Using list comprehensions and exceptions?

Question

Okay lets say i have a list, and i want to check if that list exists within another list. I can do that doing this:

all(value in some_map for value in required_values)

Which works fine, but lets say i want to the raise an exception when a required value is missing, with the value that it is missing. How can i do that using list comprehension?

I'm more or less curious, all signs seem to point to no.

EDIT Argh I meant this:

for value in required_values:
 if value not in some_map:
  raise somecustomException(value)

Looking at those i cant see how i can find the value where the error occured

Answer 1

If you don't want to consider duplicates and the values are hashable, use sets. They're easier, faster, and can extract "all" elements missing in a single operation:

required_values = set('abc') # store this as a set from the beginning
values = set('ab')
missing = required_values - values
if missing:
    raise SomeException('The values %r are not in %r' % 
                        (missing, required_values))

Answer 2

You can't use raise in a list comprehension. You can check for yourself by looking at the grammar in the Python Language Reference.

You can however, invoke a function which raises an exception for you.

Answer 3

lets say i want to the raise an exception when a required value is missing, with the value that it is missing. How can i do that using list comprehension?

List comprehensions are a syntactically concise way to create a list based on some existing list—they're not a general-purpose way of writing any for-loop in a single line. In this example, you're not actually creating a list, so it doesn't make any sense to use a list comprehension.

Answer 4

While I think using sets (like nosklo's example) is better, you could do something simple like this:

def has_required(some_map, value):
  if not value in some_map:
    raise RequiredException('Missing required value: %s' % value)

all(has_required(some_map, value) for value in required_values)

Answer 5

Another (ugly) possibility would be the error_on_false function:

def error_on_false(value)
    if value:
        return value
    else:
        raise Exception('Wrong value: %r' % value)

if all(error_on_false(value in some_map) for value in required_values):
    continue_code()
    do_something('...')

That's ugly. I'd use the set instead.