Styling multi-line conditions in 'if' stat

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?

Answer 1

I've been struggling to find a decent way to do this as well, so I just came up with an idea (not a silver bullet, since this is mainly a matter of taste).

if bool(condition1 and
        condition2 and
        ...
        conditionN):
    foo()
    bar()

I find a few merits in this solution compared to others I've seen, namely, you get exactly an extra 4 spaces of indentation (bool), allowing all conditions to line up vertically, and the body of the if statement can be indented in a clear(ish) way. This also keeps the benefits of short-circuit evaluation of boolean operators, but of course adds the overhead of a function call that basically does nothing. You could argue (validly) that any function returning its argument could be used here instead of bool, but like I said, it's just an idea and it's ultimately a matter of taste.

Funny enough, as I was writing this and thinking about the "problem", I came up with yet another idea, which removes the overhead of a function call. Why not indicate that we're about to enter a complex condition by using extra pairs of parentheses? Say, 2 more, to give a nice 2 space indent of the sub-conditions relative to the body of the if statement. Example:

if (((foo and
      bar and
      frob and
      ninja_bear))):
    do_stuff()

I kind of like this because when you look at it, a bell immediatelly rings in your head saying "hey, there's a complex thing going on here!". Yes, I know that parentheses don't help readability, but these conditions should appear rarely enough, and when they do show up, you are going to have to stop and read them carefuly anyway (because they're complex).

Anyway, just two more proposals that I haven't seen here. Hope this helps someone :)

Answer 2

I suggest moving the and keyword to the second line and indenting all lines containing conditions with two spaces instead of four:

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

This is exactly how I solve this problem in my code. Having a keyword as the first word in the line makes the condition a lot more readable, and reducing the number of spaces further distinguishes condition from action.

Answer 3

"all" and "any" are nice for the many conditions of same type case. BUT they always evaluates all conditions. As shown in this example:

def c1():
    print " Executed c1"
    return False
def c2():
    print " Executed c2"
    return False


print "simple and (aborts early!)"
if c1() and c2():
    pass

print

print "all (executes all :( )"
if all((c1(),c2())):
    pass

print

Answer 4

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.

Answer 5

(I've lightly modified the identifiers as fixed-width names aren't representative of real code – at least not real code that I encounter – and will belie an example's readability.)

if (cond1 == "val1" and cond22 == "val2"
and cond333 == "val3" and cond4444 == "val4"):
    do_something

This works well for "and" and "or" (it's important that they're first on the second line), but much less so for other long conditions. Fortunately, the former seem to be the more common case while the latter are often easily rewritten with a temporary variable. (It's usually not hard, but it can be difficult or much less obvious/readable to preserve the short-circuiting of "and"/"or" when rewriting.)

Since I found this question from your blog post about C++, I'll include that my C++ style is identical:

if (cond1 == "val1" and cond22 == "val2"
and cond333 == "val3" and cond4444 == "val4") {
    do_something
}

Answer 6

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.