Introduction to Python Michael DiRamio District School Board of Niagara Michael'Diramiodsbn'edu'on'c

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Introduction to PythonMichael DiRamioDistrict
School Board of NiagaraMichael.Diramio_at_dsbn.edu.o
n.ca

• Imperial Oil Summer Institute
• 2007

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Python

• Python is a fairly young (1990) language that is
  gaining popularity in education and industry.
• Used by YouTube, Google, Industrial Light and
  Magic.
• University of Toronto is moving to Python as
  their first year programming language.

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Why Python?

• Python is becoming more popular as a first
  programming language because
• It has a very simple, lightweight syntax.
• The interpreter window makes it easy for students
  to try ideas.
• It still has the power to cover all the high
  level concepts of computer science and has
  industry applications.

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Why not Python?

• Dynamic typing may cause problems for students.
• The built in classes and functions are not well
  documented.
• Because it is a young language, it is still a
  work in progress.

Conversely, students may find it more
straight forward to not have to declare the type
of a variable.
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Overview

• Python is an interpreted, dynamically typed,
  object oriented programming language.
• We will explore each of these concepts in this
  presentation.

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Interpreted Languages

• An interpreted language is not run directly on
  the CPU.
• There is another program running on the computer
  which then reads the lines of Python code and
  creates the appropriate output.

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Python IDE IDLE

• IDLE is a free Python integrated development
  environment.
• There are two main components we will be using
• Python Shell
• File Window

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Comments

• Commented lines in Python start with a . All
  text beyond a will be ignored.

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Output The print command

• The print command is similar to put in Turing
  or System.out.println in Java.
• It outputs what follows it and then prints a
  newline.
• To suppress the newline, put a comma (,) at the
  end of the statement.
• To concatenate output put a comma (,) between
  elements.
• Python puts a space between concatenated elements.

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Running a Program

• While in the File Window, go to
• Run ? Run Script
• IDLE will make you save any changes before
  running the program.

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Variables

• Variables in Python are dynamically typed.
• Variables are not declared,they are just assigned
  a value.
• Type information is stored with the object, not
  with the variable.
• Variables can change type by assigning it to an
  object of a different type.

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Built in Variable Types

• Integer
• Integers are unbounded in Python
• Floating Point
• Boolean True and False
• String

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Arithmetic Operations
Note Dividing two integers will still result in
integer division with both / and //.
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Strings

• Strings are immutable (cannot be changed after
  creation).
• Concatenation is done with the operator.
• There is no char type in Python, just Strings of
  length one.

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Substrings

• Indexing for Strings starts at 0.
• Examples below use the Strings 0123456789

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Either Quotes

• Strings in Python can use either single or double
  quotes.
• This allows you to use the opposite quotes inside
  a String.
• You can also use the escape character, \ (same
  as Java).

print He said Hello, I am a string."' print
"The programmer replied 'My program shouldn't be
talking!'" print "What's wrong with saying
\"Hello\"?"
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Converting Strings to Other Types
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Input

• The command raw_input(prompt) is used to get
  input from the user.
• Prompt is a message printed on the screen before
  waiting for input.
• If you dont want a prompt, call raw_input() with
  no parameter.
• Combine this method with the conversions when you
  want to read numbers or booleans.

s raw_input("Enter something ") print "You
said", s
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Comparisons
Note You can also use ltgt for inequality, but
it is considered out of date.
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Logical Operators
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No Brackets

• Python does not use brackets to denote the
  start/end of blocks of code.
• Instead, indentation is used to specify blocks.
• This forces programmers to properly indent their
  programs, otherwise the functionality changes.

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Conditional Statements

• The conditional statements are followed by colons
  ()
• Each block is indented.
• To end the block, go back out to the original
  level of indentation.

x 22 if x lt 15 print "first clause" elif
x lt 25 print "second clause" else
print "the else print "This is after the
statement"
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Conditional Loops

• Continue looping as long as the condition is true.

print from 1 to 10 x 1 while x lt 10
print x x x 1 print "Done"
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Lists

• Python has a built in List data structure that
  can hold any type.
• Indexing starts at 0.
• Unlike Strings, lists are mutable (can be
  modified).
• Elements can be changed, added, and removed from
  the list.
• List elements are not all forced to be the same
  type.

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Modifying Lists
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For Loops

• Different than what we think of as a counting
  loop like in languages such as Java.
• Loops go through each element of a set.

prints the three values for i in 10,20,14
print i
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The range Function

• The range function returns a list of the numbers
  in a given range.
• It is often used with for loops.

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For Loops and range

• Putting these two concepts together gives us our
  usual counting loop.

print 0 - 9 for i in range(10) print i
calculate 4 factorial x 4 for i in range(3, 0,
-1) x x i print x
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Functions

• Functions must be defined before they are used.
• This does not stop referencing of functions that
  have not yet been defined, but the functions can
  not be called until every part is defined.
• All functions return something. If you do not
  specify a return, it is the constant None.

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Declaring Functions

• The keyword def indicates the start of a function
  definition.
• The body of the function is indented under this
  header.
• Parameters are contained in brackets after the
  function name.
• There is no type listed with the parameters.

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Function Examples
def foo() print "Hello" print "There"
print "World"
print s n times def bar(s, n) for i in
range(n) print s
def max(a, b) if a gt b return a
else return b
Calling Functions foo() max(2, 6) bar("Hello",
4)
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Opening a File

• The function to open a file is
• open(name, mode)
• Name and mode should both be Strings

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Read Methods
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Write Methods

• write does not add a newline to output.
• The parameter to write should be a String. Output
  needs to be converted to a String using the str
  method.

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Contact and References

• If you have any questions about this
  presentation, please dont hesitate to contact
  me
• Michael.DiRamio_at_dsbn.edu.on.ca
• I used the following references to help me
  prepare this presentation
• www.python.org
• Learning Python (OReilly)
• Python in a Nutshell (OReilly)