Introduction to python-getting started

1. Introduction to python-getting started

2. What is python? • An elegant and robust programming language which delivers powerful and general applicable traditional compiled languages with the used of simpler scripting and interpreted languages. • Running python: • 3 ways • Starting the interpreter interactively , entering one line of Python at a time for execution • Running a script written in Python – this is done by invoking the interpreter on your script application • Run from graphical user interface (GUI) from within an integrated development environment (IDE/IDLE)

3. Interactive interpreter from the command line • Python can be start coded using an interactive interpreter by starting it from the command line. • This can be done from Unix, DOS etc. • Unix • At the Unix prompt (% or $), the interpreter can be started by invoking the name python (jpython), as in the following $python • Once Python started, the interpreter startup message will indicate the version and platform and be given the interpreter prompt “>>>” to enter Python commands.

4. Continue.. • Windows/DOS • From a DOS window (either DOS /Windows), the command to start python is same as Unix, the difference is that the prompt is c:\>python • Command-Line Options • Additional options may be provided to the interpreter such as listed below: • -d  provide debug output • -o  generate optimized bytecode resulting in .pyo files • file  Run Python script from given file • - c cmd  Run Python script sent in as cmd string • Etc…

5. As a Script from the Command Line • Unix- a Python script can be executed by invoking the interpreter from the command line as in the following • $ python script.py • Same as Unix

6. An integrated development environment (IDE) • Need GUI application on your system that supports Python. • Have graphical user interface (GUI), source code editor , trace and debugging facilities. • IDLE is the first Unix IDE for Python. • PythonWin is the first Windows interface for Python and is an IDE with a GUI. • Usually installed in the same directory as Python. • PythonWin features a color editor, a new and improved debugger, interactive shell window, COM (component object extensions and more.

7. Continue.. • Python extension is .py • IDLE –also available on the Windows platform • In windows, it look similar to Unix • From Windows, IDLE can be found in the Lib\idlelib subdirectory, where Python interpreter is found, c:\Python2x. • To start IDLE from a DOS command window, invoke idle.py • To start IDLE from a Windows environment, just double click on idle.pyw.

8. Continue.. • Files ending with .pyw will not open a DOS command window to run the script on it instead it is created a shortcut to c:\Python2x\Lib\idlelib\idle.pyw • Other IDEs and execution environment • Open source • IDLE –http://python.org/idle/ • PythonWin +Win32 Extensions-http://startship.python.net/crew/skippy/win32 • Etc……

9. Writing a simple python program • Prompt in Python “>>>”  primary prompt “…” secondary prompt • Primary prompt – interpreter is expecting the next python statement • Secondary prompt – interpreter is waiting for additional input to complete the current statement. • There are two primary ways that Python “does things”: 1. statements 2: expression – functions, equations, etc • A statement is a body of control which involved using keywords.

10. Continue.. • Statements may/may not lead to a result or output. • Example use print statement to print Hello World >>> print ‘Hello World!’ Output – Hello world • Expressions – do not use keywords. • Example of equations- simple equations that comes together with mathematical operators, can be a functions which are called with parentheses. • They may/may not take input, and they may/may not return a meaningful value.

11. Program Output, the print statement, and “Hello World!” >>> myString = ‘Hello World!’ >>> print myString Hello World! >>> myString ‘Hello World!’ • In this example, use a string variable, then use print to display its contents. • Given only the name reveals quotation marks around the string. • Why? This is to allow objects other than strings to be displayed in the same manner as this string. • Means that using the name of the variable, you can print/display a printable string representations of any object not just strings.

12. Continue.. • Python’s print statement, paired with the string format (%), supports string substitution similar to printf() function in C. >>> print “%s is number %d!” % (“python”,1) Pythonis number 1 • %s means to substitute a string while %d indicates an integer should be substituted. • %f is for floating point numbers. • The print statement also allows its output directed to a file. (will look into this matter later)

13. Continue.. Program input and the raw_input() built-in function. • raw_input – to get user input from the command line. • read from standard input and assigns the string value to the variable you designate. • fucntionint() can be use to convert any numeric input string to an integer representation.

14. example >>> user=raw_input(‘Enter login name: ‘) Enter login name: root >>> print ‘Your login is :’, user Your login is: root • This example is using text input

15. Example 2- a numeric string input >>> num=raw_input (‘Now enter a number:’) Now enter a number:1024 >>> print ‘Doubling your number: %d’ %(intnum)*2) Doubling your number :2048 • The int () function converts the string num to an integer so that the mathematical operation can be performed.

16. comments • Used the symbol (#) (hash or pound ) to indicate comments. • It begin with (#) and continue until the end of line. >>># one comment print ‘Hello world!’ # another comment Hello World! • Special comments called documentation strings or “doc strings” • Unlike regular comments, doc strings can be accessed at runtime and used to automatically generate documentation.

17. Operators-standard mathematical An operation that raises some given constant (base) to the power of other number (exponent).

18. Continue.. • Classic division means that if the operands are both integers, it will perform floor division • For floating numbers, it represents true division. • If true division is enabled, then the division operator will always perform that operator, regardless of operand type. • More on this later.

