Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Jeff McAninch wrote: I very often want something like a try-except conditional expression similar to the if-else conditional. I think it may be done currently with the help of next function: def guard(func, *args): try: return func() except Exception, e: for exc_type, exc_func in args: if isinstance(e, exc_type): return exc_func() raise Example usage: a, b, c = 10, 20, 0 result = a + b/c # raise ZeroDivisionError result = a + guard(lambda: b/c, (TypeError, lambda: 10), (ZeroDivisionError, lambda: b/2)) May be not very concise, but it works... -- Best regards, Alexander mailto:alexander.kozlov...@gmail.com ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
I figure I would write up the PEP draft, I have never tried writing a
pep before, but i did read PEP 1 and tried to follow it's formating
guides. If there are no additions to the idea, then it seems there
just needs to be a consensus on the syntax before submitting it to the
peps list
I posted this to the python-ideas version of this thread already, but
since more people seem to be posting to the python-dev list I will
post it here as well
PEP: pep number
Title: try-except conditional expressions
Version: svn version string
Last-Modified: svn date string
Author: Jeff McAninch mcani...@lanl.gov, Dj Gilcrease
digitalx...@gmail.com
Discussions-To: python-id...@python.org
Status: Draft
Type: Standards Track
Content-Type: text/plain
Created: 06-Aug-2009
Python-Version: 2.7/3.2
Post-History: dates of postings to python-list and python-dev
Abstract:
I very often want something like a try-except conditional
expression similar to the if-else conditional instead of resorting
to a multi-line try-except block.
Design Goals:
The new syntax should
* Be simple to read
* Be intuitive so people who may use it infrequently dont need
to go lookup the format every time
* Make it obvious what is happening
Modivation:
Often when doing calculations or string recasting (to int, float,
etc) it is required to wrap the section in a simple try-except
where the exception just assigns a default value. It would be more
readable and consise if these type of try-excepts could be written
on a single line.
Issues:
Unknown
Specification:
All 3 components would just be ordinary expressions. The exception
definition would be allowed to resolve to a single exception or a
tuple of exceptions, just as it is in a normal try/except
statement.
Syntax Ideas:
Option 1:
x = float(string) except float('nan') if ValueError
op(float(string) except float('nan') if ValueError)
Option 2:
x = float(string) except ValueError: float('nan')
op(float(string) except ValueError: float('nan'))
Option 3:
x = float(string) except ValueError else float('nan')
op(float(string) except ValueError else float('nan'))
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Steven D'Aprano writes: It's not immediately obvious to me why the last expression should be given that privileged rule. Why not the first expression? Or the second, for that matter. So find a large body of Lisp code and run grep -r prog1 | wc, grep -r prog2 | wc, and grep -r progn | wc on it. I think the pragmatic answer will be obvious. Personally, I like functional languages and style. But I admit the *need* for a progn construct (ie, block) to express procedural style, and see no particular reason why expressing that by making a syntactic distinction between expressions and statements is worse (or better) than the progn construct. That should be kept distinct from the question of whether extended assignment operators or conditional operators are appropriate for a given language. ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
On 8 Aug 2009, at 08:02 , Steven D'Aprano wrote: On Fri, 7 Aug 2009 08:22:14 pm Kristján Valur Jónsson wrote: Unless I am very much mistaken, this is the approach Ruby takes. Everything is an expression. For example, the value of a block is the value of The last expression in the block. Copying what other languages do is not necessarily a bad thing, but that would fail both explicit is better than implicit and in the face of ambiguity, avoid the temptation to guess. The first objection one might be able to give, you maybe, but the second one? Where's the ambiguity in compound statements return the result of the last evaluated expression? It's not immediately obvious to me why the last expression should be given that privileged rule. Why not the first expression? Because it wouldn't make any sense? When you're computing something, the value you want is the one at the end of the computation (usually called a result), not some random one somewhere else. ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Should be legal, right?, since syntax would be
expression except expression if exception
Dino Viehland wrote:
On option 1 is this legal then?
x = float(string) except float('nan') if some_check() else float('inf') if
ValueError
Thinking more about the syntax options: if P.J.'s if Option is used,
it should also be optional.
