Single value unpacking in Python

Just a short description of a non-surprising but non-obvious feature of tuple unpacking in Python (2.7, not even the cool new unpacking in 3.5).

Tuple unpacking is the extraction, the unpacking, of values from a tuple into an equal number of new variable names.

>>> point = (45.34, -23.12)
>>> x, y = point
>>> x
45.34
>>> y
-23.12

You can do the same thing with a tuple of one value.

>>> value = (123,)
>>> x, = value
>>> x
123

I said this is unsurprising because unpacking is kind of like pattern matching into another tuple on the left side (emphasis on “kind of”) and a tuple of one value is represented by a trailing comma like in the assignment above to value.

Here’s an alternative to unpacking this way:

>>> value = (123,)
>>> x = value[0]
>>> x
123

So what’s the benefit of single value unpacking versus using the index?

It’s certainly more idiomatic with regard to tuples so there’s that reason. It also may make for more consistent code if you’re unpacking other tuples of more than one value in the same code block. A better reason is Pythonic forgiveness seeking when extracting values with constraints.

The motivating example is handling *args for one and only one value. For example, a Django management command has a handle method which must be defined and accept an *args argument. If you want to get the first argument, you can just use the 0 index.

class SomeCommand(BaseCommand):
    def handle(self, *args, **options):
        my_value = args[0]

However you should handle the case where no arguments were passed.

class SomeCommand(BaseCommand):
    def handle(self, *args, **options):
        try:
            my_value = args[0]
        except IndexError:
            raise CommandError("No arguments provided")

This may be sufficient for cases where you have one or more arguments, but if you want to enforce only one positional argument then you’ll need to do something else.

class SomeCommand(BaseCommand):
    def handle(self, *args, **options):
        if len(args) != 1:
            raise CommandError("One argument must be provided")
        my_value = args[0]

That works(!), but it’s not Pythonic. The Pythonic way is to ask forgiveness, not permission. So we use single value unpacking and watch for a ValueError.

class SomeCommand(BaseCommand):
    def handle(self, *args, **options):
        try:
            my_value, = args
        except ValueError:
            raise CommandError("One argument must be provided")

It’s worth reiterating that the previous example is perfectly valid and absent single value unpacking is the way I’d go.