Variable arity with *args#

A function’s arity is the number of arguments it expects. For example math.sqrt has an arity of 1; the function binary_search has an arity of 2. The function print… can take 0, 1, or however many arguments. We say that print has variable arity.

Python lets you define variable arity by marking a special argument with a *; conventionally, this argument is called args. Here’s an example:

Notice a few things:

  • Every declared positional argument must be present (note the final exception).

  • The *args parameter receives any “excess” paramters.

  • The *args parameter is a tuple.

In principle, you could just use *args, never declaring positional parameters. That would be a terrible idea: positional argument document clearly what a function needs, making it an error to leave them out. Parsing out coherent arguments from *args is tedious and error-prone, too.

Supplying *args#

It’s also possible to call a function using explicit variable arity; mark the argument list with a *. Here’s an example:

Using *args when calling says, “args is iterable, and you should use is for the rest of the arguments for this function”. The first three examples show how positional variables interact; the fourth example shows that any iterable works—even a string. The last example shows that we get an error when *args doesn’t give us enough arguments.