That is, I would want this to also be legal,
expression except expression
to trap any exception when robustness is more important than catching a
specific exception.
What would be the typical next step in trying to put this forward? A
draft PEP?
--
==
Jeffrey E. McAninch, PhD
Physicist, X-2-IFD
Los Alamos National Laboratory
Phone: 505-667-0374
Email: mcani...@lanl.gov
==
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Unless I am very much mistaken, this is the approach Ruby takes. Everything is an expression. For example, the value of a block is the value of The last expression in the block. I've never understood the need to have a distinction betwen statements and expressions, not when expressions can have side effects. It's like that differentce between procedures and functions in pascal that only serves to confuse K -Original Message- From: python-dev-bounces+kristjan=ccpgames@python.org [mailto:python-dev-bounces+kristjan=ccpgames@python.org] On Behalf Of Xavier Morel Sent: 6. ágúst 2009 10:25 To: python-dev@python.org Subject: Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308) Wouldn't it be smarter to fix the issue once and for all by looking into making Python's compound statements (or even all statements without restrictions) expressions that can return values in the first place? Now I don't know if it's actually possible, but if it is the problem becomes solved not just for try:except: (and twice so for if:else:) but also for while:, for: (though that one's already served pretty well by comprehensions) and with:. ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Jeff McAninch wrote:
Should be legal, right?, since syntax would be
expression except expression if exception
Dino Viehland wrote:
On option 1 is this legal then?
x = float(string) except float('nan') if some_check() else
float('inf') if ValueError
Thinking more about the syntax options: if P.J.'s if Option is used,
it should also be optional.
That is, I would want this to also be legal,
expression except expression
to trap any exception when robustness is more important than catching a
specific exception.
Catch all exceptions:
expression except expression
Catch specific exceptions, optionally catching all others:
expression except (expression if exception)+ [else expression]
Of course, a catch-all is a bare except, with all its dangers!
What would be the typical next step in trying to put this forward? A
draft PEP?
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
I believe people now discuss this both on python-dev and python-ideas,
though since I'm new to both lists, I can't really tell where this
belongs.
I played a little with this syntax, my try_ function and @catch
decorator (which are at http://mit.edu/~unknot/www/try_cond.py):
# x = float(string) except float('nan') if ValueError
x = try_(float, string, except_ = float('nan'), if_ = ValueError)
@catch(ValueError = float('nan'))
def x1(): return float(string)
# y = float(string) except ValueError: float('nan')
y = try_(float, string, { ValueError: float('nan') })
@catch({ValueError: float('nan')})
def y1(): return float(string)
# try:
# z = open(string, 'r')
# except IOError as e:
# if e.errno == 2:
# z = 'not_exist'
# else:
# raise
#
z = try_(open, string, 'r', iocatcher({2: 'no file!'}))
@catch(iocatcher({2: 'nothing!'}))
def z1(): return open(string, 'r')
Here are my overall feelings:
(1) it would be interesting to come up with syntax for except/if
clause, but it's not obvious how to make one and this fact itself may
kill the idea.
(2) the more reasonable approach to things like this is by defining a
separate block and then performing a catch operation with it.
Unfortunately, this looks very clumsy as currently this can only be
done by defining a separate function. I think code blocks are a good
direction to explore.
2009/8/7 Kristján Valur Jónsson krist...@ccpgames.com:
Unless I am very much mistaken, this is the approach Ruby takes.
Everything is an expression. For example, the value of a block is the value
of
The last expression in the block.
