for-else in Python indicating "value not found"
There are tons of cases when we are looking for a value in a list of elements and would like to be able to indicate when we could not find that value.
In most of the languages it is a bit cumbersome.
Find the first match
For example this very simple example in which we are looking for an even number:
numbers = [3, 4, 5] for n in numbers: if n % 2 == 0: even = n break print(even)
If we run this program it will set the name even to be 4 which is what we wanted.
The problem is that if there is no match (there is no even number) then the variable even will never get a value and thus it will be undeclared.
numbers = [3, 5] for n in numbers: if n % 2 == 0: even = n break print(even)
Running this program will result in the following exception:
$ python no_even_number.py Traceback (most recent call last): File "no_even_number.py", line 8, in <module> print(even) NameError: name 'even' is not defined
Default value outside the loop
One solution to this problem employed in most programming languages is to set a default value to the even variable before we enter the loop. That way if we pass all the values without finding a match, the variable even already has a value:
numbers = [3, 5] even = None for n in numbers: if n % 2 == 0: even = n break print(even)
$ python default_result.py None
A more elegant way that can be written in Python is the use of else after the for-loop.
numbers = [3, 5] for n in numbers: if n % 2 == 0: even = n break else: even = None print(even)
The code in the else part will be executed if the loop has finished normally, without calling break.
That mean if we find a match during our loop, assign it to even and leave the loop calling break then the else part won't be executed.
Together with the name- or variable-scoping of Python this will result in the same effect, but in my humble opinion a more elegant way.
Published on 2018-03-27