5. Functions in depth

5.1. Adding new functions

So far, we have only been using the functions that come with Python, but it is also possible to add new functions. A function definition specifies the name of a new function and the sequence of statements to execute when the function is called.

Here is an example:

def print_lyrics():
    print "I'm a lumberjack, and I'm okay."
    print "I sleep all night and I work all day."

def is a keyword that indicates that this is a function definition. The name of the function is print_lyrics. The rules for function names are the same as for variable names: letters, numbers and some punctuation marks are legal, but the first character can’t be a number. You can’t use a keyword as the name of a function, and you should avoid having a variable and a function with the same name.

The empty parentheses after the name indicate that this function doesn’t take any arguments.

The first line of the function definition is called the header; the rest is called the body. The header has to end with a colon and the body has to be indented. By convention, the indentation is always four spaces. The body can contain any number of statements. We’ve now seen three compound statements that can include indented statement blocks: the for statement, the if statement, and now the def statement.

If you type a function definition in interactive mode, the interpreter prints ellipses (...) to let you know that the definition isn’t complete:

>>> def print_lyrics():
...     print "I'm a lumberjack, and I'm okay."
...     print "I sleep all night and I work all day."

To end the function, you have to enter an empty line (this is not necessary in a script).

Defining a function creates a variable with the same name.

>>> print print_lyrics
<function print_lyrics at 0xb7e99e9c>
>>> print type(print_lyrics)
<type 'function'>

The value of print_lyrics is a function object, which has type 'function'. The output from the print print_lyrics statement shows a number in hexadecimal format (the number starting "0x..."). The number refers to the memory location of the function object (i.e., where that function exists in Python’s memory space).

The syntax for calling the new function is the same as for built-in functions:

>>> print_lyrics()
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.

Once you have defined a function, you can use it inside another function. For example, to repeat the previous refrain, we could write a function called repeat_lyrics:

def repeat_lyrics():

And then call repeat_lyrics:

>>> repeat_lyrics()
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.

But that’s not really how the song goes.

5.2. Definitions and uses

Pulling together the code fragments from the previous section, the whole program looks like this:

def print_lyrics():
    print "I'm a lumberjack, and I'm okay."
    print "I sleep all night and I work all day."

def repeat_lyrics():


This program contains two function definitions: print_lyrics and repeat_lyrics. Function definitions get executed just like other statements, but the effect is to create function objects. The statements inside the function do not get executed until the function is called, and the function definition generates no output.

As you might expect, you have to create a function before you can execute it. In other words, the function definition has to be executed before the first time it is called.


  1. Move the last line of this program to the top, so the function call appears before the definitions. Run the program and see what error message you get.
  2. Move the function call back to the bottom and move the definition of print_lyrics after the definition of repeat_lyrics. What happens when you run this program? Why?

5.3. Flow of execution

In order to ensure that a function is defined before its first use, you have to know the order in which statements are executed, which is called the flow of execution.

Execution always begins at the first statement of the program. Statements are executed one at a time, in order from top to bottom.

Function definitions do not alter the flow of execution of the program, but remember that statements inside the function are not executed until the function is called.

A function call is like a detour in the flow of execution. Instead of going to the next statement, the flow jumps to the body of the function, executes all the statements there, and then comes back to pick up exactly where it left off.

That sounds simple enough, until you remember that one function can call another. While in the middle of one function, the program might have to execute the statements in another function. But while executing that new function, the program might have to execute yet another function!

Fortunately, Python is good at keeping track of where it is, so each time a function completes, the program picks up where it left off in the function that called it. When it gets to the end of the program, it terminates.

What’s the moral of this sordid tale? When you read a program, you don’t always want to read from top to bottom. Usually, you want to follow the flow of execution and read the program as Python would interpret it.

5.4. Parameters and arguments

Some of the built-in functions we have seen require arguments. For example, when you call math.sin you pass a number as an argument. Some functions take more than one argument: math.pow takes two, the base and the exponent.

Inside the function, the arguments are assigned to variables called parameters. Here is an example of a user-defined function that takes an argument:

def print_twice(bruce):
    print bruce
    print bruce

This function assigns the argument to a parameter named bruce. When the function is called, it prints the value of the parameter (whatever it is) twice.

This function works with any value that can be printed.