I've never understood the need to have a distinction betwen statements and
expressions, not when expressions can have side effects. It's like that
differentce between procedures and functions in pascal that only serves to
confuse
K
-Original Message-
From: python-dev-bounces+kristjan=ccpgames@python.org
[mailto:python-dev-bounces+kristjan=ccpgames@python.org] On Behalf
Of Xavier Morel
Sent: 6. ágúst 2009 10:25
To: python-dev@python.org
Subject: Re: [Python-Dev] (try-except) conditional expression similar
to (if-else) conditional (PEP 308)
Wouldn't it be smarter to fix the issue once and for all by looking
into making Python's compound statements (or even all statements
without restrictions) expressions that can return values in the first
place? Now I don't know if it's actually possible, but if it is the
problem becomes solved not just for try:except: (and twice so for
if:else:) but also for while:, for: (though that one's already served
pretty well by comprehensions) and with:.
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
--
http://www.ironpythoninaction.com
On 7 Aug 2009, at 12:06, ilya ilya.nikokos...@gmail.com wrote:
I believe people now discuss this both on python-dev and python-ideas,
though since I'm new to both lists, I can't really tell where this
belongs.
It definitely belongs on the ideas list...
Michael
I played a little with this syntax, my try_ function and @catch
decorator (which are at http://mit.edu/~unknot/www/try_cond.py):
# x = float(string) except float('nan') if ValueError
x = try_(float, string, except_ = float('nan'), if_ = ValueError)
@catch(ValueError = float('nan'))
def x1(): return float(string)
# y = float(string) except ValueError: float('nan')
y = try_(float, string, { ValueError: float('nan') })
@catch({ValueError: float('nan')})
def y1(): return float(string)
# try:
# z = open(string, 'r')
# except IOError as e:
# if e.errno == 2:
# z = 'not_exist'
# else:
# raise
#
z = try_(open, string, 'r', iocatcher({2: 'no file!'}))
@catch(iocatcher({2: 'nothing!'}))
def z1(): return open(string, 'r')
Here are my overall feelings:
(1) it would be interesting to come up with syntax for except/if
clause, but it's not obvious how to make one and this fact itself may
kill the idea.
(2) the more reasonable approach to things like this is by defining a
separate block and then performing a catch operation with it.
Unfortunately, this looks very clumsy as currently this can only be
done by defining a separate function. I think code blocks are a good
direction to explore.
2009/8/7 Kristján Valur Jónsson krist...@ccpgames.com:
Unless I am very much mistaken, this is the approach Ruby takes.
Everything is an expression. For example, the value of a block is
the value of
The last expression in the block.
I've never understood the need to have a distinction betwen
statements and expressions, not when expressions can have side
effects. It's like that differentce between procedures and
functions in pascal that only serves to confuse
K
-Original Message-
From: python-dev-bounces+kristjan=ccpgames@python.org
[mailto:python-dev-bounces+kristjan=ccpgames@python.org] On
Behalf
Of Xavier Morel
Sent: 6. ágúst 2009 10:25
To: python-dev@python.org
Subject: Re: [Python-Dev] (try-except) conditional expression
similar
to (if-else) conditional (PEP 308)
Wouldn't it be smarter to fix the issue once and for all by looking
into making Python's compound statements (or even all statements
without restrictions) expressions that can return values in the
first
place? Now I don't know if it's actually possible, but if it is the
problem becomes solved not just for try:except: (and twice so for
if:else:) but also for while:, for: (though that one's already
served
pretty well by comprehensions) and with:.
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
2009/8/7 Kristján Valur Jónsson krist...@ccpgames.com: Unless I am very much mistaken, this is the approach Ruby takes. Everything is an expression. For example, the value of a block is the value of The last expression in the block. I've never understood the need to have a distinction betwen statements and expressions, not when expressions can have side effects. It's like that differentce between procedures and functions in pascal that only serves to confuse If you're interested in understanding it better, research Query-Command Separation (QCS), e.g. starting at http://en.wikipedia.org/wiki/Command-query_separation and links therefrom. Alex ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
P.J. Eby wrote:
At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:
[Jeffrey E. McAninch, PhD]
I very often want something like a try-except conditional expression
similar
to the if-else conditional.
An example of the proposed syntax might be:
x = float(string) except float('nan')
or possibly
x = float(string) except ValueError float('nan')
+1 I've long wanted something like this.
