Chapter 3: Functions (Part I)

New terms from chapter 3 (Think Python: Think Like a Computer Scientist) – PART I

I found this chapter very dense. So, I will break it down in three (3) parts.

3.1 Function calls

  • Function : name + sequence of statements that perform a computation

The book “Think Python: Think Like a Computer Scientist” provides the following definition:

a function is a named sequence of statements that performs a computation

  • Function call:

Once the function’s name and sequence of statements have been specified, I can “call” the function by name.

Here is an example:

>>>type (32)
<type ‘int’>

The name of the function is type.

The expression in parenthesis is called an argument.

The result of this function is the type of the argument.

It is common to say that a function “takes” an argument and “returns” a result. The result is called the return value

Here is an illustration of a function that I found on Wikipedia:

Wikipedia Function machine

Wikipedia Function machine: A function f takes an input x, and returns an output f(x)


3.2 Type conversion functions

Python provides built-in functions that convert values from one type to another

For instance:

The int function takes any number value and converts it to an integer. If the number is a floating-point value, the function will truncate the fraction part (NOTE: The function does not round off)

For instance:

>>> int(5.25)


The float function converts integers numbers and strings numbers to floating-point numbers.

For example:

>>> float(32)
>>> float(‘3.14159’)

The str function converts its argument to a string.

Here is an illustration:

>>> str(32)
>>> str(3.14159)

3.3 Math functions

Python has a math module that provides most of the familiar mathematical functions

  • A module: A file that contains a collection of related functions. Also, before I can use the module I have to import it.

For instance:

>>> import math

>>> print math
<module ‘math’=”” from=”” ‘=”” library=”” frameworks=”” python.framework=”” versions=”” 2.7=”” lib=”” python2.7=”” <span=”” class=”hiddenSpellError” pre=””>lib-dynload/’>

Statement: >>> import math

Module object: math

The module object contains the functions and variables defined in the module

  • Dot notation the name of the module and the name of the function, separated by a dot. Do notation is used to access one of the functions in the module.

Example no1:

>>> signal_power = 10
>>> noise_power = 2.5
>>> ratio = signal_power / noise_power
>>> decibels = 10 * math.log10(ratio)
>>> print decibels

NOTE 1: This example uses the function of log10 to compute a signal-to-noise ratio in decibels.

NOTE 2: The math module provides the function log, which computes logarithms base e.

Example no2: Convert degrees to radians

>>> degrees = 45
>>> radians = degrees / 360.0 * 2 * math.pi # To convert from degrees to radians: divide the degrees by 360 and multiply it by 2∏
>>> math.sin(radians) # Find the sin of radians

NOTE: The name of the variable is a hint that sin and the other trigonometric functions (cos, tan, etc.) take arguments in radians.

The expression math.pi gets the variable pi from the math module. The value of the variable pi is an approximation of ∏, accurate to about 15 digits

3.4 Composition

Program components:

  • variables
  • expressions
  • statements

To compose: The ability to take small building blocks and build up with them.

the argument of a function can be any kind of expression, including arithmetic operators […] Almost anywhere you can put a value, you can put an arbitrary expression, with one exception:

the left side of an assignment statement has to be a variable name. Any other expression on the left side is a syntax error (we will see exceptions to this rule later)

Here is an illustration:

>>> degrees = 45
>>> x = math.sin(degrees / 360.0 * 2 * math.pi)
>>> print x
>>> y = math.exp(math.log(x+1))
>>> print y

COMPOSITION RULE: The left side of an assignment statement has to be a variable name. Any other expression on the left side is a syntax error (I will see exceptions to this rule later)

Writing an ASSIGNMENT STATEMENT this way will work:

>>> minutes = hours * 60 #The writing of this assignment statement is adequate since the left side of the assignment statement is the  variable name


Writing an ASSIGNMENT STATEMENT the way described below will generate and error:

>>> hours * 60 = minutes
File “<stdin>”, line 1
SyntaxError: can’t assign to operator

3.5 Adding new functions

– Function definition: def

A function definition specifies the name of a new function and the sequence of statements that execute when the function is called