>>> print_twice('Spam')
>>> print_twice(17)
>>> print_twice(math.pi)

The same rules of composition that apply to built-in functions also apply to user-defined functions, so we can use any kind of expression as an argument for print_twice:

>>> print_twice('Spam '*4)
Spam Spam Spam Spam
Spam Spam Spam Spam
>>> print_twice(math.cos(math.pi))

The argument is evaluated before the function is called, so in the examples the expressions 'Spam '*4 and math.cos(math.pi) are only evaluated once.

You can also use a variable as an argument:

>>> michael = 'Eric, the half a bee.'
>>> print_twice(michael)
Eric, the half a bee.
Eric, the half a bee.

The name of the variable we pass as an argument (michael) has nothing to do with the name of the parameter (bruce). It doesn’t matter what the value was called back home (in the caller); here in print_twice, we call it bruce.

5.5. Variables and parameters are local

When you create a variable inside a function, it is local, which means that it only exists inside the function. For example:

def cat_twice(part1, part2):
    cat = part1 + part2

This function takes two arguments, concatenates them, and prints the result twice. Here is an example that uses it:

>>> line1 = 'Bing tiddle '
>>> line2 = 'tiddle bang.'
>>> cat_twice(line1, line2)
Bing tiddle tiddle bang.
Bing tiddle tiddle bang.

When cat_twice terminates, the variable cat is destroyed. If we try to print it, we get an exception:

>>> print cat
NameError: name 'cat' is not defined

Parameters are also local. For example, outside print_twice, there is no such thing as bruce.

5.6. Return values

5.6.1. The special type None

Some of the built-in functions we have used, such as the math functions, produce results. When you call a function that returns a result, like math.sqrt, you almost always want to do something with the result; for example, you might assign it to a variable or use it as part of an expression:

x = math.cos(radians)
golden = (math.sqrt(5) + 1) / 2

When you call a function in interactive mode, Python displays the result:

>>> math.sqrt(5)

But in a script, if you call a function that returns a result all by itself, the return value is lost forever, and does not even show up in the console as output!


This script computes the square root of 5, but since it doesn’t store or display the result (i.e., there is no print statement), it is not very useful.

Functions that do not return anything (also called “void functions”) might display something on the screen or have some other effect, but they don’t explicitly pass back a result. However, Python will implicitly return the special value None. Say print_twice is defined as follows:

def print_twice(s):
    print s
    print s
    # nothing returned from this function

In the interactive interpreter, we call the function and assign its result to the variable result:

>>> result = print_twice('Bing')
>>> print result

The value None is not the same as the string 'None'. It is a special value that has its own type:

>>> print type(None)
<type 'NoneType'>

5.6.2. Functions with return values

If we want a function to hand back a result to the caller of the function, we can use the return statement with an expression. For example, the following function area returns the area of a circle with a given radius:

import math

def area(radius):
    temp = math.pi * radius**2
    return temp

The return statement means: “Return immediately from this function and use the following expression as a return value.” The expression can be arbitrarily complicated, so we could have written this function more concisely:

def area(radius):
    return math.pi * radius**2

On the other hand, temporary variables like temp often make debugging easier.

Sometimes it is useful to have multiple return statements, one in each branch of a conditional:

def absolute_value(x):
    if x < 0:
        return -x
        return x

Since these return statements are in an alternative conditional, only one will be executed.

As soon as a return statement executes, the function terminates without executing any subsequent statements. Code that appears after a return statement, or any other place the flow of execution can never reach, is called dead code.

In a function that returns a result, it is a good idea to ensure that every possible path through the program hits a return statement. For example:

# warning: this is problematic code!
def absolute_value(x):
    if x < 0:
        return -x
    if x > 0:
        return x

This function is incorrect because if x happens to be 0, neither condition is true, and the function ends without hitting a return statement. If the flow of execution gets to the end of a function, the return value is None, which is not the absolute value of 0.

>>> print absolute_value(0)

By the way, Python provides a built-in function called abs that computes absolute values.


  1. Write a compare function that returns 1 if x > y, 0 if x == y, and -1 if x < y.

5.7. Stack diagrams

To keep track of which variables can be used where, it is sometimes useful to draw a stack diagram. Like state diagrams, stack diagrams show the value of each variable, but they also show the function each variable belongs to.

Each function is represented by a frame (or “stack frame”). A frame is a box with the name of a function beside it and the parameters and variables of the function inside it. The stack diagram for the previous example looks like this:

Example stack diagram.