One possible spelling is:
x = float(string) except ValueError else float('nan')
I think 'as' would be better than 'else', since 'else' has a different
meaning in try/except statements, e.g.:
x = float(string) except ValueError, TypeError as float('nan')
Of course, this is a different meaning of 'as', too, but it's not as
contradictory, IMO... ;-)
(We're probably well into python-ideas territory at this point, but I'll
keep things where the thread started for now)
The basic idea appears sound to me as well. I suspect finding an
acceptable syntax is going to be the sticking point.
Breaking the problem down, we have three things we want to separate:
1. The expression that may raise the exception
2. The expression defining the exceptions to be caught
3. The expression to be used if the exception actually is caught
From there it is possible to come up with all sorts of variants.
Option 1:
Change the relative order of the clauses by putting the exception
definition last:
x = float(string) except float('nan') if ValueError
op(float(string) except float('nan') if ValueError)
I actually like this one (that's why I listed it first). It gets the
clauses out of order relative to the statement, but the meaning still
seems pretty obvious to me.
Option 2:
Follow the lamba model and allow a colon inside this form of expression:
x = float(string) except ValueError: float('nan')
op(float(string) except ValueError: float('nan'))
This has the virtue of closely matching the statement syntax, but
embedding colons inside expressions is somewhat ugly. Yes, lambda
already does it, but lambda can hardly be put forward as a paragon of
beauty.
Option 3a/3b:
Raymond's except-else suggestion:
x = float(string) except ValueError else float('nan')
op(float(string) except ValueError else float('nan'))
This has the problem of inverting the sense of the else clause relative
to the statement form (where the else clause is executed only if no
exception occurs)
A couple of extra keywords would get the sense correct again, but I'm
not sure the parser could cope with it and it is rather verbose (I much
prefer option 1 to this idea):
x = float(string) if not except ValueError else float('nan')
op(float(string) if not except ValueError else float('nan'))
Option 4:
PJE's except-as suggestion:
x = float(string) except ValueError as float('nan')
op(float(string) except ValueError as float('nan'))
Given that we now use except ValueError as ex in exception statements,
the above strikes me a really confusing idea.
Cheers,
Nick.
--
Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
---
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
On Thu, Aug 6, 2009 at 4:47 AM, Nick Coghlanncogh...@gmail.com wrote:
Option 2:
x = float(string) except ValueError: float('nan')
op(float(string) except ValueError: float('nan'))
This has the virtue of closely matching the statement syntax, but
embedding colons inside expressions is somewhat ugly. Yes, lambda
already does it, but lambda can hardly be put forward as a paragon of
beauty.
+1 on this option as it resembles the standard try/except block enough
it would be a quick edit to convert it to one if later you realize you
need to catch more exceptions*
* I recommend NOT allowing multiple exceptions in this form eg
x = float(string)/var except ValueError, ZeroDivisionError, ...: float('nan')
as it will start to reduce readability quickly
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
On 6 Aug 2009, at 00:22 , Jeff McAninch wrote: I'm new to this list, so please excuse me if this topic has been discussed, but I didn't see anything similar in the archives. I very often want something like a try-except conditional expression similar to the if-else conditional. I fear this idea is soon going to extend to all compound statements one by one. Wouldn't it be smarter to fix the issue once and for all by looking into making Python's compound statements (or even all statements without restrictions) expressions that can return values in the first place? Now I don't know if it's actually possible, but if it is the problem becomes solved not just for try:except: (and twice so for if:else:) but also for while:, for: (though that one's already served pretty well by comprehensions) and with:. ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Nick Coghlan wrote:
P.J. Eby wrote:
At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:
[Jeffrey E. McAninch, PhD]
I very often want something like a try-except conditional expression
similar
to the if-else conditional.
An example of the proposed syntax might be:
x = float(string) except float('nan')
or possibly
x = float(string) except ValueError float('nan')
+1 I've long wanted something like this.
