4.2. Using Optional and Named Arguments

Python allows function arguments to have default values; if the function is called without the argument, the argument gets its default value. Futhermore, arguments can be specified in any order by using named arguments. Stored procedures in SQL Server Transact/SQL can do this, so if you're a SQL Server scripting guru, you can skim this part.

Here is an example of info, a function with two optional arguments:

def info(object, spacing=10, collapse=1):

spacing and collapse are optional, because they have default values defined. object is required, because it has no default value. If info is called with only one argument, spacing defaults to 10 and collapse defaults to 1. If info is called with two arguments, collapse still defaults to 1.

Say you want to specify a value for collapse but want to accept the default value for spacing. In most languages, you would be out of luck, because you would need to call the function with three arguments. But in Python, arguments can be specified by name, in any order.

Example 4.4. Valid Calls of info

info(odbchelper)                    1
info(odbchelper, 12)                2
info(odbchelper, collapse=0)        3
info(spacing=15, object=odbchelper) 4
1 With only one argument, spacing gets its default value of 10 and collapse gets its default value of 1.
2 With two arguments, collapse gets its default value of 1.
3 Here you are naming the collapse argument explicitly and specifying its value. spacing still gets its default value of 10.
4 Even required arguments (like object, which has no default value) can be named, and named arguments can appear in any order.

This looks totally whacked until you realize that arguments are simply a dictionary. The “normal” method of calling functions without argument names is actually just a shorthand where Python matches up the values with the argument names in the order they're specified in the function declaration. And most of the time, you'll call functions the “normal” way, but you always have the additional flexibility if you need it.

The only thing you need to do to call a function is specify a value (somehow) for each required argument; the manner and order in which you do that is up to you.

Further Reading on Optional Arguments