The frames are arranged in a stack that indicates which function called which, and so on. In this example, print_twice was called by cat_twice, and cat_twice was called by __main__, which is a special name for the topmost frame. When you create a variable outside of any function, it belongs to __main__.

Each parameter refers to the same value as its corresponding argument. So, part1 has the same value as line1, part2 has the same value as line2, and bruce has the same value as cat.

If an error occurs during a function call, Python prints the name of the function, and the name of the function that called it, and the name of the function that called that, all the way back to __main__.

For example, if you try to access cat from within print_twice, you get a NameError:

Traceback (innermost last):
  File "test.py", line 13, in __main__
    cat_twice(line1, line2)
  File "test.py", line 5, in cat_twice
  File "test.py", line 9, in print_twice
    print cat
NameError: name 'cat' is not defined

This list of functions is called a traceback. It tells you what program file the error occurred in, and what line, and what functions were executing at the time. It also shows the line of code that caused the error (i.e., line 9, in the function print_twice).

The order of the functions in the traceback is the same as the order of the frames in the stack diagram. The function that is currently running is at the bottom. [1]

5.8. Boolean functions

Functions can return booleans, which is often convenient for hiding complicated tests inside functions. For example:

def is_divisible(x, y):
    if x % y == 0:
        return True
        return False

It is common to give boolean functions names that sound like yes/no questions; is_divisible returns either True or False to indicate whether x is divisible by y.

Here is an example:

>>> is_divisible(6, 4)
>>> is_divisible(6, 3)

The result of the == operator is a boolean, so we can write the function more concisely by returning it directly:

def is_divisible(x, y):
    return x % y == 0

Boolean functions are often used in conditional statements:

if is_divisible(x, y):
    print 'x is divisible by y'

It might be tempting to write something like:

if is_divisible(x, y) == True:
    print 'x is divisible by y'

But the extra comparison is unnecessary.


  1. Write a function is_between(x, y, z) that returns True if x \le y \le z or False otherwise.

5.9. Debugging

5.9.1. Adding print statements

Breaking a large program into smaller functions creates natural checkpoints for debugging. If a function is not working, there are three possibilities to consider:

  • There is something wrong with the arguments the function is getting.
  • There is something wrong with the function.
  • There is something wrong with the return value or the way it is being used.

To rule out the first possibility, you can add a print statement at the beginning of the function and display the values of the parameters (and maybe their types). Or you can write code that checks the preconditions explicitly.

If the parameters look good, add a print statement before each return statement that displays the return value. If possible, check the result by hand. Consider calling the function with values that make it easy to check the result.

If the function seems to be working, look at the function call to make sure the return value is being used correctly (or used at all!).

5.9.2. Unit testing and assert

After you think you’ve solved a problem, how do you know your program behaves as intended? You’ve probably run it once or twice to make sure it does something, and maybe you’ve even tested it out with a few different inputs. But we can do better.

Especially when writing functions that perform a specific task, it is common to create test cases to call the function with specific inputs to ensure that the function works correctly and returns the “right” thing. This approach to testing is called unit testing because of the focus on testing individual functional units (i.e., the functions!) in a program.

You can think of testing your program in this way as something of an experiment. First, you decide on the inputs (parameters) you want to pass to your function. The output you expect from the function is basically a hypothesis which can easily be tested by running the function with your chosen input: if it produces the expected output, then your hypothesis was correct. If not, then there is probably something wrong with the function. (There may also be something wrong with the output you expected — for this reason you need to be very careful when devising tests!)

There is a built-in function called assert that can help with developing unit tests to ensure a function works as expected. The assert function takes a Boolean expression as a parameter. If the expression evaluates to False, the assert function will cause your program to crash. This is a good thing! The crash lets you know that something is wrong and needs to be fixed! If the expression in the assert function call evaluates to True, essentially nothing happens — the next line in the program will be executed.

Here’s an example. The following function is supposed to take one string as a parameter, and count up and return the number of upper- and lower-case 'A's in the string. It has two bugs. Before you read on, see if you can figure out what they are.

# function should count up all the lower- and upper-case
# A's in a string and return the count.
def count_As(mystring):
    count = 0
    for char in mystring:
        if char == 'a':
            count += 1

Let’s think of four test cases for our unit test of this function:

  • If we call count_As with 'xyz' (or an empty string), it should return 0.
  • If we call count_As with 'abc', it should return 1.
  • If we call count_As with 'ABC', it should also return 1.
  • If we call count_As with 'Abracadabra', it should return 5.

