How to exit an if clause

Posted on

Question :

How to exit an if clause

What sorts of methods exist for prematurely exiting an if clause?

There are times when I’m writing code and want to put a break statement inside of an if clause, only to remember that those can only be used for loops.

Lets take the following code as an example:

if some_condition:
   ...
   if condition_a:
       # do something
       # and then exit the outer if block
   ...
   if condition_b:
       # do something
       # and then exit the outer if block
   # more code here

I can think of one way to do this: assuming the exit cases happen within nested if statements, wrap the remaining code in a big else block. Example:

if some_condition:
   ...
   if condition_a:
       # do something
       # and then exit the outer if block
   else:
       ...
       if condition_b:
           # do something
           # and then exit the outer if block
       else:
           # more code here

The problem with this is that more exit locations mean more nesting/indented code.

Alternatively, I could write my code to have the if clauses be as small as possible and not require any exits.

Does anyone know of a good/better way to exit an if clause?

If there are any associated else-if and else clauses, I figure that exiting would skip over them.

Asked By: Roman

||

Answer #1:

(This method works for ifs, multiple nested loops and other constructs that you can’t break from easily.)

Wrap the code in its own function. Instead of break, use return.

Example:

def some_function():
    if condition_a:
        # do something and return early
        ...
        return
    ...
    if condition_b:
        # do something else and return early
        ...
        return
    ...
    return

if outer_condition:
    ...
    some_function()
    ...
Answered By: Drew Dormann

Answer #2:

from goto import goto, label

if some_condition:
   ...
   if condition_a:
       # do something
       # and then exit the outer if block
       goto .end
   ...
   if condition_b:
       # do something
       # and then exit the outer if block
       goto .end
   # more code here

label .end

(Don’t actually use this, please.)

Answered By: ephemient

Answer #3:

while some_condition:
   ...
   if condition_a:
       # do something
       break
   ...
   if condition_b:
       # do something
       break
   # more code here
   break
Answered By: Thomas Eding

Answer #4:

You can emulate goto’s functionality with exceptions:

try:
    # blah, blah ...
    # raise MyFunkyException as soon as you want out
except MyFunkyException:
    pass

Disclaimer: I only mean to bring to your attention the possibility of doing things this way, while in no way do I endorse it as reasonable under normal circumstances. As I mentioned in a comment on the question, structuring code so as to avoid Byzantine conditionals in the first place is preferable by far. 🙂

Answered By: Micha? Marczyk

Answer #5:

may be this?

if some_condition and condition_a:
       # do something
elif some_condition and condition_b:
           # do something
           # and then exit the outer if block
elif some_condition and not condition_b:
           # more code here
else:
     #blah
if
Answered By: ghostdog74

Answer #6:

For what was actually asked, my approach is to put those ifs inside a one-looped loop

while (True):
    if (some_condition):
        ...
        if (condition_a):
            # do something
            # and then exit the outer if block
            break
        ...
        if (condition_b):
            # do something
            # and then exit the outer if block
            break
        # more code here
    # make sure it is looped once
    break

Test it:

conditions = [True,False]
some_condition = True

for condition_a in conditions:
    for condition_b in conditions:
        print("n")
        print("with condition_a", condition_a)
        print("with condition_b", condition_b)
        while (True):
            if (some_condition):
                print("checkpoint 1")
                if (condition_a):
                    # do something
                    # and then exit the outer if block
                    print("checkpoint 2")
                    break
                print ("checkpoint 3")
                if (condition_b):
                    # do something
                    # and then exit the outer if block
                    print("checkpoint 4")
                    break
                print ("checkpoint 5")
                # more code here
            # make sure it is looped once
            break
Answered By: izzulmakin

Answer #7:

Generally speaking, don’t. If you are nesting “ifs” and breaking from them, you are doing it wrong.

However, if you must:

if condition_a:
   def condition_a_fun():
       do_stuff()
       if we_wanna_escape:
           return
   condition_a_fun()
if condition_b:
   def condition_b_fun():
       do_more_stuff()
       if we_wanna_get_out_again:
           return
   condition_b_fun()

Note, the functions don’t HAVE to be declared in the if statement, they can be declared in advance 😉 This would be a better choice, since it will avoid needing to refactor out an ugly if/then later on.

Answered By: Enki

Answer #8:

Effectively what you’re describing are goto statements, which are generally panned pretty heavily. Your second example is far easier to understand.

However, cleaner still would be:

if some_condition:
   ...
   if condition_a:
       your_function1()
   else:
       your_function2()

...

def your_function2():
   if condition_b:
       # do something
       # and then exit the outer if block
   else:
       # more code here
Answered By: Smashery

Leave a Reply

Your email address will not be published.