C-like assignment expression?

[boblatest@googlemail.com]

Hello,

I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:

if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match

....buy this is illegal in python. The other way is to open up an else:
block in each level, do the assignment and then the test. This
unneccessarily leads to deeper and deeper nesting levels which I find
ugly. Just as ugly as first testing against the RE in the elif: clause
and then, if it matches, to re-evaluate the RE to access the match
groups.

Thanks,
robert

[Diez B. Roggisch]

Re: C-like assignment expression?

boblatest@googl email.com wrote:

Hello,
>
I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:
>
if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match
>
...buy this is illegal in python. The other way is to open up an else:
block in each level, do the assignment and then the test. This
unneccessarily leads to deeper and deeper nesting levels which I find
ugly. Just as ugly as first testing against the RE in the elif: clause
and then, if it matches, to re-evaluate the RE to access the match
groups.

This might help:

-----------
s = "foo"

class Tester(object):

def __call__(self, pattern):
self.m = re.match(patter n, s)
return self.m is not None

def __getattr__(sel f, name):
return getattr(self.m, name)

test = Tester()

if test("bar"):
print "wrong"
elif test("foo"):
print "right"
-------------


Diez

[Ulrich Eckhardt]

Re: C-like assignment expression?

boblatest@googl email.com wrote:

I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:
>
if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match
>
...buy this is illegal in python. The other way is to open up an else:
block in each level, do the assignment and then the test. This
unneccessarily leads to deeper and deeper nesting levels which I find
ugly.

How about this (untested) code:

for re in (re1, re2, re3):
match = re.match(line)
if match:
# use it

This requires that "use it" means the same for each regular expression
though...

Uli

--
Sator Laser GmbH
Geschäftsführ er: Thorsten Föcking, Amtsgericht Hamburg HR B62 932

[Bruno Desthuilliers]

Re: C-like assignment expression?

boblatest@googl email.com a écrit :

Hello,
>
I have an if-elif chain in which I'd like to match a string against
several regular expressions. Also I'd like to use the match groups
within the respective elif... block. The C-like idiom that I would
like to use is this:
>
if (match = my_re1.match(li ne):
# use match
elsif (match = my_re2.match(li ne)):
# use match
elsif (match = my_re3.match(li ne))
# use match

<ot>
Isn't it the third or fourth time this very same question pops up here ?
Starts to look like a FAQ.
</ot>

The canonical solution is to iterate over a list of expression,func tion
pairs, ie:

def use_match1(matc h):
# code here

def use_match2(matc h):
# code here

def use_match3(matc h):
# code here

for exp, func in [
(my_re1, use_match1),
(my_re2, use_match2),
(my_re3, use_match3)
]:
match = exp.match(line)
if match:
func(match)
break


The alternate solution is Diez's Test object.

HTH

[Hrvoje Niksic]

Re: C-like assignment expression?

Bruno Desthuilliers <bruno.42.desth uilliers@websit eburo.invalid>
writes:

The canonical solution is to iterate over a list of
expression,func tion pairs, ie:

Although that solution is pretty, it is not the canonical solution
because it doesn't cover the important case of "if" bodies needing to
access common variables in the enclosing scope. (This will be easier
in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
IMHO closer to a canonical solution to this FAQ.

[boblatest@googlemail.com]

Re: C-like assignment expression?

On May 21, 1:47 pm, Hrvoje Niksic <hnik...@xemacs .orgwrote:

Although that solution is pretty, it is not the canonical solution
because it doesn't cover the important case of "if" bodies needing to
access common variables in the enclosing scope. (This will be easier
in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
IMHO closer to a canonical solution to this FAQ.

Hello everybody,

thanks for the various answers. I'm actually pretty puzzled because I
expected to see some obvious solution that I just hadn't found before.
In general I find Python more elegant and syntactically richer than C
(that's where I come from), so I didn't expect the solutions to be a
lot more verbose and/or ugly (no offense) than the original idea which
would have worked if Python's assignment statement would double as
expression, as in C.

