lines counting whitespace ;) Would a 'while' outside of a 'do' block (but without the colon) then be a syntax error ? 'do:' would just be syntactic sugar for 'while True:' I guess. That's the slight issue I still have with the idea - you could end up with multiple ways of spelling some of the basic loop forms, such as these 3 flavours of infinite loop: do: pass # Is there an implicit 'while True' at the end of the loop body? do: while True while True: pass Following the current idiom, isn't it more natural to repeat the loop 'until' a condition is met. If we introduced two new keywords, it would avoid ambiguity in the use of 'while'. do: loop body until condition A do loop could require an 'until', meaning 'do' is not *just* a replacement for an infinite loop. (Assuming the parser can be coerced into co-operation.) It is obviously still a new construct in terms of Python syntax (not requiring a colon after 'condition'.) I'm sure this has been suggested, but wonder if it has already been ruled out. An 'else' block could then retain its current meaning (execute if the loop is not terminated early by an explicit break.) Michael Foord http://www.voidspace.org.uk This message contains information that may be privileged or confidential and is the property of the Capgemini Group. It is intended only for the person to whom it is addressed. If you are not the intended recipient, you are not authorized to read, print, retain, copy, disseminate, distribute, or use this message or any part thereof. If you receive this message in error, please notify the sender immediately and delete all copies of this message. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
You are all wasting your time on this. It won't go in. -- --Guido van Rossum (home page: http://www.python.org/~guido/) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
On 10/4/06, Guido van Rossum [EMAIL PROTECTED] wrote: You are all wasting your time on this. It won't go in. +1 from me. Should you mark PEP 315 as rejected? Jeremy -- --Guido van Rossum (home page: http://www.python.org/~guido/) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/jeremy%40alum.mit.edu ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
I'll mark it as withdrawn. Raymond -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jeremy Hylton Sent: Wednesday, October 04, 2006 8:44 AM To: Guido van Rossum Cc: Hans Polak; python-dev@python.org Subject: Re: [Python-Dev] PEP 315 - do while On 10/4/06, Guido van Rossum [EMAIL PROTECTED] wrote: You are all wasting your time on this. It won't go in. +1 from me. Should you mark PEP 315 as rejected? Jeremy -- --Guido van Rossum (home page: http://www.python.org/~guido/) ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/jeremy%40alum.mit.edu ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/rhettinger%40ewtllc.co m ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Hans Polak wrote: Ok, I see your point. Really, I've read more about Python than worked with it, so I'm out of my league here. Can I combine your suggestion with mine and come up with the following: do: setup code loop body while condition else: loop completion code In my example, the 3 sections (setup code, loop body and loop completion code are all optional. A basic do-while loop would look like this: do: setup code while condition (That is, setup code is still repeated each time around the loop - it's called that because it is run before the loop evaluated condition is evaluated) Cheers, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://www.boredomandlaziness.org ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Nick Coghlan wrote: Hans Polak wrote: Ok, I see your point. Really, I've read more about Python than worked with it, so I'm out of my league here. Can I combine your suggestion with mine and come up with the following: do: setup code loop body while condition else: loop completion code In my example, the 3 sections (setup code, loop body and loop completion code are all optional. A basic do-while loop would look like this: do: setup code while condition (That is, setup code is still repeated each time around the loop - it's called that because it is run before the loop evaluated condition is evaluated) +1 This looks good. The current idiom works fine, but looks unnatural : while True: if condition: break Would a 'while' outside of a 'do' block (but without the colon) then be a syntax error ? 'do:' would just be syntactic sugar for 'while True:' I guess. Michael Foord http://www.voidspace.org.uk Cheers, Nick. ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Fuzzyman wrote: Nick Coghlan wrote: In my example, the 3 sections (setup code, loop body and loop completion code are all optional. A basic do-while loop would look like this: do: setup code while condition (That is, setup code is still repeated each time around the loop - it's called that because it is run before the loop evaluated condition is evaluated) +1 This looks good. I'm pretty sure it was proposed by someone else a long time ago - I was surprised to find it wasn't mentioned in PEP 315. That said, Guido's observation on PEP 315 from earlier this year holds for me too: I kind of like it but it doesn't strike me as super important [1] The current idiom works fine, but looks unnatural : while True: if condition: break There's the rationale for the PEP in a whole 5 lines counting whitespace ;) Would a 'while' outside of a 'do' block (but without the colon) then be a syntax error ? 