Python Jump Start
                                                 Bob Brown
                                College of Computing and Software Engineering
                                           Kennesaw State University


About Reading and Writing
We learned to write English by first reading. We started with simple material and progressed to
increasingly complex material. Then we wrote, also starting with simple material. We should
learn to write computer programs in the same way. The purpose of this “Jump Start” is to
provide just enough information to let you read many Python programs. As with reading
English, when you come upon something you don’t understand in a Python program, look it up!
You will increase your Python vocabulary just as you increased your English vocabulary. This
document uses some programming terms of art, i.e. jargon. If you come across a term that’s
unfamiliar to you, look it up!
This is not a reference document; you should read and absorb all of it, then start reading, and
perhaps writing, Python programs. You will expand your knowledge of Python as you go along.
Why is it Called “Python?”
“When he began implementing Python, Guido van Rossum was also reading the published
scripts from “Monty Python’s Flying Circus”, a BBC comedy series from the 1970s. Van
Rossum thought he needed a name that was short, unique, and slightly mysterious, so he decided
to call the language Python.” 1 So, despite the many graphics you will see, Python has nothing
to do with snakes.
Getting Help
If you are learning Python, you are probably working in an IDE – Integrated Development
Environment. The first thing you should do is to read the documentation… yes, really, read the
documentation! Many IDEs have built-in help. Many Python IDEs present two windows, one in
which you type your program and another into which you can type Python statements to See
What Happens. ™
There is a wealth of information on the Web. Start with https://docs.python.org/.
Python Identifiers
Identifiers are the names of things. Identifiers can name variables, functions, objects, and other
entities that are a part of Python. Python identifiers must begin with a letter, then consist of
letters, digits, and underscores; other special characters are not allowed. Python identifiers are
not length-limited, but are case sensitive, so Number and number are not the same.

1
    From the General Python FAQ: https://docs.python.org/2/faq/general.html



                                Licensed under the Creative Commons BY-SA License
                                                                                     Revised: 11/9/22
Identifiers should be meaningful. While i = 1 is perfectly valid, index = 1 is preferable.
When an identifier comprises two or more words, use camelCase (rowIndex), CapWords
(RowIndex), or underscores (row_index.) Don’t mix them; pick a style and stick with it.
Python Keywords and Operators
There are more than thirty Python keywords, including things like if, else, while, and
import. The two Boolean constants True and False are capitalized; the other keywords are
not. Rather than memorizing a list of keywords, learn them as you need them.
Python arithmetic operators include the usual suspects, +, -, *, and / for addition, subtraction,
multiplication, and division. You can also use // for floor division, which returns the integer
part of the quotient, ** for exponentiation, and % for modulus.
The assignment operator is the equal sign, =. There are numerous compound assignment
operators, so x += 1 is the same as x = x + 1.
The comparison operators are == (two consecutive equal signs) for equality, != for not equal,
and < and > for less than and greater than. There are two other comparison operators and several
other types of Python operators. Learn them as you need them.
Comments
Everything following the hash symbol, #, to the end of the line is a comment. Comments are
ignored by the Python interpreter; they exist help those reading the program, including the
original programmer, to understand it.
       # This is a comment that takes up the entire line
       rowIndex += offset   # This comment might explain "offset"

Resist the urge to comment the obvious; it clutters your programs and makes them hard to read.
       rowIndex += 1        # Add 1 to rowIndex

An easy way to remove code temporarily when testing is to “comment it out,” that is, to prefix
each line with a #-symbol. Many Python IDEs have a function that does this automagically.
You can also enclose such a block in triple apostrophes.
Variables, Data Types
Python variable names follow the rules for identifiers, as described earlier. A variable name is
nothing more than a name for an area in the computer’s memory. How the contents of that area
are used depends on the data type associated with the variable.
Python has four primitive data types: integers, floating-point numbers, strings, and Booleans.
Integers are whole numbers, including zero and the negative numbers. Floating point numbers
can have a whole number part and fractional part. Strings hold sequences of characters, like
"Hello, world!" Booleans have only two possible values: True and False.



                                               – 2–
Python also has many compound data types, including arrays and lists, data types for date and
time, and for enumeration. You will learn these as you need them.
Assigning a value to a variable for the first time declares the variable, that is, creates it.
        answer = 'yes'            # The variable "answer" exists and has a value

Python is dynamically typed. That means the data type of a variable can change when a new
value is assigned to it.
        answer = 'yes'            # The variable "answer" is a string
        answer = 42               # And now it's an integer

It is, in general, good practice not to change the type of a variable during the execution of a
program. Python has a built-in type( ) that will return the type of a variable is passed to it. 2
        >>> answer = 'yes'
        >>> type(answer)
        <class 'str'>
        >>> answer = 42
        >>> type(answer)
        <class 'int'>