Thanks again,
robert

PS: Since I'm testing only three REs, and I only need the match
results from one of them, I just re-evaluate that one.

[cokofreedom@gmail.com]

Re: C-like assignment expression?

On May 21, 3:12 pm, "boblat...@goog lemail.com"
<boblat...@goog lemail.comwrote :

On May 21, 1:47 pm, Hrvoje Niksic <hnik...@xemacs .orgwrote:
>

Although that solution is pretty, it is not the canonical solution
because it doesn't cover the important case of "if" bodies needing to
access common variables in the enclosing scope. (This will be easier
in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
IMHO closer to a canonical solution to this FAQ.

>
Hello everybody,
>
thanks for the various answers. I'm actually pretty puzzled because I
expected to see some obvious solution that I just hadn't found before.
In general I find Python more elegant and syntactically richer than C
(that's where I come from), so I didn't expect the solutions to be a
lot more verbose and/or ugly (no offense) than the original idea which
would have worked if Python's assignment statement would double as
expression, as in C.
>
Thanks again,
robert
>
PS: Since I'm testing only three REs, and I only need the match
results from one of them, I just re-evaluate that one.

Is it really a lot to change to have it

if my_re1.match(li ne):
match = my_re1.match(li ne)
elseif my_re2.match(li ne):
match = my_re2.match(li ne)
elseif my_re3.match(li ne):
match = my_re3.match(li ne)

?

That reads clearly to me...

[Diez B. Roggisch]

Re: C-like assignment expression?

boblatest@googl email.com wrote:

On May 21, 1:47 pm, Hrvoje Niksic <hnik...@xemacs .orgwrote:
>

>Although that solution is pretty, it is not the canonical solution
>because it doesn't cover the important case of "if" bodies needing to
>access common variables in the enclosing scope. (This will be easier
>in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
>IMHO closer to a canonical solution to this FAQ.

>
Hello everybody,
>
thanks for the various answers. I'm actually pretty puzzled because I
expected to see some obvious solution that I just hadn't found before.
In general I find Python more elegant and syntactically richer than C
(that's where I come from), so I didn't expect the solutions to be a
lot more verbose and/or ugly (no offense) than the original idea which
would have worked if Python's assignment statement would double as
expression, as in C.

Well, it's a design-decision - and I'm pretty ok with it being a bit verbose
here - as it prevents a *great* deal of programming errors that would
otherwise happen from accidentally writing a = b where a == b was meant.

One could argue that regular expressions - which seem to be THE case where
it bugs people - should offer a standard way that essentially works as my
solution - by keeping state around, making series of tests easier.


Diez

[Diez B. Roggisch]

Re: C-like assignment expression?

cokofreedom@gma il.com wrote:

On May 21, 3:12 pm, "boblat...@goog lemail.com"
<boblat...@goog lemail.comwrote :

>On May 21, 1:47 pm, Hrvoje Niksic <hnik...@xemacs .orgwrote:
>>

Although that solution is pretty, it is not the canonical solution
because it doesn't cover the important case of "if" bodies needing to
access common variables in the enclosing scope. (This will be easier
in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
IMHO closer to a canonical solution to this FAQ.

>>
>Hello everybody,
>>
>thanks for the various answers. I'm actually pretty puzzled because I
>expected to see some obvious solution that I just hadn't found before.
>In general I find Python more elegant and syntactically richer than C
>(that's where I come from), so I didn't expect the solutions to be a
>lot more verbose and/or ugly (no offense) than the original idea which
>would have worked if Python's assignment statement would double as
>expression, as in C.
>>
>Thanks again,
>robert
>>
>PS: Since I'm testing only three REs, and I only need the match
>results from one of them, I just re-evaluate that one.

>
Is it really a lot to change to have it
>
if my_re1.match(li ne):
match = my_re1.match(li ne)
elseif my_re2.match(li ne):
match = my_re2.match(li ne)
elseif my_re3.match(li ne):
match = my_re3.match(li ne)
>
?
>
That reads clearly to me...

