Question

I’m looking for a way to easily determine if all not None items in a list occur in a single continuous slice. I’ll use integers as examples of not None items.

For example, the list [None, None, 1, 2, 3, None, None] meets my requirements for continuous integer entries. By contrast, [1, 2, None, None, 3, None] is not continuous, because there are None entries between integers.

Some more examples to make this a clear as possible.

Continuous:
[1, 2, 3, None, None]
[None, None, 1, 2, 3]
[None, 1, 2, 3, None]

Not Continuous:
[None, 1, None, 2, None, 3]
[None, None, 1, None, 2, 3]
[1, 2, None, 3, None, None]

My first approach was to use variables to keep track of whether or not we had come across a None yet, and whether or not we had come across an int yet — this ends up with a highly nested and very difficult to follow series of if/else statements embedded in a for loop. (On top of the ugliness, I confess I haven’t gotten it to work in every case).

Anyone know an easier way to figure out if the not None items in a list occur in a single continuous slice?

Answer 1

def contiguous(seq):
    seq = iter(seq)
    all(x is None for x in seq)        # Burn through any Nones at the beginning
    any(x is None for x in seq)        # and the first group
    return all(x is None for x in seq) # everthing else (if any) should be None.

Here are a couple of examples. You can use next(seq) to get the next item from an iterator. I’ll put a mark pointing to the next item after each

example1:

seq = iter([None, 1, 2, 3, None])        #  [None, 1, 2, 3, None]
                                         # next^
all(x is None for x in seq)            
                                         #        next^
any(x is None for x in seq)            
                                         #                    next^ (off the end)
return all(x is None for x in seq)       # all returns True for the empty sequence

example2:

seq = iter([1, 2, None, 3, None, None])  #    [1, 2, None, 3, None, None]
                                         # next^
all(x is None for x in seq)            
                                         #    next^
any(x is None for x in seq)            
                                         #             next^  
return all(x is None for x in seq)       # all returns False when 3 is encountered