Loop once... (f)or else! (Or, Python pro-tip number 2)

Once upon a time, some seven years ago, I wrote a post about a Python trick for accessing the first element of any kind of sequence.

The gist of that article is that just using indexing isn’t general enough: if the sequence is lazily computed by a generator, then indexing won’t work. What does work, however, is to iterate over the sequence using a for loop. But since we just want the first element, let’s immediately break out of the loop.

for x in seq:
    print(x) # or do whatever
    break

But there’s something I missed in that post! What if the sequence is empty?

An obvious solution is to use a boolean:

entered = False
for x in seq:
    entered = True
    print(x)
    break
if not entered:
    pass # do whatever

But that’s pretty ugly if not outright disgusting. Instead, we can take advantage of a little-known feature of Python’s for-loops: in Python, a for-loop can have an else block! The semantics are that if the loop terminates normally, then the else-block is run; else, if the loop exits early, e.g. by a break, then the else-block is skipped.

These semantics are frankly weird to me, but they end up working out marvellously for our current use case. It almost makes me think that this is why else was allowed in the first place for a for-loop. Consider this.

for x in seq:
    print(x)
    break
else:
    print("nothing in the sequence")

If the sequence is nonempty, then we will print the first item in the sequence and abort the loop, skipping the else-block. If the sequence is empty, then the loop will exit normally, so the else-block will run!