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 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()

Answer 2

Plain and simple, also passes pep8 checks:

if (
    cond1 and
    cond2
):
    print("Hello World!")

---

In recent times I have been preferring the all and any functions, since I rarely mix And and Or comparisons this works well, and has the additional advantage of Failing Early with generators comprehension:

if all([
    cond1,
    cond2,
]):
    print("Hello World!")

Just remember to pass in a single iterable! Passing in N-arguments is not correct.

Note: any is like many or comparisons, all is like many and comparisons.

---

This combines nicely with generator comprehensions, for example:

# Check if every string in a list contains a substring:
my_list = [
    'a substring is like a string', 
    'another substring'
]

if all('substring' in item for item in my_list):
   print("Hello World!")

# or

if all(
    'substring' in item
    for item in my_list
):
    print("Hello World!")

More on: generator comprehension

Answer 3

Here's what I do, remember that "all" and "any" accepts an iterable, so I just put a long condition in a list and let "all" do the work.

condition = [cond1 == 'val1', cond2 == 'val2', cond3 == 'val3', cond4 == 'val4']

if all(condition):
   do_something

Answer 4

Personally, I like to add meaning to long if-statements. I would have to search through code to find an appropriate example, but here's the first example that comes to mind: let's say I happen to run into some quirky logic where I want to display a certain page depending on many variables.

English: "If the logged-in user is NOT an administrator teacher, but is just a regular teacher, and is not a student themselves..."

if not user.isAdmin() and user.isTeacher() and not user.isStudent():
    doSomething()

Sure this might look fine, but reading those if statements is a lot of work. How about we assign the logic to label that makes sense. The "label" is actually the variable name:

displayTeacherPanel = not user.isAdmin() and user.isTeacher() and not user.isStudent()
if displayTeacherPanel:
    showTeacherPanel()

This may seem silly, but you might have yet another condition where you ONLY want to display another item if, and only if, you're displaying the teacher panel OR if the user has access to that other specific panel by default:

if displayTeacherPanel or user.canSeeSpecialPanel():
    showSpecialPanel()

Try writing the above condition without using variables to store and label your logic, and not only do you end up with a very messy, hard-to-read logical statement, but you also just repeated yourself. While there are reasonable exceptions, remember: Don't Repeat Yourself (DRY).

Answer 5

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

or if this is clearer:

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

There is no reason indent should be a multiple of 4 in this case, e.g. see "Aligned with opening delimiter":

http://google-styleguide.googlecode.com/svn/trunk/pyguide.html?showone=Indentation#Indentation

Answer 6

I guess something like this is the most readable option:

for pet in zoo:
    cute = every_pet()
    furry = hair is 'over9000'
    small = size < min_size
    if cute and furry and small:
        return 'I need to feed it!'