Block structure and Indentation
In many languages, indentation is used for pretty-printing, that is to make the program more
readable, but is ignored by the language processor. In Python, indentation is used to delineate
syntactic structures called blocks, and so it has meaning to the Python language processor. It
also has the effect of making the program more readable.
        if i < 0:                   # The colon starts a block
            print "i is negative" # This is within the block
        else:                       # A new block at the same level
           print "i is nonnegative"
           if i < 10:               # An inner block; indented further
             print "i has one digit"
           else:
             print "i has multiple digits"

Indentation must be consistent; “ragged” indentation will cause an error, as will mixing tabs and
spaces. Python’s specified best practice in PEP-8 is to use four spaces for each block level. 3
Defining and Using Functions
Python has hundreds of functions, like the type( ) function described earlier. You can write
your own functions, too. You’d do that when the same bit of code would otherwise have to

2
 The Python interpreter uses >>> to prompt for input from the user.
3
 That’s right… four keystrokes when one ought to do. Happily, autopep8 will fix things up for you. Some editors
and IDEs can also help with this.


                                                     – 3–
appear in two or more places within your program. A function means the code appears in only
one place and is called from several places as needed. 4
A function is defined by the keyword def (for “define”) followed by the function name, the
parameters of the function enclosed in parentheses, and a colon. The colon begins a block, and
the function body – the code and data that comprise the function – are the contents of the block.
Parameters are placeholders for data; when the function is called, the parameters are replaced by
the arguments with which the function was called. If there are no parameters, the parentheses
are empty but must still be present.
Here is a tiny Python program with an example of defining and using a function.
        def greet(who, when):
            print("Good " + when + ", " + who + "!")

        greet("Bob","afternoon")                     # Prints "Good afternoon, Bob!"
        player = "Gina"
        time = "evening"
        greet(player,time)                           # Prints "Good evening, Gina!"


In this program, the function is called greet and the parameters are who and when. There’s
something a little strange, too. The print function is printing strings, but there are plus signs in
there. The reason is that the plus operator is overloaded in Python to serve as the string
concatenation operator, too. If plus is given two numbers, it adds; if it’s given two strings, it
concatenates. Mixing strings and numbers gives an error. z = 3 + "george"                 # Error!

The first time the function is called, the arguments are two strings, “Bob” and “afternoon.” The
program prints, “Good afternoon, Bob!”
Suppose we were writing a game in which a troll snarkily greets any player who approaches the
troll bridge. We’d need variables to hold the player name and the time, and the last three lines in
the example above show how that might work.
Scope of Variables and Other Objects
Before you can use a named object, such as a variable name or function name, it must be
defined.
print (answer)                     # Doesn’t work because "answer" is not defined.
answer = 42                        # This defines "answer"
print (answer)                     # Prints 42

4
  And that has the happy property that there’s only one place to change things if you find an error or improve your
code. Otherwise, you’d have to search the entire program and make the same changes in many places, a process that
is fraught with opportunity for error.




                                                      – 4–
To use an object, it must be defined either within its own scope or within an enclosing scope.
“Scope” only means whether an object is visible to a particular piece of code. Let’s look at an
example:
# Program to illustrate "scope"
name = "Gina"

def greetTest():
    print ("Entering greetTest")
    print (name)        # Prints "Gina"                                                  Outer scope
    when = "morning"
                                                                Scope of
                                                                greetTest                – the entire
    print (when)        # Prints "morning"
                                                                                         program
    print ("Leaving greetTest")

greetTest()
print (name)    # Works as expected, prints "Gina"
print (when)    # Name Error: name 'when' is not defined
# End of the program

There are two important things to notice about this example. The variable name is defined in the
outer scope, and so is available anywhere in the program, including within the function
greetTest. The variable when is defined within greetTest; it is available within the scope
of greetTest, including any inner scopes – there aren’t any inner scopes in the example – but it
is not available outside the scope of greetTest.
If the same name is defined in an outer and inner scope, the definition in the current scope is
used. Such a definition makes the name a local variable. Let’s look at the example again,
slightly changed.
name = "Gina"
def greetTest():
    name = "George"              # Redefines name, now a local variable
    print (name)
greetTest()                      # Prints "George"
print (name)                     # Prints "Gina"

The variable name is defined in two different places, and has two different meanings. Within the
function greetTest, name is a local variable, and changes to it do not propagate to the outer
scope.

The fact the declaring a variable within a function creates a new variable is a Good Thing ™
and can prevent hard-to-find errors. However, sometimes you want to be able a variable defined
outside a function from within the function. The global keyword does that.




                                               – 5–
name = "Gina"
def greetTest():
    global name                         # 'name' will NOT be redefined
    name = "George"                     # The variable OUTSIDE the function changes
    print (name)
greetTest()                            # Prints "George"
print (name)                           # Prints "George" again

Python’s predefined functions and objects have global scope. That is, they are available
everywhere. It is possible to re-use the names of Python’s predefined objects by writing new
definitions. Doing so is very bad practice because it leads to subtle and hard-to-find errors.
Importing Modules
Python has a huge number of pre-written and pre-tested modules to perform various functions.
The ability to import modules means you don’t have to write code that’s already been written by
someone else. You can write your own modules, too, but doing so is beyond the scope 5 of this
document.
You get access to these modules using Python’s import keyword. For example, the time
module has a number of useful methods 6, including pausing the program for a given number of
seconds. Let’s look at an example:
         import time
         print ("Here comes the punch line!")
         print ("Wait for it…")
         time.sleep (5)
         print ("He's not that shaggy.")

