Partial list unpack in Python

Question

In Python, the assignment operator can unpack a list or a tuple into variables, like this:

l = (1, 2) a, b = l # Here goes auto unpack 

But I need to specify exactly the same amount of names to the left as an item count in the list to the right. But sometimes I don't know the size of the list to the right, for example, if I use split().

Example:

a, b = "length=25".split("=") # This will result in a="length" and b=25 

But the following code will lead to an error:

a, b = "DEFAULT_LENGTH".split("=") # Error, list has only one item 

Is it possible to somehow unpack the list in the example above so I can get a = "DEFAULT_LENGTH" and b equals to None or not set? A straightforward way looks kind of long:

a = b = None if "=" in string :   a, b = string.split("=") else :   a = string 

Answer 1

This may be of no use to you unless you're using Python 3. However, for completeness, it's worth noting that the extended tuple unpacking introduced there allows you to do things like:

>>> a, *b = "length=25".split("=") >>> a,b ("length", ['25']) >>> a, *b = "DEFAULT_LENGTH".split("=") >>> a,b ("DEFAULT_LENGTH", []) 

I.e. tuple unpacking now works similarly to how it does in argument unpacking, so you can denote "the rest of the items" with *, and get them as a (possibly empty) list.

Partition is probably the best solution for what you're doing however.

Answer 2

# this will result in a="length" and b="25" a, b = "length=25".partition("=")[::2]  # this will result in a="DEFAULT_LENGTH" and b="" a, b = "DEFAULT_LENGTH".partition("=")[::2]