One possible spelling is:
x = float(string) except ValueError else float('nan')
I think 'as' would be better than 'else', since 'else' has a different
meaning in try/except statements, e.g.:
x = float(string) except ValueError, TypeError as float('nan')
Of course, this is a different meaning of 'as', too, but it's not as
contradictory, IMO... ;-)
(We're probably well into python-ideas territory at this point, but I'll
keep things where the thread started for now)
The basic idea appears sound to me as well. I suspect finding an
acceptable syntax is going to be the sticking point.
Breaking the problem down, we have three things we want to separate:
1. The expression that may raise the exception
2. The expression defining the exceptions to be caught
3. The expression to be used if the exception actually is caught
From there it is possible to come up with all sorts of variants.
Option 1:
Change the relative order of the clauses by putting the exception
definition last:
x = float(string) except float('nan') if ValueError
op(float(string) except float('nan') if ValueError)
I actually like this one (that's why I listed it first). It gets the
clauses out of order relative to the statement, but the meaning still
seems pretty obvious to me.
A further extension (if we need it):
result = foo(arg) except float('inf') if ZeroDivisionError else
float('nan')
The 'else' part handles any other exceptions (not necessarily a good idea!).
or:
result = foo(arg) except float('inf') if ZeroDivisionError else
float('nan') if ValueError
Handles a number of different exceptions.
Option 2:
Follow the lamba model and allow a colon inside this form of expression:
x = float(string) except ValueError: float('nan')
op(float(string) except ValueError: float('nan'))
This has the virtue of closely matching the statement syntax, but
embedding colons inside expressions is somewhat ugly. Yes, lambda
already does it, but lambda can hardly be put forward as a paragon of
beauty.
A colon is also used in a dict literal.
Option 3a/3b:
Raymond's except-else suggestion:
x = float(string) except ValueError else float('nan')
op(float(string) except ValueError else float('nan'))
[snip]
-1
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
What about catching specific error numbers? Maybe an option so that
the dictionary elements can also be dictionaries with integers as the
keys:
filedata = try_3(open, randomfile, except = { IOError, {2: None} } )
If it isn't found in the dictionary, then we raise the error.
On Thu, Aug 6, 2009 at 07:03, ilyailya.nikokos...@gmail.com wrote:
I took a look at the options 1 and 2:
x = float(string) except float('nan') if ValueError
y = float(string) except ValueError: float('nan')
and I think this can be done just as easily with existing syntax:
x = try_1(float, string, except_ = float('nan'), if_ = ValueError)
y = try_2(float, string, { ValueError: float('nan') })
Here's the full example:
- example starts -
def try_1(func, *args, except_ = None, if_ = None):
try:
return func(*args)
except if_ as e:
return except_
def try_2(func, *args):
'The last argument is a dictionary {exception type: return value}.'
dic = args[-1]
try:
return func(*args[:-1])
except Exception as e:
for k,v in dic.items():
if isinstance(e, k):
return v
raise
for string in ['5', 'five']:
# x = float(string) except float('nan') if ValueError
x = try_1(float, string, except_ = float('nan'), if_ = ValueError)
# y = float(string) except ValueError: float('nan')
y = try_2(float, string, { ValueError: float('nan') })
print(x, y)
- example ends -
As a side note, if I just subscribed to python-dev, is it possible to
quote an old email? Below is my manual cut-and-paste quote:
-- my quote --
Nick Coghlan wrote:
P.J. Eby wrote:
At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:
[Jeffrey E. McAninch, PhD]
I very often want something like a try-except conditional expression
similar
to the if-else conditional.
An example of the proposed syntax might be:
x = float(string) except float('nan')
or possibly
x = float(string) except ValueError float('nan')
+1 I've long wanted something like this.
