Using default None values in Python when assigning split() to a tuple

Question

Let's say I want to parse audio track information into two variables like this:

• '2/15' -> track = 2, num_tracks = 15
• '7' -> track = 7, num_tracks = None

What's an elegant way to do this in Python? For example:

track, num_tracks = track_info.split('/')

This works for the first format, but not for the second, raising ValueError: need more than 1 value to unpack. So I came up with this:

try:
    track, num_tracks = track_info.split('/', 1)
except:
    track, num_tracks = track_info, None

How can this be more elegant or better?

Answer 1

try:
    track, num_tracks = track_info.split('/', 1)
except:
    track, num_tracks = track_info, None

Honestly, this is not a terrible solution. But you should almost never use except:; you should catch a specific exception.

Here is another way:

tracks, _, num_tracks = text.partition('/')
return int(tracks), int(num_tracks) if num_tracks else None

Answer 2

You can concatenate an array with your default value.

track, num_tracks = (track_info.split('/', 1) + [None])[:2]

Answer 3

This may be overly generic, but could be reused. It "pads" a sequence by making it long enough to fit the requested length, adding as many repetitions as necessary of a given padding item.

def right_pad(length, seq, padding_item=None):
    missing_items = length - len(seq)
    return seq + missing_items * [ padding_item]

If the sequence is long enough already, nothing will be added. So the idea is to extend to 2 elements with None, if the second is not present:

track, num_tracks = right_pad(2, track_info.split('/'))