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On Thu, 24 Jul 2003 23:44:57 +0200, Egbert Bouwman <> wrote:

>While looping over a long list (with file records)
>I use an (also long) if..elif sequence.
>One of these elif's tests a regular expression, and
>if the test succeeds, I want to use a part of the match.
>Something like this:
>
>pat = re.compile(r'...')
>for line in mylist:
> if ... :
> ....
> elif ... :
> ....
> elif pat.search(line):
> mat = pat.search(line)
> elif ... :
> ...
> else ...:
> ...
>Is there a way to to do this job with only one pat.search(line) ?
Hack #1 (list comprehension abuse):

>>> import re
>>> pat = re.compile(r'...')
>>> mylist = ' a bc def ghij'.split(' ')
>>> mylist
['', 'a', 'bc', 'def', 'ghij']
>>> for line in mylist:
... if 1==2: 'naw'
... elif 3==4: 'neither'
... elif [1 for mat in [pat.search(line)] if mat]:
... print repr(mat), mat.groups(), mat.group()
... elif 4==5: 'haw'
... else:
... print 'final else'
...
final else
final else
final else
<_sre.SRE_Match object at 0x007F1260> () def
<_sre.SRE_Match object at 0x007F5900> () ghi

Hack #2: (attach temporary value to an instance of something
that can accept attributes (almost any object):

>>> import re
>>> pat = re.compile(r'...')
>>> mylist = ' a bc def ghij'.split(' ')
>>> mylist
['', 'a', 'bc', 'def', 'ghij']
>>> obj = type('Any',(),{})()
>>> for line in mylist:
... if 1==2: 'naw'
... elif 3==4: 'neither'
... elif setattr(obj,'mat', pat.search(line)) or obj.mat:
... print repr(obj.mat), obj.mat.groups(), obj.mat.group()
... elif 4==5: 'haw'
... else:
... print 'final else'
...
final else
final else
final else
<_sre.SRE_Match object at 0x007F69C0> () def
<_sre.SRE_Match object at 0x007F6980> () ghi

The trick above is that setattr(...) return None, so the expression always continues to the or part.
>>> setattr(obj,'xxx',123)
>>> repr(setattr(obj,'xxx',123))
'None'

You can spell

... elif setattr(obj,'mat', pat.search(line)) or obj.mat:
... print repr(obj.mat), obj.mat.groups(), obj.mat.group()

a little slicker if you make a special object to hold a binding to
the pat.search result, e.g., h is the Holder instance in the following,
which remembers the last thing passed to it and immediately returns it,
and also returns that last thing on being called without an arg:

>>> mylist
['', 'a', 'bc', 'def', 'ghij']
>>> h = Holder()
>>> for line in mylist:
... if 1==2: 'naw'
... elif 3==4: 'neither'
... elif h(pat.search(line)):
... print repr(h()), h().groups(), h().group()
... elif 4==5: 'haw'
... else:
... print 'final else'
...
final else
final else
final else
<_sre.SRE_Match object at 0x007F6FC0> () def
<_sre.SRE_Match object at 0x007F6F80> () ghi

Obviously
... print repr(h()), h().groups(), h().group()
could have been
... mat=h(); print repr(mat), mat.groups(), mat.group()
instead.

E.g., using the leftover value in h:

>>> print repr(h()), h().groups(), h().group()
<_sre.SRE_Match object at 0x007F6F80> () ghi
>>> mat=h(); print repr(mat), mat.groups(), mat.group()
<_sre.SRE_Match object at 0x007F6F80> () ghi


>Of course I can do this:
>
>for line in mylist:
> mat = pat.search(line)
> if ...:
> ....
> elif ...:
> ....
> elif mat:
> ...
>but the test is relevant in only a relatively small number of cases.
>And i would like to know if there exists a general solution
>for this kind of problem.

Take a pick, but not the list comprehension

Regards,
Bengt Richter

On 25 Jul 2003 02:17:24 GMT, (Bengt Richter) wrote:
[...]
>You can spell
>
> ... elif setattr(obj,'mat', pat.search(line)) or obj.mat:
> ... print repr(obj.mat), obj.mat.groups(), obj.mat.group()
>
>a little slicker if you make a special object to hold a binding to
>the pat.search result, e.g., h is the Holder instance in the following,
>which remembers the last thing passed to it and immediately returns it,
>and also returns that last thing on being called without an arg:
>
> >>> mylist
> ['', 'a', 'bc', 'def', 'ghij']
> >>> h = Holder()
> >>> for line in mylist:
> ... if 1==2: 'naw'
> ... elif 3==4: 'neither'
> ... elif h(pat.search(line)):
> ... print repr(h()), h().groups(), h().group()
> ... elif 4==5: 'haw'
> ... else:
> ... print 'final else'
> ...
> final else
> final else
> final else
> <_sre.SRE_Match object at 0x007F6FC0> () def
> <_sre.SRE_Match object at 0x007F6F80> () ghi
>
>Obviously
> ... print repr(h()), h().groups(), h().group()
>could have been
> ... mat=h(); print repr(mat), mat.groups(), mat.group()
>instead.
>
Sorry, I seem to have snipped out a Holder definition, in case anyone cares:

>>> class Holder(object):
... def __call__(self, *args):
... if args: self.val = args[0]
... return self.val
...
>>> h = Holder()
>>> h(123)
123
>>> h()
123
>>> h(345),h(),h(67,h() # result depends on left-to-right eval here
(345, 345, 678, 67

Regards,
Bengt Richter