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Date: Sat, 27 Aug 2011 11:24:33 +0100
Subject: Re: how to format long if conditions
From: Arnaud Delobelle <arnodel@gmail.com>
To: "Steven D'Aprano" <steve+comp.lang.python@pearwood.info>
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On 27 August 2011 08:24, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> Arnaud Delobelle wrote:
>
>> Hi all,
>>
>> I'm wondering what advice you have about formatting if statements with
>> long conditions (I always format my code to <80 colums)
>>
>> Here's an example taken from something I'm writing at the moment and
>> how I've formatted it:
>>
>>
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 if (isinstance(left, PyCompare) and isinstan=
ce(right, PyCompare)
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 and left.complis=
t[-1] is right.complist[0]):
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 py_and =3D PyCompare(left.comp=
list + right.complist[1:])
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 else:
>> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 py_and =3D PyBooleanAnd(left, =
right)
>>
>> What would you do?
>
> I believe that PEP 8 now suggests something like this:
>
> =C2=A0 =C2=A0 =C2=A0 =C2=A0if (
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0isinstance(left, P=
yCompare) and isinstance(right, PyCompare)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0and left.complist[=
-1] is right.complist[0]):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyCompare(left.compli=
st + right.complist[1:]
> =C2=A0 =C2=A0 =C2=A0 =C2=A0else:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyBooleanAnd(left, ri=
ght)
>
>
> I consider that hideous and would prefer to write this:
>
>
> =C2=A0 =C2=A0 =C2=A0 =C2=A0if (isinstance(left, PyCompare) and isinstance=
(right, PyCompare)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0and left.complist[-1] is right.c=
omplist[0]):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyCompare(left.compli=
st + right.complist[1:]
> =C2=A0 =C2=A0 =C2=A0 =C2=A0else:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyBooleanAnd(left, ri=
ght)
>
>
> Or even this:
>
> =C2=A0 =C2=A0 =C2=A0 =C2=A0tmp =3D (
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0isinstance(left, PyCompare) and =
isinstance(right, PyCompare)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0and left.complist[-1] is right.c=
omplist[0])
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0if tmp:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyCompare(left.compli=
st + right.complist[1:]
> =C2=A0 =C2=A0 =C2=A0 =C2=A0else:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyBooleanAnd(left, ri=
ght)
>
>
> But perhaps the best solution is to define a helper function:
>
> def is_next(left, right):
> =C2=A0 =C2=A0"""Returns True if right is the next PyCompare to left."""
> =C2=A0 =C2=A0return (isinstance(left, PyCompare) and isinstance(right, Py=
Compare)
> =C2=A0 =C2=A0 =C2=A0 =C2=A0and left.complist[-1] is right.complist[0])
> =C2=A0 =C2=A0# PEP 8 version left as an exercise.
>
>
> # later...
> =C2=A0 =C2=A0 =C2=A0 =C2=A0if is_next(left, right):
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyCompare(left.compli=
st + right.complist[1:]
> =C2=A0 =C2=A0 =C2=A0 =C2=A0else:
> =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0py_and =3D PyBooleanAnd(left, ri=
ght)
>

Thanks Steven and Hans for you suggestions.  For this particular
instance I've decided to go for a hybrid approach:

* Add two methods to PyCompare:

class PyCompare(PyExpr):
    ...
    def extends(self, other):
        if not isinstance(other, PyCompare):
            return False
        else:
            return self.complist[0] =3D=3D other.complist[-1]
    def chain(self, other):
        return PyCompare(self.complist + other.complist[1:])

* Rewrite the if as:

        if isinstance(right, PyCompare) and right.extends(left):
            py_and =3D left.chain(right)
        else:
            py_and =3D PyBooleanAnd(left, right)


The additional benefit is to hide the implementation details of
PyCompare (I suppose this could illustrate the thread on when to
create functions).

--=20
Arnaud