Variable Number of Arguments

Sometimes when you write a function, it's not possible to know ahead of time how many arguments you will be providing to the function. Suppose we wish to write a function which will examine an arbitrary number of strings, and return the length of the longest string. (An alternative would be to write a function which accepts a list of values instead of individual arguments, but we'll continue this approach to illustrate the idea of a function with an arbitrary number of arguments.) By providing a function argument whose name begins with an asterisk, Python will collect any unnamed arguments passed to the function in a tuple, which can be accessed by the argument's name. To write a function which returns the length of the longest of a variable number of arguments, we can use this feature by including an argument whose name begins with an asterisk (in this case, we'll use the name *strings), which will become a tuple containing all the arguments passed to the function:

def longlen(*strings):
   max = 0
   for s in strings:
     if len(s) > max:
        max = len(s)

   return max

Notice that inside the function, strings is just an ordinary tuple; the asterisk before its name is only needed when the function is defined. If we call the function with a collection of strings as arguments, it will check them all, and return the maximum length of any of them:

>>> longlen('apple','banana','cantaloupe','cherry')
10

A similar technique can be used to create functions which can deal with an unlimited number of keyword/argument pairs. If an argument to a function is preceded by two asterisks, then inside the function, Python will collect all keyword/argument pairs which were not explicitly declared as arguments into a dictionary. When such an argument is used, it must be the last argument in the definition of the function. In this way, you can write a function to accept any named parameter, even if its name is not known when you are writing the function.

To illustrate, consider a trivial function which simply gathers all the keyword/value pairs which were not explicitly declared in the function definition into a dictionary called dict, and then prints the value of each declared argument. Two named arguments are provided before the final argument to show how Python deals with named arguments in the presence of an argument with two asterisks.

def printargs(a,b,**dict):
    print 'a=%s' % a
    print 'b=%s
    for k in dict.keys():
        print '%s=%s' % (k,dict[k])

We can test the function as follows:

>>> printargs(x='seven',a='six',b='five',next='four',last='three')
a=six
b=five
next=four
x=seven
last=three

Notice that arguments a and b were not placed in the dictionary, since they were explicitly specified as part of the function definition.

In more complex situations, both kinds of special arguments (single and double asterisks) can be used in the same function. Once again, a trivial program will illustrate how this works:

def allargs(one,*args,**dict):
    print 'one=%s' % str(one)
    print 'Unnamed arguments:'
    for a in args:
        print '%s' % str(a)
    print 'Named arguments:'
    for k in dict.keys():
        print '%s: %s' % (k,str(dict[k]))   

Since there is one named argument, allargs must be called with at least one argument, and the first argument will always be interpreted as the value of one. Here is the result when we call the function with more than one argument:

>>> allargs(12,'dog','cat',a=10,name='fred')
one=12
Unnamed arguments:
dog
cat
Named arguments:
name: fred
a: 10