'do:' would just be syntactic sugar for 'while True:' I guess. That's the slight issue I still have with the idea - you could end up with multiple ways of spelling some of the basic loop forms, such as these 3 flavours of infinite loop: do: pass # Is there an implicit 'while True' at the end of the loop body? do: while True while True: pass The other issue I have is that I'm not yet 100% certain it's implementable with Python's parser and grammar. I *think* changing the definition of the while statement from: while_stmt ::= while expression : suite [else : suite] to while_stmt ::= while expression [: suite [else : suite]] And adding a new AST node and a new type of compiler frame block DO_LOOP would do the trick (the compilation of a while statement without a trailing colon would then check that it was in a DO_LOOP block and raise an error if not). Cheers, Nick. [1] http://mail.python.org/pipermail/python-dev/2006-February/060711.html -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://www.boredomandlaziness.org ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Nick Coghlan wrote: [snip..] The current idiom works fine, but looks unnatural : while True: if condition: break There's the rationale for the PEP in a whole 5 lines counting whitespace ;) Would a 'while' outside of a 'do' block (but without the colon) then be a syntax error ? 'do:' would just be syntactic sugar for 'while True:' I guess. That's the slight issue I still have with the idea - you could end up with multiple ways of spelling some of the basic loop forms, such as these 3 flavours of infinite loop: do: pass # Is there an implicit 'while True' at the end of the loop body? do: while True while True: pass Following the current idiom, isn't it more natural to repeat the loop 'until' a condition is met. If we introduced two new keywords, it would avoid ambiguity in the use of 'while'. do: loop body until condition A do loop could require an 'until', meaning 'do' is not *just* a replacement for an infinite loop. (Assuming the parser can be coerced into co-operation.) It is obviously still a new construct in terms of Python syntax (not requiring a colon after 'condition'.) I'm sure this has been suggested, but wonder if it has already been ruled out. An 'else' block could then retain its current meaning (execute if the loop is not terminated early by an explicit break.) Michael Foord http://www.voidspace.org.uk ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Nick Coghlan wrote: Fuzzyman wrote: Nick Coghlan wrote: In my example, the 3 sections (setup code, loop body and loop completion code are all optional. A basic do-while loop would look like this: do: setup code while condition (That is, setup code is still repeated each time around the loop - it's called that because it is run before the loop evaluated condition is evaluated) +1 This looks good. I'm pretty sure it was proposed by someone else a long time ago - I was surprised to find it wasn't mentioned in PEP 315. That said, Guido's observation on PEP 315 from earlier this year holds for me too: I kind of like it but it doesn't strike me as super important [1] I looked though a few files in the library for different while usage patterns and there really wasn't as many while loops that would fit this pattern as I expected. There are much more while loops with one or more exit conditions in the middle as things in the loop are calculated or received. So it might be smart to find out just how many places in the library it would make a difference. Ron ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
Re: [Python-Dev] PEP 315 - do while
Hans Polak wrote: Hi Nick, Yep, PEP 315. Sorry about that. Now, about your suggestion do: setup code while condition loop body else: loop completion code This is pythonic, but not logical. The 'do' will execute at least once, so the else clause is not needed, nor is the loop completion code. The loop body should go before the while terminator. This objection is based on a misunderstanding of what the else clause is for in a Python loop. The else clause is only executed if the loop terminated naturally (the exit condition became false) rather than being explicitly terminated using a break statement. This behaviour is most commonly useful when using a for loop to search through an iterable (breaking when the object is found, and using the else clause to handle the 'not found' case), but it is also defined for while loops. Regards, Nick. -- Nick Coghlan | [EMAIL PROTECTED] | Brisbane, Australia --- http://www.boredomandlaziness.org ___ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
