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Introduction to python

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Introduction to python

  2. 2. COURSE OUTCOMES:  On successful completion of the course, students will be able to:  CO1: Illustrate the basics of Python environment, data types and operations.  CO2: Develop simple Python scripts using control statements and data types.  CO3: Construct object oriented programs using functions.  CO4: Outline the concepts of Inheritance and Polymorphism.  CO5: Apply overloading concepts in various applications.h  CO6: Outline the concept of handling the exception through handlers. 2
  3. 3. Syllabus 3
  4. 4. Books & References 4
  5. 5. Introduction to Python  Python is a high-level programming language  Open source and community driven  “Batteries Included”  a standard distribution includes many modules  Dynamic typed  Source can be compiled or run just-in-time  Similar to perl, tcl, ruby 5
  6. 6. Why Python?  Unlike AML and Avenue, there is a considerable base of developers already using the language  “Tried and true” language that has been in development since 1991  Can interface with the Component Object Model (COM) used by Windows  Can interface with Open Source GIS toolsets 6
  7. 7. Python!  Created in 1991 by Guido van Rossum (now at Google)  Named for Monty Python  Useful as a scripting language  script: A small program meant for one-time use  Targeted towards small to medium sized projects  Used by:  Google, Yahoo!, Youtube  Many Linux distributions  Games and apps (e.g. Eve Online)
  8. 8. Installing Python Windows:  Download Python from http://www.python.org  Install Python.  Run Idle from the Start Menu. Mac OS X:  Python is already installed.  Open a terminal and run python or run Idle from Finder. Linux:  Chances are you already have Python installed. To check, run python from the terminal.  If not, install from your distribution's package system.Note: For step by step installation instructions, see the course web site.
  9. 9. Interpreted Languages  interpreted  Not compiled like Java  Code is written and then directly executed by an interpreter  Type commands into interpreter and see immediate results Computer Runtime Environment CompilerCodeJava: ComputerInterpreterCodePython:
  10. 10. The Python Interpreter  Allows you to type commands one-at-a-time and see results  A great way to explore Python's syntax  Repeat previous command: Alt+P
  11. 11. Our First Python Program  Python does not have a main method like Java  The program's main code is just written directly in the file  Python statements do not end with semicolons hello.py 1 print("Hello, world!")
  12. 12. The print Statement print("text") print() (a blank line)  Escape sequences such as " are the same as in Java  Strings can also start/end with ' swallows.py 1 2 3 4 print("Hello, world!") print() print("Suppose two swallows "carry" it together.") print('African or "European" swallows?')
  13. 13. Comments  Syntax: # comment text (one line) swallows2.py 1 2 3 4 5 6 # Suzy Student, CSE 142, Fall 2097 # This program prints important messages. print("Hello, world!") print() # blank line print("Suppose two swallows "carry" it together.") print('African or "European" swallows?')
  14. 14. IDLE – Development Environment  IDLE helps you program in Python by:  color-coding your program code  debugging  auto-indent  interactive shell 14
  15. 15. More than just printing  Python is an object oriented language  Practically everything can be treated as an object  “hello world” is a string  Strings, as objects, have methods that return the result of a function on the string 15
  16. 16. What Is a Program?  Usually, one or more algorithms written in a programming language that can be translated to run on a real machine  We sometimes call programs software
  17. 17. What Is a Programming Language?  A programming language is somewhat like a natural language, but with a very limited set of statements and strict syntax rules.  Has statements to implement sequential, conditional and iterative processing - algorithms  Examples: FORTRAN, COBOL, Lisp, Basic, Pascal, C, C++, Java, C#, Python, …
  18. 18. Compiler  A compiler is a program that converts a program written in a programming language into a program in the native language, called machine language, of the machine that is to execute the program.
  19. 19. From Algorithms to Hardware (with compiler) Algorithm Program A real computer Translate (by a human being) Translate (by compiler program)
  20. 20. The Program Development Process (Data Flow) Editor Compiler A real computer Algorithm Program in programming language Program in machine’s language Input Output
  21. 21. The Program Development Process (Control Flow) Edit Compile Run Syntax errors Input Output Runtime errors
  22. 22. Three kinds of errors  Syntax error : Some statement in the program is not a legal statement in the language.  Runtime error : An error occurs while the program is executing, causing the program to terminate (divide by zero, etc.)  Logic error : The program executes to completion, but gives incorrect results.
  23. 23. Interpreter  An alternative to a compiler is a program called an interpreter. Rather than convert our program to the language of the computer, the interpreter takes our program one statement at a time and executes a corresponding set of machine instructions.
