One way the we can handle optional parameters is by providing a default value for a parameter. If no argument is supplied for the parameter, the default value is used.

def report( spin, count=1 ):
    print spin, count, "times in  a row"

This silly function can be used in two ways:

report( n )
report( n, 2 )

The first form provides a default argument of 1 for the count parameter. The second form has an explicit argument value of 2 for the count parameter.

If a parameter has no default value, it is not optional. If a parameter has a default value, it is optional. In order to disambiguate the assignment of arguments to parameters, Python uses a simple rule: all required parameters must be first, all optional parameters must come after the required parameters.

The int function does this. We can say int("23") to do decimal conversion and int("23",16) to do hexadecimal conversion. Clearly, the second argument to int has a default value of 10.

When we look at the Python range function, we see a more sophisticated version of this.

range (x) is the same as range (0, x, 1)

range (x, y) is the same as range (x, y, 1)

It appears from these examples that the first parameter is optional. The authors of Python use a pretty slick trick for this that you can use also. The range function behaves as though the following function is defined.

def range(x, y=None, z=None):
    if y==None:
        start, stop, step = 0, x, 1
    elif z==None:
        start, stop, step = x, y, 1
    else:
        start, stop, step = x, y, z
    Real work is done with start, stop and step

By providing a default of None, the function can determine whether a value was supplied or not supplied. This allows for complex default handling within the body of the function.

Conclusion. Python must find a value for all parameters. The basic rule is that the values of parameters are set in the order in which they are declared. Any missing parameters will have their default values assigned. These are called positional parameters, since the position is the rule used for assigning argument values when the function is applied.

If a mandatory parameter (a parameter without a default value) is missing, this is a basic TypeError. For example.

#!/usr/bin/env python
def hack(a,b):
    print a+b

hack(3)

When we run this example, we see the following.

$ python badcall.py

Traceback (most recent call last):
  File "badcall.py", line 5, in ?
    hack(3)
TypeError: hack() takes exactly 2 arguments (1 given)

$ 

In addition to supplying argument values by position, Python also permits argument values to be specified by name. Using explicit keywords can make programs much easier to read.

First, we'll define a function with a simple parameter list:

import random

def averageDice( samples=100 ):
    """Return the average of a number of throws of 2 dice."""
    s = 0
    for i in range(samples):
        d1,d2 = random.randrange(6)+1,random.randrange(6)+1
        s += d1+d2
    return float(s)/float(samples)

Next, we'll show three different kinds of arguments: keyword, positional, and default.

test1 = averageDice( samples=200 )
test2 = averageDice( 300 )
test3 = averageDice()

When the averageDice function is evaluated to set test1, the keyword form is used. The second call of the averageDice function uses the positional form. The final example relies on a default for the parameter.

Conclusion. This gives us a number of variations including positional parameters and keyword parameters, both with and without defaults. Positional parameters work well when there are few parameters and their meaning is obvious. Keyword parameters work best when there are a lot of parameters, especially when there are optional parameters.

Good use of keyword parameters mandates good selection of keywords. Single-letter parameter names or obscure abbreviations do not make keyword parameters helpfully informative.

Here are the rules we've seen so far:

There are still more options available for handling variable numbers of parameters. It's possible for additional positional parameters to be collected into a sequence object. Further, additional keyword parameters can be collected into a dictionary object. We'll get to them when we cover dictionaries in the section called “Advanced Parameter Handling For Functions”.

One common desire among programmers is a feature that allows a function to return multiple values. Python has some built-in functions that have this property. For example, divmod returns the divisor and remainder in division. We could imagine a function, rollDice that would return two values showing the faces of two dice.

In Python, it is done by returning a tuple. We'll wait for Chapter 13, Tuples for complete information on tuples. The following is a quick example of how multiple assignment works with functions that return multiple values.

import random

def rollDice():
    return ( 1 + random.randrange(6), 1 + random.randrange(6) )

d1,d2=rollDice()
print d1,d2

This shows a function that creates a two-valued tuple. You'll recall from the section called “Multiple Assignment Statement” that Python is perfectly happy with multiple expressions on the right side of =, and multiple destination variables on the left side. This is one reason why multiple assignment is so handy.