Pythonic way to determine whether not null list entries are 'continuous'

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

Answer 1

I did some profiling to compare @gnibbler's approach with the groupby approach. @gnibber's approach is consistently faster, esp. for longer lists. E.g., I see about a 50% performance gain for random inputs with length 3-100, with a 50% chance of containing a single int sequence (randomly selected), and otherwise with random values. Test code below. I interspersed the two methods (randomly selecting which one goes first) to make sure any caching effects get cancelled out. Based on this, I'd say that while the groupby approach is more intuitive, @gnibber's approach may be appropriate if profiling indicates that this is an important part of the overall code to optimize -- in that case, appropriate comments should be used to indicate what's going on with the use of all/any to consumer iterator values.

from itertools import groupby
import random, time

def contiguous1(seq):
    # gnibber's approach
    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.

def contiguous2(seq):
    return sum(1 for k,g in groupby(seq, lambda x: x is not None) if k) == 1

times = {'contiguous1':0,'contiguous2':0}

for i in range(400000):
    n = random.randint(3,100)
    items = [None] * n
    if random.randint(0,1):
        s = random.randint(0,n-1)
        e = random.randint(0,n-s)
        for i in range(s,e):
            items[i] = 3
    else:
        for i in range(n):
            if not random.randint(0,2):
                items[i] = 3
    if random.randint(0,1):
        funcs = [contiguous1, contiguous2]
    else:
        funcs = [contiguous2, contiguous1]
    for func in funcs:
        t0 = time.time()
        func(items)
        times[func.__name__] += (time.time()-t0)

print
for f,t in times.items():
    print '%10.7f %s' % (t, f)