  24. 24. Interpreter Edit Interpreter Syntax or runtime errors Input Output
  25. 25. Python  Python is a real-world, production language that is freely available for most computers. http:www.python.org  If you want a copy of Python to use with this course, go to http://code.google.com/p/mediacomp-jes/ . We are using JES (Jython Environment for Students) which has a lot of special multimedia functionality.  Note: Our textbook covers a limited amount of Python. There are many excellent online tutorials. For example, see http://en.wikibooks.org/wiki/Non-Programmer's_Tutorial_for_Python/Contents
  26. 26. Python  Python uses an interpreter. Not only can we write complete programs, we can work with the interpreter in a statement by statement mode enabling us to experiment quite easily.  Python is especially good for our purposes in that it does not have a lot of “overhead” before getting started.  It is easy to jump in and experiment with Python in an interactive fashion.
  27. 27. Language terminology  Syntax: The formal rules for legal statements in the language.  Semantics: The meaning of the statements - what happens when the statement is executed.
  28. 28. Three major control constructs of programming (Execution flow of instructions)  Sequential: Simply do steps one after the other in order they are listed.  Conditional: Decide which statement to do next based on some true/false test.  Iterative: A set of statements is repeated over and over until some condition is met.
  29. 29. Sequential Operations “Atomic”  Input  Computation  Output
  30. 30. The Big Plan  We want to get some experience of programming simple algorithms in a real programming language. This gives us an understanding of how software is written and allows us to test our algorithms to see if they work.  We’ll first work with programs where the variables have numbers as values.  Later we’ll work with programs dealing with pictures and sound.  In lab we’ll work with some simple statements and small programs.
  31. 31. The Basic Pattern  Most of our programs will use the basic pattern of  Get some user input  Perform some algorithm on the input  Provide results as output
  32. 32. Identifiers  Identifiers are names of various program elements in the code that uniquely identify the elements. They are the names of things like variables or functions to be performed. They're specified by the programmer and should have names that indicate their purpose.  In Python, identifiers  Are made of letters, digits and underscores  Must begin with a letter or an underscore  Examples: temperature, myPayrate, score2
  33. 33. Keywords  Keywords are reserved words that have special meaning in the Python language. Because they are reserved, they can not be used as identifiers. Examples of keywords are if, while, class, import.
  34. 34. Variables in Python  A variable has  A name – identifier  A data type - int, float, str, etc.  Storage space sufficient for the type.
  35. 35. Numeric Data Types  int This type is for whole numbers, positive or negative. Examples: 23, -1756  float This type is for numbers with possible fraction parts. Examples: 23.0, -14.561
  36. 36. Integer operators The operations for integers are: + for addition - for subtraction * for multiplication / for integer division: The result of 14/5 is 2 % for remainder: The result of 14 % 5 is 4  *, /, % take precedence over +, - x + y * z will do y*z first  Use parentheses to dictate order you want. (x+y) * z will do x+y first.
  37. 37. Integer Expressions  Integer expressions are formed using  Integer Constants  Integer Variables  Integer Operators  Parentheses
  38. 38. Python Assignment Statements  In Python, = is called the assignment operator and an assignment statement has the form <variable> = <expression>  Here  <variable> would be replaced by an actual variable  <expression> would be replaced by an expression  Python: age = 19
  39. 39. Python Assignment Statement  Syntax: <variable> = <expression>  Note that variable is on left  Semantics: Compute value of expression Store this as new value of the variable  Example: Pay = PayRate * Hours Payrate 10 Hours 40 Pay 400
  40. 40. Assignment Example Before X 3 Z 12 Y 5 After X 3 Z 11 Y 5 Execute Z = X * 3 + Z / Y
  41. 41. Python Session
  42. 42. Python Session
  43. 43. What about floats?  When computing with floats, / will indicate regular division with fractional results.  Constants will have a decimal point.  14.0/5.0 will give 2.8 while 14/5 gives 2.
  44. 44. Comments  Often we want to put some documentation in our program. These are comments for explanation, but not executed by the computer.  If we have # anywhere on a line, everything following this on the line is a comment – ignored
  45. 45. Numerical Input  To get numerical input from the user, we use an assignment statement of the form <variable> = input(<prompt>)  Here  <prompt> would be replaced by a prompt for the user inside quotation marks  If there is no prompt, the parentheses are still needed  Semantics  The prompt will be displayed  User enters number  Value entered is stored as the value of the variable
  46. 46. Print Statement  For output we use statements of the form print <expression>  Semantics  Value of expression is computed  This value is displayed  Several expressions can be printed – separate them by commas
  47. 47. Example - Fahrenheit to Centigrade  We want to convert a Fahrenheit temperature to Centigrade.  The formula is C = (F -32) x 5/9  We use type float for the temperatures.
  48. 48. Python Session