And wastes time. regular expressions can become expensive to match - doing
it twice might be hurtful.

Diez

[cokofreedom@gmail.com]

Re: C-like assignment expression?

>

And wastes time. regular expressions can become expensive to match - doing
it twice might be hurtful.
>
Diez

match = (my_re1.match(l ine) or my_re2.match(li ne)) or
my_re3.match(li ne)

?

[Diez B. Roggisch]

Re: C-like assignment expression?

cokofreedom@gma il.com wrote:

>>
>And wastes time. regular expressions can become expensive to match -
>doing it twice might be hurtful.
>>
>Diez

>
match = (my_re1.match(l ine) or my_re2.match(li ne)) or
my_re3.match(li ne)

How do you know *which* of the three has matched then?

Diez

[cokofreedom@gmail.com]

Re: C-like assignment expression?

On May 21, 4:09 pm, "Diez B. Roggisch" <de...@nospam.w eb.dewrote:

cokofree...@gma il.com wrote:
>

And wastes time. regular expressions can become expensive to match -
doing it twice might be hurtful.

>

Diez

>

match = (my_re1.match(l ine) or my_re2.match(li ne)) or
my_re3.match(li ne)

>
How do you know *which* of the three has matched then?
>
Diez

Depends if the OP wants to know that...

[inhahe]

Re: C-like assignment expression?

one of the few things i miss from C is being able to use assignment in
expressions. that's the only thing, really.
also there's no switch/case, you have to use a dictionary of functions
instead, although i rarely need that, usually i just use elif.

<boblatest@goog lemail.comwrote in message
news:7df99fd4-21c9-49a1-ba65-e55794fd4c12@26 g2000hsk.google groups.com...

On May 21, 1:47 pm, Hrvoje Niksic <hnik...@xemacs .orgwrote:
>

>Although that solution is pretty, it is not the canonical solution
>because it doesn't cover the important case of "if" bodies needing to
>access common variables in the enclosing scope. (This will be easier
>in Python 3 with 'nonlocal', though.) The snippet posted by Diez is
>IMHO closer to a canonical solution to this FAQ.

>
Hello everybody,
>
thanks for the various answers. I'm actually pretty puzzled because I
expected to see some obvious solution that I just hadn't found before.
In general I find Python more elegant and syntactically richer than C
(that's where I come from), so I didn't expect the solutions to be a
lot more verbose and/or ugly (no offense) than the original idea which
would have worked if Python's assignment statement would double as
expression, as in C.
>
Thanks again,
robert
>
PS: Since I'm testing only three REs, and I only need the match
results from one of them, I just re-evaluate that one.

[Diez B. Roggisch]

Re: C-like assignment expression?

cokofreedom@gma il.com wrote:

On May 21, 4:09 pm, "Diez B. Roggisch" <de...@nospam.w eb.dewrote:

>cokofree...@gm ail.com wrote:
>>

>And wastes time. regular expressions can become expensive to match -
>doing it twice might be hurtful.

>>

>Diez

>>

match = (my_re1.match(l ine) or my_re2.match(li ne)) or
my_re3.match(li ne)

>>
>How do you know *which* of the three has matched then?
>>
>Diez

>
Depends if the OP wants to know that...

Well, in *general* one wants that. So as a general-purpose solution this is
certainly *not* the way to go.

Diez

[cokofreedom@gmail.com]

Re: C-like assignment expression?

On May 21, 4:57 pm, "inhahe" <inh...@gmail.c omwrote:

one of the few things i miss from C is being able to use assignment in
expressions. that's the only thing, really.
also there's no switch/case, you have to use a dictionary of functions
instead, although i rarely need that, usually i just use elif.

One thing I hate from C is the assignment in expressions...F orcing
myself to write
0 == Something
rather than
Something == 0
just to make sure I was mistakenly assigning values in statements is
annoying, it ruins the ease of reading.

I kind of agree with the select:case, but I think a key issue is how
to implement it. Elif is reasonable for now.

Diez, true I guess, but then we haven't seen what these expressions
are, and why there has to be three.