Python - Arbitrary or, Variable-length Arguments

Arbitrary Arguments (*args)

You may want to define a function that is able to accept arbitrary or variable number of arguments. Moreover, the arbitrary number of arguments might be positional or keyword arguments.

  • An argument prefixed with a single asterisk * for arbitrary positional arguments.

  • An argument prefixed with two asterisks ** for arbitrary keyword arguments.

Arbitrary Arguments Example

Given below is an example of arbitrary or variable length positional arguments −

# sum of numbers
def add(*args):
   s=0
   for x in args:
      s=s+x
   return s
result = add(10,20,30,40)
print (result)

result = add(1,2,3)
print (result)

The args variable prefixed with "*" stores all the values passed to it. Here, args becomes a tuple. We can run a loop over its items to add the numbers.

It will produce the following output −

100
6

Required Arguments With Arbitrary Arguments

It is also possible to have a function with some required arguments before the sequence of variable number of values.

Example

The following example has avg() function. Assume that a student can take any number of tests. First test is mandatory. He can take as many tests as he likes to better his score. The function calculates the average of marks in first test and his maximum score in the rest of tests.

The function has two arguments, first is the required argument and second to hold any number of values.

#avg of first test and best of following tests
def avg(first, *rest):
   second=max(rest)
   return (first+second)/2
   
result=avg(40,30,50,25)
print (result)

Following call to avg() function passes first value to the required argument first, and the remaining values to a tuple named rest. We then find the maximum and use it to calculate the average.

It will produce the following output −

45.0

Arbitrary Keyword Arguments (**kwargs)

If a variable in the argument list has two asterisks prefixed to it, the function can accept arbitrary number of keyword arguments. The variable becomes a dictionary of keyword:value pairs.

Example

The following code is an example of a function with arbitrary keyword arguments. The addr() function has an argument **kwargs which is able to accept any number of address elements like name, city, phno, pin, etc. Inside the function kwargs dictionary of kw:value pairs is traversed using items() method.

def addr(**kwargs):
   for k,v in kwargs.items():
      print ("{}:{}".format(k,v))

print ("pass two keyword args")
addr(Name="John", City="Mumbai")
print ("pass four keyword args")

# pass four keyword args
addr(Name="Raam", City="Mumbai", ph_no="9123134567", PIN="400001")

It will produce the following output −

pass two keyword args
Name:John
City:Mumbai
pass four keyword args
Name:Raam
City:Mumbai
ph_no:9123134567
PIN:400001

Multiple Arguments With Arbitrary Keyword Arguments

If the function uses mixed types of arguments, the arbitrary keyword arguments should be after positional, keyword and arbitrary positional arguments in the argument list.

Example

Imagine a case where science and maths are mandatory subjects, in addition to which student may choose any number of elective subjects.

The following code defines a percent() function where marks in science and marks are stored in required arguments, and the marks in variable number of elective subjects in **optional argument.

def percent(math, sci, **optional):
   print ("maths:", math)
   print ("sci:", sci)
   s=math+sci
   for k,v in optional.items():
      print ("{}:{}".format(k,v))
      s=s+v
   return s/(len(optional)+2)

result=percent(math=80, sci=75, Eng=70, Hist=65, Geo=72)
print ("percentage:", result)

It will produce the following output −

maths: 80
sci: 75
Eng:70
Hist:65
Geo:72
percentage: 72.4
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