Lecture 20

More on functions, arguments, and assignment

MCS 260 Fall 2021
David Dumas

Reminders

• Project 2 due 6pm central today
• Project 2 solution will be posted next Friday
• Homework 7 posted, due 10am Tue

Terminal

I unified the "mini-terminal" examples from the 10am and 2pm lectures in terminal.py.

Criticisms

It's a good start, but:

• Adding a new command requires a new elif
• List of all commands (e.g. for `help`) must be manually updated

Functions are values

• Functions are values in Python, just like float, int, etc.
• Functions can be assigned to variables, used as parameters, stored in lists, used as keys or values

Returning multiple values

        def sumprod(x,y):
            """Return the sum and product of two numbers"""
            return x+y, x*y
        
        s,p = sumprod(5,8)
        # now s==13 and p==40
    

Why this works

A comma separated list (either bare or in parentheses) in Python is a tuple.

Tuples are like lists but immutable. They are iterable.

Tuple assignment lets you assign an iterable of values to a tuple of names as

        name0, name1, name2 = value0, value1, value2
        name0, name1, name2 = L  # if L has length 3
        

Example: swap

        x = 19
        y = 52
        x,y = y,x  # swap their values!
    

In other languages you would need a temporary place to store one of the values.

Returning multiple values?

        def sumprod(x,y):
            """Return the sum and product of two numbers"""
            return x+y, x*y
        
        s,p = sumprod(5,8)
        # now s==13 and p==40
    

From Python's perspective, sumprod returns one value (a tuple), and then tuple assignment stores those in s and p, respectively.

Variadic functions

A Python function can indicate that it will accept however many arguments the caller decides to give it:

        def f(x,y,*args):
            """function that accepts 2 or more arguments"""
            # body of function here
            # probably examine len(args) and args[i], i=0,1,...
    

This example requires at least 2 arguments, but allows more. Arguments 3 and on are "packed" into a tuple called args.

Argument unpacking

Conversely, what if you know all the arguments you want to give a function, but they are in a list rather than separate variables?

        L = ["Users","ddumas","teaching","mcs260","example.py"]
        os.path.join(L) # FAILS
    

Use * to tell Python to unpack the list (or other iterable) into separate arguments:

        L = ["Users","ddumas","teaching","mcs260","example.py"]
        os.path.join(*L) # equivalent to os.path.join(L[0],L[1],...)
    

Wrong number of arguments

If you pass a function a number of arguments that it cannot accept, it raises TypeError. E.g.

        def f(x,y,*args):
            print()

        def g(x):
            print()
        
        f()      # TypeError
        f(1)     # TypeError
        f(1,2)   # OK
        f(1,2,3) # OK

        g()      # TypeError
        g(1)     # OK
        g(1,2)   # TypeError
        

Back to the mini-terminal

Let's unify the many similar if/elif in our terminal example as follows:

• Make a dictionary to store all the commands
• Keys are command names
• Values are functions that perform the actions
• Main loop uses the command name to look up the right function to call. No if/elif/elif/...