One possible spelling is:
x = float(string) except ValueError else float('nan')
I think 'as' would be better than 'else', since 'else' has a different
meaning in try/except statements, e.g.:
x = float(string) except ValueError, TypeError as float('nan')
Of course, this is a different meaning of 'as', too, but it's not as
contradictory, IMO... ;-)
(We're probably well into python-ideas territory at this point, but I'll
keep things where the thread started for now)
The basic idea appears sound to me as well. I suspect finding an
acceptable syntax is going to be the sticking point.
Breaking the problem down, we have three things we want to separate:
1. The expression that may raise the exception
2. The expression defining the exceptions to be caught
3. The expression to be used if the exception actually is caught
From there it is possible to come up with all sorts of variants.
Option 1:
Change the relative order of the clauses by putting the exception
definition last:
x = float(string) except float('nan') if ValueError
op(float(string) except float('nan') if ValueError)
I actually like this one (that's why I listed it first). It gets the
clauses out of order relative to the statement, but the meaning still
seems pretty obvious to me.
A further extension (if we need it):
result = foo(arg) except float('inf') if ZeroDivisionError else
float('nan')
The 'else' part handles any other exceptions (not necessarily a good idea!).
or:
result = foo(arg) except float('inf') if ZeroDivisionError else
float('nan') if ValueError
Handles a number of different exceptions.
Option 2:
Follow the lamba model and allow a colon inside this form of expression:
x = float(string) except ValueError: float('nan')
op(float(string) except ValueError: float('nan'))
This has the virtue of closely matching the statement syntax, but
embedding colons inside expressions is somewhat ugly. Yes, lambda
already does it, but lambda can hardly be put forward as a paragon of
beauty.
A colon is also used in a dict literal.
Option 3a/3b:
Raymond's except-else suggestion:
x = float(string) except ValueError else float('nan')
op(float(string) except ValueError else float('nan'))
[snip]
-1
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
In article d28975e8-6706-4515-9c9e-fb7f90775...@masklinn.net, Xavier Morel catch-...@masklinn.net wrote: On 6 Aug 2009, at 00:22 , Jeff McAninch wrote: I'm new to this list, so please excuse me if this topic has been discussed, but I didn't see anything similar in the archives. I very often want something like a try-except conditional expression similar to the if-else conditional. I fear this idea is soon going to extend to all compound statements one by one. Wouldn't it be smarter to fix the issue once and for all by looking into making Python's compound statements (or even all statements without restrictions) expressions that can return values in the first place? Now I don't know if it's actually possible, but if it is the problem becomes solved not just for try:except: (and twice so for if:else:) but also for while:, for: (though that one's already served pretty well by comprehensions) and with:. I like this idea a lot. -- Russell ___ 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] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
On option 1 is this legal then?
x = float(string) except float('nan') if some_check() else float('inf') if
ValueError
-Original Message-
From: python-dev-bounces+dinov=microsoft@python.org
[mailto:python-dev-bounces+dinov=microsoft@python.org] On Behalf Of Nick
Coghlan
Sent: Thursday, August 06, 2009 3:48 AM
To: P.J. Eby
Cc: python-dev@python.org; Jeff McAninch
Subject: Re: [Python-Dev] (try-except) conditional expression similar to
(if-else) conditional (PEP 308)
P.J. Eby wrote:
At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:
[Jeffrey E. McAninch, PhD]
I very often want something like a try-except conditional expression
similar to the if-else conditional.
An example of the proposed syntax might be:
x = float(string) except float('nan') or possibly
x = float(string) except ValueError float('nan')
+1 I've long wanted something like this.
One possible spelling is:
x = float(string) except ValueError else float('nan')
I think 'as' would be better than 'else', since 'else' has a different
meaning in try/except statements, e.g.:
x = float(string) except ValueError, TypeError as float('nan')
Of course, this is a different meaning of 'as', too, but it's not as
contradictory, IMO... ;-)
(We're probably well into python-ideas territory at this point, but I'll keep
things where the thread started for now)
The basic idea appears sound to me as well. I suspect finding an acceptable
syntax is going to be the sticking point.
Breaking the problem down, we have three things we want to separate:
1. The expression that may raise the exception 2. The expression defining the
exceptions to be caught 3. The expression to be used if the exception actually
is caught
From there it is possible to come up with all sorts of variants.
