C-like assignment expression?

[boblatest]

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]

bo*******@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]

bo*******@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]

bo*******@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]

Bruno Desthuilliers <br************ ********@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]

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]

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]

bo*******@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]

co*********@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]

>

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)

?