We can construct calls to assert by including a Boolean expression that calls the function, and compares the return value to the expected output. In the assert calls, we are making assertions (duh!) about what the output should be:

def unit_tests():
    # three test cases using three different
    # strings to test whether the count_As
    # function works correctly
    assert(count_As("xyz") == 0)
    assert(count_As("abc") == 1)
    assert(count_As("ABC") == 1)
    assert(count_As("Abracadabra") == 5)


When we run this program, we’ll first call unit_tests, then we’ll call each of the assert statements, in order. Because of the bugs in count_As, we’ll crash on the first assert call:

Traceback (most recent call last):
  File "test2.py", line 16, in <module>
  File "test2.py", line 11, in unit_tests
    assert(count_As("xyz") == 0)

We see from the function stack traceback that the program died on line 11, which was the first call to assert. The program crashes because the return value of calling count_As("xyz") is not 0 (although it should be!) If we look carefully at the count_As function, we’ll see one problem: there’s no return statement! The function currently always returns None. Easy to fix:

def count_As(mystring):
    count = 0
    for char in mystring:
        if char == 'a':
            count += 1
    return count # added a return statement!

When we run the program now, we hit another AssertionError:

Traceback (most recent call last):
  File "test2.py", line 17, in <module>
  File "test2.py", line 14, in unit_tests
    assert(count_As("ABC") == 1)

Now, the program crashes on the assert on line 14. If we carefully examine the code (and perhaps add a print statement or two to help us to see what’s going on in the function), we can see that we’re only counting lower case 'a's, not upper case. Once we fix that problem, all the Boolean expressions in the assert calls will evaluate to True, and the program will finish without crashing. This will indicate that all our tests passed successfully.

Interestingly, an increasingly common practice within the software industry is to specify a set of test cases before writing a function. The idea is that the activity of specifying a set of test cases helps to clarify what a function should do. Once the test cases are specified, the function can be written. Once all the test cases successfully pass, the function is done.

The hard part of testing a program is figuring out what a good set of test cases should be. Here are some rules of thumb:

  • Pick one or two “normal” inputs and expected outputs. Think of parameters you expect to be commonly passed to the function and ensure that the function works for those parameters.
  • Think about any “corner cases” — parameters that are just outside any “normal” or expected values. For example, if you usually expect to get the integers 1-10 as parameters to a function, write a tests for the valid bounds (1 and 10) as well as values just outside those bounds (0 and 11).
  • Think deviously. What sorts of inputs might cause problems for a function? For example, if a function expects a string as input, what happens if an empty string ("") gets passed in?

You can also read more about unit testing on Wikipedia: http://en.wikipedia.org/wiki/Unit_testing.

5.10. Glossary

A named sequence of statements that performs some useful operation. Functions may or may not take arguments and may or may not produce a result.
function definition:
A statement that creates a new function, specifying its name, parameters, and the statements it executes.
function object:
A value created by a function definition. The name of the function is a variable that refers to a function object.
function header:
The first line of a function definition.
function body:
The sequence of statements inside a function definition.
A name used inside a function to refer to the value passed as an argument.
function call:
A statement that executes a function. It consists of the function name followed by an argument list.
A value provided to a function when the function is called. This value is assigned to the corresponding parameter in the function.
local variable:
A variable defined inside a function. A local variable can only be used inside its function.
return value:
The result of a function. If a function call is used as an expression, the return value is the value of the expression.
flow of execution:
The order in which statements are executed during a program run.
stack diagram:
A graphical representation of a stack of functions, their variables, and the values they refer to.
A box in a stack diagram that represents a function call. It contains the local variables and parameters of the function.
A list of the functions that are executing, printed when an exception occurs.
temporary variable:
A variable used to store an intermediate value in a complex calculation.
dead code:
Part of a program that can never be executed, often because it appears after a return statement.
A special value returned by functions that have no return statement or a return statement without an argument.
test case:
A set of parameters (inputs) and expected outputs for a function that can test whether the function behaves as expected.
unit testing:
The idea of testing smaller pieces of a program, like a function, rather than testing the whole program at once.
A propositional statement that you expect to be True at some point in a program. The built-in assert function can be used to test Boolean propositional statements.

5.11. Exercises

  1. Fix the last bug in the count_As function. Can you think of any additional test cases that should be added for this function?

