Introduction to Python

1
Introduction to Python

• Lecture 1

2
Big Picture Language Features

• Python is interpreted
• Not compiled
• Object-oriented language
• There are no declarations of variables
• Variables take a retain the type of the data
  assigned to it
• Whitespace is used as delimiters
• no main method

3
First Statement

• Hello World
• gtgtgt print 'Hello World!'
• Usually double and single quotes can be used
  interchangeably
• A string in double quotes cannot contain a "
• A string in single quotes cannot contain a '
• Put strings in """ if you need both or newlines

Try outputting Hello World on two lines
4
Python Calculator

• You can print the results of expressions
  (variables and functions)
• gtgtgt print 6 5
• gtgtgt print ((8 3) (7 / 3)) ( 5 1)
• Integer vs. Floating point division
• gtgtgt print 15 / 4
• gtgtgt print 15 / 4.0

5
Using Formatted Strings

• Combining strings and numbers
• gtgtgt print 'The total is ', 2453
• Combining strings with many numbers
• gtgtgt print "The sum of d and d is d" (7, 18,
  (7 18))
• Just like printf in C, other example codes
• s String
• x Hexadecimal
• 0.2f A real number with two figures following
  the decimal point
• 4d Leading spaces to pad the number to at
  least 4 digits
• 04d Leading 0's to pad the number to at least
  4 digits

6
Other important features

• Comments begin with a
• gtgtgt print 7 5 This should print 12
• Making use of libraries or other functions and
  methods you have written
• gtgtgt import sys
• gtgtgt sys.exit()
• gtgtgt from random import
• gtgtgt from pyrobot.brain import Brain

7
Python Error Messages

• gtgtgt print 'fred' 7
• Traceback (innermost last)
• File "ltstdingt", line 1, in ?
• TypeError cannot concatenate 'str' and 'int'
  objects
• Errors only reported when line is encountered
• Python's error messages are often wrong
• Inspect the line of the error and a few above

8
Variables

• Variables are not declared
• Can change types with new assignments
• gtgtgt q 7 q is currently a number
• gtgtgt print q
• gtgtgt q 'Seven' q is reassign to a string
• gtgtgt print q

9
String Operators

• operator String Concatenation
• gtgtgt print 'Again' ' and again'
• operator String repetition
• gtgtgt print 'repeat ' 3
• Combining operators
• gtgtgt print 'Again' (' and again' 3)
• String methods are documented here
  http//docs.python.org/lib/string-methods.html

10
Boolean Operators
C/Java Operator Python Operator
A B A and B
A B A or B
A B A B
A ! B A ! B A ltgt B
!A not A

• Relational operators are all the same
• 5 lt 6
• Boolean values
• True (case sensitive)
• False

Try print ((5 lt 6) true)
11
Lists

• A sequence of items
• Has the ability to grow (unlike array)
• Use indexes to access elements (array notation)
• examples
• aList
• another 1,2,3
• You can print an entire list or an element
• print another
• print another0
• index -1 accesses the end of a list

12
List operation

• append method to add elements (don't have to be
  the same type)
• aList.append(42)
• aList.append(another)
• del removes elements
• del aList0 removes 42
• Concatenate lists with
• Add multiple elements with
• zerolist 0 5
• Multiple assignments
• point 1,2
• x , y point
• More operations can be found at
  http//docs.python.org/lib/types-set.html

13
Control Statements

• Conditionals
• if ltconditiongt
• lttabgtltstatement(s)gt
• elif ltconditiongt
• lttabgtltstatement(s)gt
• else
• lttabgtltstatement(s)gt
• Example
• name "Jane"
• if "a" in name
• print "There is an 'a'"
• else
• print "There is not an 'a'"
• There is no switch statement

14
Control Statements

• Loops
• while ltconditiongt
• lttabgtltstatement(s)gt
• for ltvargt in range(i,j) up to but
• not including j
• lttabgtltstatement(s)gt
• Example
• name "Jane"
• for c in name
• print c
• no do while loops

15
Interacting with user

• Obtaining data from a user
• Use function raw_input for strings or input for
  numbers
• Example
• name raw_input("What's your name?")
• Command Line arguments
• Example
• import sys
• for item in sys.argv
• print item
• Remember sys.argv0 is the program name

16
Defining New Functions

• Syntax
• def ltfunction namegt(ltargument listgt)
• lttabgtltstatementsgt
• Remember there is not type checking
• Function may or may not return something
• Example
• def times(n)
• for i in range(1,13)
• print "2d x 2d 3d" (i, n, in)

17
Classes

• Begin with header line
• class ltclassNamegt
• Constructor and methods
• tabbed over once
• first argument is "self"
• Constructor
• def __init__(self, ltother argumentsgt)
• lttabgtltstatement(s)gt
• Methods
• def ltmethod namegt (self, ltother argumentsgt)
• lttabgtltstatement(s)gt
• Class variables
• self.ltvariable namegt

18
Example Class

• class Message
• def __init__(self, aString)
• self.text aString
• def printIt(self)
• print self.text
• def getMessage(self)
• return self.text

Use from Message import msg
Message("hi") msg.printIt()
19
Using Files

• Function open opens a file in the mode specified
  by the argument
• ltfile handlegt open(ltfile namegt, ltmodegt)
• file name is a string with path and name of the
  file
• Possible modes
• "r" to read
• "w" to write
• "rb" to read a binary file
• "wb" to write a binary file
• Example
• input open("myFile.txt", "r")
• output open("copy.txt", "w")
• for line in input.readlines()
• output.write(line)
• input.close()
• output.close()

20
Sample Program

• file createDict.py
• by Dawn Lawrie
• date September 8, 2005
• Reads the standard dictionary and creates a
  simpler
• dictionary that only includes words in all
  lowercase letters
• without aprostrophes
• Opens the input and output files
• input open("/usr/share/dict/american-english",
  "r")
• output open("simpleDict", "w")
• Reads each lines of the dictionary
• for line in input.readlines()
• Assumes that the word should not be in the
  simpler
• dictionary
• good False

21
Sample Program

• Accepts the word if it contains at least
  one lowercase
• letter
• lowercase "abcdefghijklmnopqrstuvwxyz"
• for c in lowercase
• if not good and c in line
• good True
• Rejects the word if there is an
  apostrophe
• if "'" in line
• good False
• Rejects the word if it contains a uppercase
  letter
• if good
• caps "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
• for c in caps
• if good and c in line
• good False

22
Sample Program con't

• Outputs the word if it is acceptable
• if good
• output.write(line)

23
Using String Methods

• file createDict2.py
• by Dawn Lawrie
• date September 8, 2005
• code the opens files has been removed
• Reads each lines of the dictionary
• for line in input.readlines()
• Removes all leading and trailing whitespace
• word line.strip()
• Outputs the word if all the characters are
  alphabetic
• and they are all lowercase
• if word.isalpha() and word.islower()
• output.write(word "\n")

24
Pitfalls

• My common mistakes
• Forgetting "" in a condition or loop especially
  after else
• Emacs helps because it won't tab over
• Forgetting "self" before class variables
• Without it python creates a new variable
• Forgetting the return statement
• This turned up as a type mismatch when I tried to
  use the results of the function which I had
  stored in another variable (Very Confusing)

25
Lab Exercise

• Write a program in python that plays hangman