This prints the first two lines, pauses for five seconds, then prints the punch line. The name of
the sleep method is qualified by prefixing it with the module name and a dot: time.sleep.
(Your Python code will generally not have embedded links especially not to shaggy dog stories.)
Suppose all we wanted was the sleep method. We could write something like this:
         from time import sleep
         print ("Here comes the punch line!")
         print ("Wait for it…")
         sleep (5)
         print ("He's not that shaggy.")

5
  “Beyond the scope” means either that the writer doesn't know enough to write about it, or that it won't fit in the
space available. The question of which one is always left to the reader to decide. Some writers make “beyond the
scope” implicit, omitting any mention of the subject in the hope that readers will ignore it or refrain from wondering
about it. This depends for its success upon the phenomenon Douglas Adams called “S.E.P. – someone else's
problem.”
6
  Functions encapsulated within objects are called methods.



                                                        – 6–
We no longer have to qualify sleep by prefixing the module name, so we type less. Cool! The
whole time module is still imported, but only the sleep() method becomes a part of the current
program’s name space.
Instantiating and Using Objects
Python is an object-oriented language, and in Python, everything is an object, even the variables
and functions we’ve described so far. Object-oriented languages include the concept of classes.
A class is a description of an object in much the same way a set of blueprints is a description of a
house. We create an object from its class, similar to the way in which a house is built from a set
of blueprints. Creating an object is called instantiation.
An object can hold properties, that is data, and methods, that is functions, both of which are
described in the class definition. We create an object with the assignment operator, with the
object name on the left and the class name, with optional arguments, on the right. By
convention, Python objects are named using the CapWords 7 style. Here’s an example:
           account = CheckingAccount("Bob Brown",                              "12345", 50.00)

Such a statement is a constructor for the object, analogous to constructing a house from
blueprints. Executing the constructor is called instantiating the object. When it has been
instantiated, account becomes an object of class CheckingAccount. It can have properties
like name, accountNumber, and balance, and methods like deposit( ) and withdraw( ).
The properties and methods were defined in the class. We might be able to use it like this:
           account.deposit(25.00)                  # Deposit $25

Each module that you might import can define one or more classes. Their properties and
methods are described in the documentation for that module. Let’s take a quick look at the LED
object from the gpiozero module for Python on the Raspberry Pi. It is documented here. We
instantiate an LED object this way:
           from gpiozero import LED
           redLed = LED(17)   # The LED will use GPIO pin 17

We could have used any name we wanted in place of redLed. The only required argument is
the pin number but there are three others. To see what they do, click the link above and read the
documentation. Once the redLed object has been instantiated, it will have methods that we can
use to make it do things using only one line of code each. Examples are redLed.on() and
redLed.blink(3, 1). Check the documentation to see what those will do.

You can write your own class definitions and modules, and will begin doing that as you advance
with Python.



7
    CapWords differs from camelCase in that the initial word is capitalized.


                                                          – 7–
The Error Messages Are Your Friends
One of the sad things about learning to program is that we make mistakes. If we’re lucky, the
mistakes lead to error messages. 8 Since we don’t like making mistakes, those error messages
sometimes make our brains slam shut, and that’s a bad thing.
So, read the error messages; they exist to help you. Python’s error messages will tell you where
(at what line) the error was detected, how Python got there, and something about the error. Note
that where the error was detected isn’t always where the error is. You may have to put in some
effort! For example, suppose a message says, “Name Error: name 'when' is not defined.” The
actual error is almost certainly somewhere else. Perhaps when wasn’t defined at all, or it was
misspelled, perhaps as wjen, when it was defined. On the other hand, if the message says,
“Name Error: name 'wjen' is not defined” then the error is probably right there, in the form of a
misspelled variable name.
A Sample Program
Here is a Python program taken from https://www.programiz.com/python-
programming/examples/prime-number Read it carefully. Identify those features you learned
here and look up anything that’s new. That’s how you learn to read and write!
        # Python program to check if the input number is prime or not
        # take input from the user
        num = int(input("Enter a number: ")) # Look up int and input
        # prime numbers are greater than 1
        if num > 1:
           # check for factors
           for i in range(2,num):             # Look up for…in
               if (num % i) == 0:
        # Look back at Keywords and Operators; look up modulus, maybe
                   print(num,"is not a prime number")
                   print(i,"times",num//i,"is",num)
                   break
           else:
               print(num,"is a prime number")

        # if input number is less than
        # or equal to 1, it is not prime
        else:
           print(num,"is not a prime number")

8
 If we’re unlucky, our programs run, but give wrong answers with no indication of error. Language processors like
Python can detect syntax errors, that is, errors in the expression of the language. They usually can’t detect logic
errors, in which we do the wrong thing, but in the right way.




                                                      – 8–