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Styling multi-line conditions in 'if' statements?


Question

Sometimes I break long conditions in ifs onto several lines. The most obvious way to do this is:

  if (cond1 == 'val1' and cond2 == 'val2' and
      cond3 == 'val3' and cond4 == 'val4'):
      do_something

Isn't very very appealing visually, because the action blends with the conditions. However, it is the natural way using correct Python indentation of 4 spaces.

For the moment I'm using:

  if (    cond1 == 'val1' and cond2 == 'val2' and
          cond3 == 'val3' and cond4 == 'val4'):
      do_something

But this isn't very pretty. :-)

Can you recommend an alternative way?

2017/05/30
1
699
5/30/2017 5:35:39 PM

Accepted Answer

You don't need to use 4 spaces on your second conditional line. Maybe use:

if (cond1 == 'val1' and cond2 == 'val2' and 
       cond3 == 'val3' and cond4 == 'val4'):
    do_something

Also, don't forget the whitespace is more flexible than you might think:

if (   
       cond1 == 'val1' and cond2 == 'val2' and 
       cond3 == 'val3' and cond4 == 'val4'
   ):
    do_something
if    (cond1 == 'val1' and cond2 == 'val2' and 
       cond3 == 'val3' and cond4 == 'val4'):
    do_something

Both of those are fairly ugly though.

Maybe lose the brackets (the Style Guide discourages this though)?

if cond1 == 'val1' and cond2 == 'val2' and \
   cond3 == 'val3' and cond4 == 'val4':
    do_something

This at least gives you some differentiation.

Or even:

if cond1 == 'val1' and cond2 == 'val2' and \
                       cond3 == 'val3' and \
                       cond4 == 'val4':
    do_something

I think I prefer:

if cond1 == 'val1' and \
   cond2 == 'val2' and \
   cond3 == 'val3' and \
   cond4 == 'val4':
    do_something

Here's the Style Guide, which (since 2010) recommends using brackets.

2016/05/27
775
5/27/2016 1:32:28 PM

I've resorted to the following in the degenerate case where it's simply AND's or OR's.

if all( [cond1 == 'val1', cond2 == 'val2', cond3 == 'val3', cond4 == 'val4'] ):

if any( [cond1 == 'val1', cond2 == 'val2', cond3 == 'val3', cond4 == 'val4'] ):

It shaves a few characters and makes it clear that there's no subtlety to the condition.

2008/10/08

Someone has to champion use of vertical whitespace here! :)

if (     cond1 == val1
     and cond2 == val2
     and cond3 == val3
   ):
    do_stuff()

This makes each condition clearly visible. It also allows cleaner expression of more complex conditions:

if (    cond1 == val1
     or 
        (     cond2_1 == val2_1
          and cond2_2 >= val2_2
          and cond2_3 != bad2_3
        )
   ):
    do_more_stuff()

Yes, we're trading off a bit of vertical real estate for clarity. Well worth it IMO.

2008/10/08

I prefer this style when I have a terribly large if-condition:

if (
    expr1
    and (expr2 or expr3)
    and hasattr(thingy1, '__eq__')
    or status=="HappyTimes"
):
    do_stuff()
else:
    do_other_stuff()
2018/11/06

Here's my very personal take: long conditions are (in my view) a code smell that suggests refactoring into a boolean-returning function/method. For example:

def is_action__required(...):
    return (cond1 == 'val1' and cond2 == 'val2'
            and cond3 == 'val3' and cond4 == 'val4')

Now, if I found a way to make multi-line conditions look good, I would probably find myself content with having them and skip the refactoring.

On the other hand, having them perturb my aesthetic sense acts as an incentive for refactoring.

My conclusion, therefore, is that multiple line conditions should look ugly and this is an incentive to avoid them.

2016/08/04

This doesn't improve so much but...

allCondsAreOK = (cond1 == 'val1' and cond2 == 'val2' and
                 cond3 == 'val3' and cond4 == 'val4')

if allCondsAreOK:
   do_something
2008/10/08

Source: https://stackoverflow.com/questions/181530
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