– Function names rule of thumb:

  • letters, numbers and some punctuation marks are legal,
  • the first character of the function name can’t be a number
  • keyword as the name of a function is illegal
  • avoid having a variable and a function with the same name

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

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

>>> def print_lyrics():

Header: It is the first line of the function definition and it has to end with a colon

– Body: All the subsequent lines after the first line of the function definition and they have to be indented. There is no limit as to the number of statements a function definition may have.
NOTE: By convention, the indentation is always four spaces (see Section 3.14)

– Strings: The strings in the print statements are enclosed in double quotes.
NOTE: Single quotes and double quotes do the same thing; most people use single quotes except in cases like this where a single quote (which is also an apostrophe) appears in the string.

– End: To end a function, I have to enter an empty line (just hit enter). This is not necessary in a script mode

Here is an example:

>>> def print_lyrics():
…     print “I’m a lumberjack, and I’m okay.”
…     print “I sleep all night and work all day.”


I can recognize the keyword that indicates that it is a function definition: def

  • I can also recognize the name of the function: print_lyrics
  • The header of the definition function is: >>> def print_lyrics():    Please note that it ends with a colon.
  • The body of the definition function is everything that follows and it is indented
  • The strings in the print statements of the function definition are enclosed in double quotes and are indented after the ellipsis (the three dots) :

…     print “I’m a lumberjack, and I’m okay.”
…     print “I sleep all night and work all day.”

  • The end of the function: After the third (3rd) ellipsis, I hit enter again and it comes back to >>>

– Defining a function: After I defined my function definition (name, header, body, strings, end), I can ask python to print it (or recall it). Python will create a variable with the same name. See below:

>>> print print_lyrics
<function print_lyrics at 0x10049acf8>

– Value type of the function definition we just created can be found as illustrated below:

>>> type (print_lyrics)
<type ‘function’>

In other words, the value of print_lyrics is a function object, which has type ‘function’.

The syntax for calling the new function is the same as for built-in functions […] Once you have defined a function, you can use it inside another function

Here is a fun example of how pop music (Lady Gaga / Poker face) and Python can mix.

  • Step 1: Define the function print_chorus_pf

>>> def print_chorus_pf():
…     print “P-p-p-poker face, p-p-poker face”
…     print “(Mum mum mum mah)”

  • Step 2: Define the function repeat_chorus_pf

>>> def repeat_chorus_pf():
…     print_chorus_pf()
…     print_chorus_pf()

  • Step 3: Call function repeat_chorus_pf

>>> repeat_chorus_pf()
P-p-p-poker face, p-p-poker face
(Mum mum mum mah)
P-p-p-poker face, p-p-poker face
(Mum mum mum mah)

3.6 Definitions and uses

This program contains two function definitions: print_chorus_pf and repeat_chorus_pf.

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.

Exercise 3.1) 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.

>>> repeat_chorus_pf()
Traceback (most recent call last):
File “<stdin>”, line 1, in <module>
NameError: name ‘repeat_chorus_pf’ is not defined

Exercise 3.2. Move the function call back to the bottom and move the definition of print_chorus_pf after the definition of repeat_chorus_pf. What happens when you run this program?

>>> def repeat_chorus_pf():
…     print_chorus_pf()
…     print_chorus_pf()

>>> def print_chorus_pf():
…     print “P-p-p-poker face, p-p-poker face”
…     print “(Mum mum mum mah)”

>>> repeat_chorus_pf
<function repeat_chorus_pf at 0x10049acf8>



Acknowledgments :

These notes represent my understanding from the book Think Python: How to Think Like a Computer Scientist written by Allen B. Downey.

Part of the chapter is transcribed and all the quotes unless specified otherwise come directly from his book.

Thank you Professor Downey for making this knowledge available.


Also, I would like to thank the open source community for their valuable contribution in making resources on programming available.

Thank you

One thought on “Chapter 3: Functions (Part I)

  1. Pingback: Exercises from chapter 3 – Think Python | Python Project

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