Introduction to Python Sajal Desai

What is Python? Versitile, simple, high level language No linking and compilation “By the way, the language is named after the BBC show “Monty Python’s Flying Circus” and has nothing to do with reptiles. Making references to Monty Python skits in documentation is not only allowed, it is encouraged!”

Using the Interpreter On Tux, /usr/local/bin/python quit() or crtl+d to quit >>> waiting for next command... continuation lines # denotes everything trailing on the rest of the line is commented

Starting Python python Python 2.6 (#1, Feb , 00:02:06) Type "help", "copyright", "credits" or "license" for more information. >>>

Some Math! The interpreter acts as a simple calculator: –>>> 7/3 2 ‘=‘ assigns the value to a variable –>>> width = 20 –>>> height = 5 * 9 –>>> width * height 900 simultaneously: –x = y= z = 0 **Note** Variables must be defined before used!!! Integer math, double math, float math all work the same as they do in C++ _ is the value of the last variable

Strings! Single or double quotes \ to display escape characters –“doesn\’t’ doesn’t To continue input across a number of lines: – use the \ character: hello = “Hullo? This is a super long string\ hullo hullo.\ all of this is still going into the variable hello. –Alternatively use triple quotes “”” Hello = “”” This is a super super long string but it is being stored in hello. “”” “\n” denotes newline, and still must be embedded in a string

Strings (cont.) Use r to have “raw” input –string = r”Super duper long string\n\n\n” “Super duper long string\n\n\n” Concatenated with + Repeated with * Strings are indexed, 0 is the start For substrings, use slice notation –string[0:6] Super d –string [:3] Supe –string [1:] per duper long string\n\n\n –string[-1] n –string [:-5] Super duper long string\ **NOTE** Strings cannot be changed

Lists Comma separated, do not have to be the same type –a = [hi, bye, 6] Can be indexed, can use slice, concatenated Can change Can use slice to assign, even resize Can nest strings –a = [hi, bye, 6, b] –b = [1,2,3]

if statements >>> x = int (raw_input(“Enter: “)) Enter: 42 >>> int x < 0: x = 0 print “Negative changed to 0” elif x == 0: print x; else: print “X is more than 0”

For statements For statements, unlike C++ Iterates over a sequence of items (think lists), not over an arithmetic progression (i++) Not wise to modify the list being interated, make a copy of the list for x in a[:]: ….stuff….

range() Builds a list of arithmetic progressions –range(10) [0,1…10] Some useful examples: –range(5,10) [5…10] –range (0,10,3) [0,3,6,9] –range (-10, -100, -30) [-10,-40,-70]

Functions def –function definition keyword def fib(n): –contains def name(list of parameters): –everything the function contains must be indented

Functions (cont.) Default arguments –def ask_ok(prompt, retries=4, complaint='Yes or no, please!'): can be called any of these ways: –ask_ok(“Do you really want to quit”) –ask_ok(“Do you really want to quit, 2”) Keyword Arguments –def parrot(voltage, state='a stiff', action='voom', ='Norwegian Blue'): can be called: –parrot(1000) –parrot(action = 'VOOOOOM', voltage = ) –parrot('a thousand', state = 'pushing up the daisies') –parrot('a million', 'bereft of life', 'jump') DocStrings –The first line after defining a function is a comment –Use to write a short description of the function

Conventions 4-space indentations Lines should not exceed 79 characters Use blank lines to separate functions and classes Comment on separate lines Use DocStrings Use spaces around operators, after commas Use CamelCase for classes, lower_case_with_underscores for functions

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