Python: Unpacking works with one item!

One of the advantages in python is that it is very practical to unpack variables out of short lists and tuples. Some pieces of code which would otherwise be repetitive and ugly (a = lst[0]; b = lst[1]) end up clean, short and easy to read.

>>> a,b = [1,2]
>>> a
1
>>> b
2
>>> 

It's the reason behind python's multi-return values making sense. When we create code that is going to be used by other modules, it opens up a lot of possibilities, and eases the writing and understanding of the client code that uses them.

>>> #create_monkey will use the monkey count in its calculations.
... This value is very useful for client code, but it is rather expensive to obtain.
... monkey, new_monkey_count = create_monkey()
>>>

My discovery today is that you can "unpack" a list or tuple even when it contains only one item. Check it out:

ActivePython 2.7.2.5 (ActiveState Software Inc.) based on
Python 2.7.2 (default, Jun 24 2011, 12:22:14) [MSC v.1500 64 bit (AMD64)] on win
32
Type "help", "copyright", "credits" or "license" for more information.
>>> a, = [1]
>>> a
1
>>> a, = []
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 0 values to unpack
>>> 

This opens up some possibilities. If we are one hundred percent sure that the list contains a single value, we can unpack that into a variable instead of using [0], which is inherently ugly.

When you are not sure whether the container is empty (or more than one item), you can always catch the resulting ValueError. Or use this:

>>> (a,) = [] or ['default']
>>> a
'default'
>>> 

However, the comma is very subtle and future code readers (including the smartass who decided to use this obscure thang) will probably not notice it. This effectively makes refactoring affected code tricky.

Furthermore, someone may think that it is a syntax error, edit the comma out, and TypeErrors and ValueErrors start popping up everywhere. Subtle bug hunting fun!

There is a little workaround for these readability issues, which is to use full tuple syntax:

>>> (a,) = [1]
>>> a
1
>>>

So it seems like I can use this single-unpacking to do good and not just evil.
It seems to be really interesting, but I am not sure whether I should use it in real code. Seems like I have some meditation to do.
In the meantime, I can show off a little.

>>> a, = 1,
>>> a
1
>>>