Part 1: Functions as Objects

First, lets start Python. For Part 1, we will do everything using ipython, which provides a nice interactive python shell. We start ipython using the command

ipython

(note that you must be using Python 2 for this workshop and not using Python 3. Complete this workshop using Python 2, then read about the small changes if you are interested in using Python 3)

Functional programming is based on treating a function in the same way as you would a variable or object. So, to start, we should first create a function. This will be a simple function that just adds together two numbers. Please type in ipython

def sum(x,y):
    """Simple function returns the sum of the arguments"""
    return x+y

This is a very simple function that just returns the sum of its two arguments. Call the function using, e.g.

result = sum(3,7)
print(result)

which should print out 10.

In functional programming, a function is treated in exactly the same way as a variable or an object. This means that a function can be assigned to a variable, e.g. type

a = sum
result = a(3,7)
print(result)

This should print 10 again. Here, we have assigned the function sum to the variable a. So how does this work?

For variables, you should be comfortable with the idea that a variable is a container for a piece of data. For example,

b = 10

would create a piece of data (the integer 10) and will place it into the container (the variable b). When we type

a = b

we are copying the data from b and placing it into the variable a. Now both a and b contain (or point to) the same data.

For functional programming, the code of a function is also treated like a piece of data. The code

def sum(x,y):
    """Simple function returns the sum of the arguments"""
    return x+y

creates a new piece of data (the code to sum together x and y), and places that code into a container (the variable sum). When we then typed

a = sum

we copied the code data from sum and placed it into the variable a. Now both a and sum contain (or point to) the same data, i.e. the same code that sums together the two arguments (e.g. sum(3,7) and a(3,7) will call the same code, and give the same result).

This means that “code of a function” is a type, in the same way that “integer”, “string” and “floating point number” are types.

Properties of a Function

Just as “integer” and “string” have properties, so to does “function”. Type into ipython

sum.__[TAB]

(where [TAB] means that you should press the tab key)

This should show something like

sum.__call__          sum.__dict__          sum.__hash__          sum.__reduce_ex__
sum.__class__         sum.__doc__           sum.__init__          sum.__repr__
sum.__closure__       sum.__format__        sum.__module__        sum.__setattr__
sum.__code__          sum.__get__           sum.__name__          sum.__sizeof__
sum.__defaults__      sum.__getattribute__  sum.__new__           sum.__str__
sum.__delattr__       sum.__globals__       sum.__reduce__        sum.__subclasshook__

(exactly what you see will depend on your version of python)

This is the list of properties (functions and variables) of a function. The most interesting variables are __name__ and __doc__. Try typing

print(sum.__name__)
print(sum.__doc__)

From the output, can you guess what these two variables contain?

Functions as Arguments

As well as assigning functions to variables, you can also pass functions as arguments. Type this into ipython;

def call_function( func, arg1, arg2 ):
    """Simple function that calls the function 'func' with  
       arguments 'arg1' and 'arg2', returning the result"""
    return func(arg1, arg2)

result = call_function( sum, 3, 7 )
print(result)

This should print out 10. Can you see why?

The function call_function takes three arguments. The first is the function to be called. The second two arguments are the arguments that will be passed to that function. The code in call_function simply calls func using the arguments arg1 and arg2. So far, so useless…

However, let us now create another function, called difference. Please type into ipython

def diff(x, y):
    """Simple function that returns the difference of
       its arguments"""
    return x-y

and then type

result = call_function(diff, 9, 2)
print(result)

What do you now see? What has happened here?

Now we have passed diff to call_function, and so func(arg1,arg2) has used the code contained in diff, e.g. calculating the difference of the two numbers. The result, 7, should be printed.

You are probably now wondering how has this helped? Well, let us now change call_function. Please type into ipython

def call_function(func, arg1, arg2):
    """Simple function that returns the difference of
       its arguments"""
    print("Calling function %s with arguments %s and %s." % \
            (func.__name__, arg1, arg2) )
    result = func(arg1, arg2)
    print("The result is %s" % result)
    return result

Now type

result = call_function(sum, 3, 7)

You should see printed to the screen

Calling function sum with arguments 3 and 7.
The result is 10

Now try

result = call_function(diff, 9, 2)

You should now see

Calling function diff with arguments 9 and 2.
The result is 7

The new call_function is now doing something useful. It is printing out extra information about our functions, and can do that for any function (which accepts two arguments) that we pass. For example, now type

def multiply(x, y):
    """Simple function that returns the multiple of the
       two arguments"""
    return x * y

result = call_function( multiply, 4, 5 )

You should see

Calling function multiply with arguments 4 and 5.
The result is 20

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