Option 1:
Change the relative order of the clauses by putting the exception definition
last:
x = float(string) except float('nan') if ValueError
op(float(string) except float('nan') if ValueError)
I actually like this one (that's why I listed it first). It gets the clauses
out of order relative to the statement, but the meaning still seems pretty
obvious to me.
Option 2:
Follow the lamba model and allow a colon inside this form of expression:
x = float(string) except ValueError: float('nan')
op(float(string) except ValueError: float('nan'))
This has the virtue of closely matching the statement syntax, but embedding
colons inside expressions is somewhat ugly. Yes, lambda already does it, but
lambda can hardly be put forward as a paragon of beauty.
Option 3a/3b:
Raymond's except-else suggestion:
x = float(string) except ValueError else float('nan')
op(float(string) except ValueError else float('nan'))
This has the problem of inverting the sense of the else clause relative to the
statement form (where the else clause is executed only if no exception occurs)
A couple of extra keywords would get the sense correct again, but I'm not sure
the parser could cope with it and it is rather verbose (I much prefer option 1
to this idea):
x = float(string) if not except ValueError else float('nan')
op(float(string) if not except ValueError else float('nan'))
Option 4:
PJE's except-as suggestion:
x = float(string) except ValueError as float('nan')
op(float(string) except ValueError as float('nan'))
Given that we now use except ValueError as ex in exception statements, the
above strikes me a really confusing idea.
Cheers,
Nick.
--
Nick Coghlan | ncogh...@gmail.com | Brisbane, Australia
---
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
MRAB wrote:
Dino Viehland wrote:
On option 1 is this legal then?
x = float(string) except float('nan') if some_check() else float('inf') if
ValueError
Well, is this is legal?
try:
x = float(string)
except some_check():
x = float('nan')
except ValueError:
x = float('inf')
I was thinking this was would be equal to:
x = float(string) except (float('nan') if some_check() else float('inf')) if
ValueError
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Dino Viehland wrote:
MRAB wrote:
Dino Viehland wrote:
On option 1 is this legal then?
x = float(string) except float('nan') if some_check() else float('inf') if
ValueError
Well, is this is legal?
try:
x = float(string)
except some_check():
x = float('nan')
except ValueError:
x = float('inf')
I was thinking this was would be equal to:
x = float(string) except (float('nan') if some_check() else float('inf')) if
ValueError
I suppose it depends on the precedence of 'x except y if z' vs 'x if y
else y'.
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
At 05:59 PM 8/5/2009 -0700, Raymond Hettinger wrote:
[Jeffrey E. McAninch, PhD]
I very often want something like a try-except conditional expression similar
to the if-else conditional.
An example of the proposed syntax might be:
x = float(string) except float('nan')
or possibly
x = float(string) except ValueError float('nan')
+1 I've long wanted something like this.
One possible spelling is:
x = float(string) except ValueError else float('nan')
I think 'as' would be better than 'else', since 'else' has a
different meaning in try/except statements, e.g.:
x = float(string) except ValueError, TypeError as float('nan')
Of course, this is a different meaning of 'as', too, but it's not
as contradictory, IMO... ;-)
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Re: [Python-Dev] (try-except) conditional expression similar to (if-else) conditional (PEP 308)
Raymond Hettinger wrote:
If accepted, this would also solve the feature requests for various
functions to have default arguments.
For example:
x = min(seq) except ValueError else 0 # default to zero for
empty sequences
It would also be helpful in calculations that have algebraic
restrictions:
sample_std_deviation = sqrt(sum(x - mu for x in seq) / (len(seq)-1))
except ZeroDivisionError else float('Inf')
Raymond
Yes, exactly the situations I keep coding around.
--
==
Jeffrey E. McAninch, PhD
Physicist, X-2-IFD
Los Alamos National Laboratory
Phone: 505-667-0374
Email: mcani...@lanl.gov
==
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