4.3 Functions with Multiple Parameters

Let’s define a maximum function that determines and returns the largest of three values—the following session calls the function three times with integers, floating-point numbers and strings, respectively.

In [1]: def maximum(value1, value2, value3):
   ...:     """Return the maximum of three values."""
   ...:     max_value = value1
   ...:     if value2 > max_value:
   ...:         max_value = value2
   ...:     if value3 > max_value:
   ...:         max_value = value3
   ...:     return max_value
   ...:

In [2]: maximum(12, 27, 36)
Out[2]: 36

In [3]: maximum(12.3, 45.6, 9.7)
Out[3]: 45.6

In [4]: maximum('yellow', 'red', 'orange')
Out[4]: 'yellow'

We did not place blank lines above and below the if statements, because pressing return on a blank line in interactive mode completes the function’s definition.

You also may call maximum with mixed types, such as ints and floats:

In [5]: maximum(13.5, -3, 7)
Out[5]: 13.5

The call maximum(13.5, 'hello', 7) results in TypeError because strings and numbers cannot be compared to one another with the greater-than (>) operator.

Function maximum’s Definition

Function maximum specifies three parameters in a comma-separated list. Snippet [2]’s arguments 12, 27 and 36 are assigned to the parameters value1, value2 and value3, respectively.

To determine the largest value, we process one value at a time:

• Initially, we assume that value1 contains the largest value, so we assign it to the local variable max_value. Of course, it’s possible that value2 or value3 contains the actual largest value, so we still must compare each of these with max_value.

• The first if statement then tests value2 > max_value, and if this condition is True assigns value2 to max_value.

• The second if statement then tests value3 > max_value, and if this condition is True assigns value3 to max_value.

Now, max_value contains the largest value, so we return it. When control returns to the caller, the parameters value1, value2 and value3 and the variable max_value in the function’s block—which are all local variables—no longer exist.

Python’s Built-In max and min Functions

For many common tasks, the capabilities you need already exist in Python. For example, built-in max and min functions know how to determine the largest and smallest of their two or more arguments, respectively:

In [6]: max('yellow', 'red', 'orange', 'blue', 'green')
Out[6]: 'yellow'

In [7]: min(15, 9, 27, 14)
Out[7]: 9

Each of these functions also can receive an iterable argument, such as a list or a string. Using built-in functions or functions from the Python Standard Library’s modules rather than writing your own can reduce development time and increase program reliability, portability and performance. For a list of Python’s built-in functions and modules, see

https://docs.python.org/3/library/index.html

Self Check

1. (Fill-In) A function with multiple parameters specifies them in a(n)      .
   Answer: comma-separated list.

2. (True/False) When defining a function in IPython interactive mode, pressing Enter on a blank line causes IPython to display another continuation prompt so you can continue defining the function’s block.
   Answer: False. When defining a function in IPython interactive mode, pressing Enter on a blank line terminates the function definition.

3. (IPython Session) Call function max with the list [14, 27, 5, 3] as an argument, then call function min with the string 'orange' as an argument.
   Answer:

   In [1]: max([14, 27, 5, 3])
   Out[1]: 27
   
   In [2]: min('orange')
   Out[2]: 'a'