how to format long if conditions

[Arnaud Delobelle]

On 27 August 2011 08:24, Steven D'Aprano
<steve+comp.lang.python at 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:
>>
>>
>>         if (isinstance(left, PyCompare) and isinstance(right, PyCompare)
>>                 and left.complist[-1] is right.complist[0]):
>>             py_and = PyCompare(left.complist + right.complist[1:])
>>         else:
>>             py_and = PyBooleanAnd(left, right)
>>
>> What would you do?
>
> I believe that PEP 8 now suggests something like this:
>
>        if (
>                isinstance(left, PyCompare) and isinstance(right, PyCompare)
>                and left.complist[-1] is right.complist[0]):
>            )
>            py_and = PyCompare(left.complist + right.complist[1:]
>        else:
>            py_and = PyBooleanAnd(left, right)
>
>
> I consider that hideous and would prefer to write this:
>
>
>        if (isinstance(left, PyCompare) and isinstance(right, PyCompare)
>            and left.complist[-1] is right.complist[0]):
>            py_and = PyCompare(left.complist + right.complist[1:]
>        else:
>            py_and = PyBooleanAnd(left, right)
>
>
> Or even this:
>
>        tmp = (
>            isinstance(left, PyCompare) and isinstance(right, PyCompare)
>            and left.complist[-1] is right.complist[0])
>            )
>        if tmp:
>            py_and = PyCompare(left.complist + right.complist[1:]
>        else:
>            py_and = PyBooleanAnd(left, right)
>
>
> But perhaps the best solution is to define a helper function:
>
> def is_next(left, right):
>    """Returns True if right is the next PyCompare to left."""
>    return (isinstance(left, PyCompare) and isinstance(right, PyCompare)
>        and left.complist[-1] is right.complist[0])
>    # PEP 8 version left as an exercise.
>
>
> # later...
>        if is_next(left, right):
>            py_and = PyCompare(left.complist + right.complist[1:]
>        else:
>            py_and = PyBooleanAnd(left, right)
>

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] == 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 = left.chain(right)
        else:
            py_and = 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).

-- 
Arnaud