Assignment and comparison in one statement

[Paul McGuire]

On May 24, 6:12 am, Johannes Bauer <dfnsonfsdu... at gmx.de> wrote:
> Carl Banks schrieb:
>
> > p = myfunction()
> > if p:
> >     print p
>
> > (I recommend doing it this way in C, too.)
>
> This is okay for if-clauses, but sucks for while-loops:
>
> while (fgets(buf, sizeof(buf), f)) {
>         printf("%s\n", buf);
>
> }
>
> is much shorter than
>
> char *tmp;
> tmp = fgets(buf, sizeof(buf), f);
> while (tmp) {
>         printf("%s\n", buf);
>         tmp = fgets(buf, sizeof(buf), f);
>
> }
> > For the case where you want to do this in an elif-clause, look for the
> > recent thread "C-like assignment expression?" which list some
> > workarounds.  Or just Google this newsgroup for "assignment
> > expression".  It's one of few minor irritating things in Python
> > syntax.
>
> Alright, I will. Thanks.
>
> Regards,
> Johannes
>
> --
> "Wer etwas kritisiert muss es noch lange nicht selber besser können. Es
> reicht zu wissen, daß andere es besser können und andere es auch
> besser machen um einen Vergleich zu bringen."     -     Wolfgang Gerber
>        in de.sci.electronics <47fa8447$0$11545$9b622... at news.freenet.de>

I posted this to the other thread, but I'm afraid it might get buried
over there.  See if this works for you.

class TestValue(object):
    """Class to support assignment and test in single operation"""
    def __init__(self,v=None):
        self.value = v

    """Add support for quasi-assignment syntax using '<<' in place of
'='."""
    def __lshift__(self,other):
        self.value = other
        return self

    def __bool__(self):
        return bool(self.value)
    __nonzero__ = __bool__


import re

tv = TestValue()
integer = re.compile(r"[-+]?\d+")
real = re.compile(r"[-+]?\d*\.\d+")
word = re.compile(r"\w+")

for inputValue in ("123 abc -3.1".split()):
    if (tv << real.match(inputValue)):
        print "Real", float(tv.value.group())
    elif (tv << integer.match(inputValue)):
        print "Integer", int(tv.value.group())
    elif (tv << word.match(inputValue)):
        print "Word", tv.value.group()

Prints:

Integer 123
Word abc
Real -3.1

In your examples, this could look like:

tv = TestValue()
while (tv << fgets(buf, sizeof(buf), f)) {
         printf("%s\n", tv.value);

or:

if (tv << myfunction()):


What do you think?  Is '<<' close enough to '=' for you?

-- Paul