19. Operator precedence >>> print -2*4 +3**2 1 In this example, 3**2 is calculated first followed by (-2*4).

20. Standard comparison operators • This operators return a Boolean value indicating the truthfulness of the expression.

21. example >>> 2 < 4 True >>> 2 == 4 False >>> 2 > 4 False 6.2 <= 6.2 True >>> 6.2 <= 6.20001 True

22. Conjunction operators • can use these operations to chain together arbitrary expressions and logically combine the Boolean results: • >>> 2 < 4 and 2 == 4 • False • >>> 2 > 4 or 2 < 4 • True • >>> not 6.2 <= 6 • True • >>> 3 < 4 < 5 • True • >>> 3 < 4 < 5 is a short way of saying >>> 3<4 and 4<5

23. Variables and assignments • Variables in Python is similar to other high level languages. • Variables are simply identifier names with an alphabetic first character , upper or lowercase letters including the underscore (_). • Other alphanumeric or underscore. • Python is case sensitive means that “CASE” is not same as “case”. • Python is dynamically typed means that no pre-declaration of a variable or its type is necessary. • The type (and value) are initialized on assignment. • Assignments are performed using the equal sign.

24. example >>> counter =0 # the integer >>> miles =1000.0 # the floating point >>> name =‘Bob’ # string >>> counter = counter +1 #an incremental statement for integer >>>kilometers = 1.609*miles #floating point operation and assignment >>> print “%f miles is the same as %f km’ %(miles,kilometers) 1000.000000 miles is the same as 1.609.000000 km In this example, there are 5 variable assignments.

25. Continue.. • Python also supports augmented assignments – statement that both refer to and assign values to variables. Example n =n *10  shortcut n *= 10 • Python does not support increment/decrement operators like in C : n++ /--n. Because +, -- are also unary operators, Python will interpret --n as -(-n) == n and the same is true for ++n.

26. numbers • Python support 5 basic numerical types Boolean values are a special case of integer. If put in a numeric context such as addition with other numbers, true is treated as the integer with value 1 and false as value 0. Complex number , number that involve the square root of -1, so called “imaginary” numbers. More on numbers in later chapter

27. Strings • String in Python are identified as a contiguous set of characters in between quotation marks. • Python allows 8 pairs of single /double quotes. • Triple quotes (three consecutive single/double quotes) can be used to escape special characters. • Subsets of strings can be taken using the index ([ ] ) and slice ([ : ]) operators, which works with indexes starting at 0 in the beginning of the string and working their way from -1 at the end. • The plus (+) sign is the string concatenation operator, and the asterisk (*) is the repetition operator.

28. example >>> pystr = ’PYTHON’ >>> iscool = ‘is cool!’ >>> pystr[0] ‘P’ >>> pystr[2:5] ‘tho’ # the output start at 2 up to 4 not include the character at end. >>> iscool[:2] 'is‘ >>> pystr + iscool ‘PYTHON is cool!' >>> pystr*2 ‘PYTHONPYTHON' >>> iscool[-1] '!‘ Counting forward Counting backward More on string in coming chapter

29. Lists and tuples • Lists and tuples can be though of as generic “ arrays” with which to hold an arbitrary number of arbitrary Python objects • The items are ordered and accessed via index offsets, similar to array except that lists and tuples can store different types of objects. • Lists are enclosed in brackets ([ ]) and their elements and size can be changed. • Tuples are enclosed in parentheses (()) and cannot be updated. • Tuples can be thought of for now as “read-only” lists. • Subsets can be taken with the slice operator ([] and [ : ] in the same manner as string.

30. examples >>> alist =[1,2,3,4] >>> alist [1, 2, 3, 4] >>> alist[0] 1 >>> alist[2:] [3, 4] >>> alist[:3] [1, 2, 3] >>> alist[1]=5 >>> alist [1, 5, 3, 4]

31. examples >>> atuple=('robots',77,93,'try') >>> atuple ('robots', 77, 93, 'try') >>> atuple[:3] ('robots', 77, 93) >>> atuple[1]=5 # not allowed to change the element in tuple Traceback (most recent call last): File "<pyshell#25>", line 1, in <module> atuple[1]=5 TypeError: 'tuple' object does not support item assignment

32. Code blocks • Code blocks are identified by indentation rather than using symbols like {} • Indentation clearly identifies which block of code a statement belongs to. if statement if expression : if _suite • If the expression is non-zero or true, thenthe statementif_suiteis executed, otherwise execution continues on the first statement after. • Suite is the term used in python to refer to a sub-block of code and can consists of single or multiple statements. Example: >>> if x < .0: print '"x must be at least 10!'

33. Continue.. • Python also support an else statement that is used with if : if expression: if_suite else: else_suite • Python has an “else-if” spelled as elif : if expression1: if_suite elseif expression2: elif_suite else: else_suite

34. While loop whileexpression: while_suite • The statement while_suite is executed continuously in a loop until the expression becomes zero or false. • Execution then continues on the first succeeding statement. counter =0 while counter <3: print 'loop #%d'% (counter) counter +=1 loop #0 loop #1 loop #2

35. For loop • Python’s for takes an iterable (such as sequence or iterator) and traverses each element once. >>> print 'i like to use the internet for:' i like to use the internet for: >>> for item in ['e-mail','net-surfing','homework','chat']: print item e-mail net-surfing homework chat • More on if and loops in coming chapter