  2. Write a function named compare_ab that takes one string as parameters, and counts the occurrences of 'a's and 'b's in the string. The function should return True if the number of 'a's and 'b's is the same, and False otherwise. Think of a set of test cases for this function, and write them.

  3. Write a function named right_justify that takes a string named s as a parameter and prints the string with enough leading spaces so that the last letter of the string is in column 60 of the display.

    >>> right_justify('allen')
  4. Write a function called is_leap that takes a year value as a parameter, and returns True if the year is a leap year or False if it is not. Refer to one of the exercises from the last chapter for the definition of a leap year.

  5. A function object is a value you can assign to a variable or pass as an argument. For example, do_twice is a function that takes a function object as an argument and calls it twice:

    def do_twice(f):

    Here’s an example that uses do_twice to call a function named print_spam twice.

    def print_spam():
        print 'spam'
    1. Type this example into a script and test it.
    2. Modify do_twice so that it takes two arguments, a function object and a value, and calls the function twice, passing the value as an argument.
    3. Write a more general version of print_spam, called print_twice, that takes a string as a parameter and prints it twice.
    4. Use the modified version of do_twice to call print_twice twice, passing 'spam' as an argument.
    5. Define a new function called do_four that takes a function object and a value and calls the function four times, passing the value as a parameter. There should be only two statements in the body of this function, not four.
  6. This exercise [2] can be done using only the statements and other features we have learned so far.

    Write a function that draws a grid like the following:

    + - - - - + - - - - +
    |         |         |
    |         |         |
    |         |         |
    |         |         |
    + - - - - + - - - - +
    |         |         |
    |         |         |
    |         |         |
    |         |         |
    + - - - - + - - - - +

    Hint: to print more than one value on a line, you can print a comma-separated sequence:

    print '+', '-'

    If the sequence ends with a comma, Python leaves the line unfinished, so the value printed next appears on the same line.

    print '+',
    print '-'

    The output of these statements is '+ -'.

    A print statement all by itself ends the current line and goes to the next line.

  7. Use the previous function to draw a similar grid with four rows and four columns.

  8. Write a function that takes one integer named size as a parameter and prints an equilateral triangle composed of asterisks of length size. For example, the call make_triangle(4) should result in the following triangle printed:

      * *
     * * *
    * * * *
  9. Draw a stack diagram for the following program. What does the program print?

    def b(z):
        prod = a(z, z)
        print z, prod
        return prod
    def a(x, y):
        x = x + 1
        return x * y
    def c(x, y, z):
        sum = x + y + z
        pow = b(sum)**2
        return pow
    x = 1
    y = x + 1
    print c(x, y+3, x+y)
  10. Fermat’s Last Theorem says that there are no integers a, b, and c such that:

    a^n + b^n = c^n

    for any values of n greater than 2.

    1. Write a function named check_fermat that takes four parameters—a, b, c and n—and that checks to see if Fermat’s theorem holds. If n is greater than 2 and it turns out to be true that

      a^n + b^n = c^n

      the function should return True. Otherwise, the function should return False.

    2. Write a function that prompts the user to input values for a, b, c and n, converts them to integers, and uses check_fermat to check whether they violate Fermat’s theorem. If the result of calling check_fermat is False, this function should print “Holy smokes, Fermat was wrong!”. Otherwise, it should print “No, that doesn’t work.”

  11. If you are given three sticks, you may or may not be able to arrange them in a triangle. For example, if one of the sticks is 12 inches long and the other two are one inch long, it is clear that you will not be able to get the short sticks to meet in the middle. For any three lengths, there is a simple test to see if it is possible to form a triangle:

    If any of the three lengths is greater than the sum of the other two, then you cannot form a triangle. Otherwise, you can.

    1. Write a function named is_triangle that takes three integers as arguments, and that returns True or False, depending on whether you can or cannot form a triangle from sticks with the given lengths.
    2. Write a function that prompts the user to input three stick lengths, converts them to integers, and uses is_triangle to check whether sticks with the given lengths can form a triangle.
[1]Stack diagrams can either be drawn starting from the top, working down, or from the bottom, working up. It depends which hemisphere (northern or southern) you come from. (Just kidding. As long as you’re consistent, it doesn’t matter which way you draw it.)
[2]Based on an exercise in Oualline, Practical C Programming, Third Edition, O’Reilly (1997)