Xi Wang Yang Zhang

1. Strings 2. Text Files 3. Exceptions 4. Classes  Refer to basics 2.py for the example codes for this section.

 Strings are made up of characters.  Slice a string >>> fruit = 'apple' >>> fruit[1] 'p' >>> fruit[0] 'a' apple index

>>> fruit[-1] 'e' >>> fruit[0:2] 'ap' >>> fruit[2:] 'ple' >>> len(fruit) 5

 Strings are immutable: >>> fruit[0] = "A" Traceback (most recent call last): File " ", line 1, in TypeError: 'str' object does not support item assignment

 Use for loop to process a string: >>> for char in fruit: print char a p p l e

>>> prefixes = "JKL" >>> suffix = "ack" >>> for letter in prefixes: print letter + suffix Jack Kack Lack

 The in operation tests if one string is a substring of another: >>> 'a' in fruit True >>> 'o' in fruit False >>> 'o' not in fruit True >>> 'pp' in fruit True

>>> def removeP(s): newString = ' ' for letter in s: if letter != 'p': newString += letter return newString >>> removeP(fruit) 'ale'

 String methods are functions for strings. >>> greeting = "Good morning!" >>> greeting.upper() GOOD MORNING! >>> greeting.lower() good morning!

>>> badGreeting = " GOOd mOrning! " >>> badGreeting.strip() 'GOOd mOrning!' >>> badGreeting.replace(“O",“o") ' Good morning! ' >>> badGreeting.split() ['GOOd', 'mOrning!']

 When a program is running, its data is stored in random access memory (RAM).  If you want to use the data after ending the program, you need to write the data to a file in a non-volatile storage medium (such as hard drive and flash drive).

 A text file is a file that contains printable characters and whitespace, organized into lines separated by newline characters (\n).  Working with a text file is like working with a notebook.

>>> file = open('test.txt', 'w') >>> print file >>> file.write("# Line one \n") >>> file.write("# Line two \n") >>> file.write("# Line three \n") >>> file.write("Now is the time to close the file. \n") >>> file.close()

>>> file = open('test.txt', 'r') >>> file = open('test.dat', 'r') IOError: [Errno 2] No such file or directory: 'test.dat' >>> text = file.read() >>> print text Line one Line two Line three Now is the time to close the fille.

>>> print file.read(5) # Line >>> print file.readline() one >>> print file.readline() # Line two

>>> print file.readlines() ['# Line three \n', 'Now is the time to close the file. \n'] >>> file.close()

def filter(oldfile, newfile): infile = open(oldfile, 'r') outfile = open(newfile, 'w') while True: text = infile.readline() if text == "": break if text[0] == '#': continue outfile.write(text) infile.close() outfile.close() return

>>> filter("test.txt", "newfile.txt") >>> newfile = open("newfile.txt") >>> print newfile.read() Now is the time to close the file.

1. Tuples 2. Lists 3. Dictionaries  They are used to store data.

 Tuples can be made up of anything. >>> myTuple = ('apple', 3, True)  Tuples and strings are indexed in a similar way. >>> myTuple[0] 'apple' >>> myTuple[-1] True >>> myTuple[0:2] ('apple', 3)

 Like strings, tuples are immutable. >>> myTuple[-1] = False Traceback (most recent call last): File " ", line 1, in TypeError: ‘tuple' object does not support item assignment

 Iteration over a tuple: >>> fruitTuple = ('apples', 'bananas', 'oranges') >>> for fruit in fruitTuple: print 'I love', fruit I love apples I love bananas I love oranges

 Lists are just like tuples except that lists are mutable: >>> myList = ['apple', 3, True] >>> myList[-1] = False >>> myList ['apple', 3, False]

 Side effect of mutability >>> numList1 = [10, 20, 30, 40] >>> numList2 = numList1 >>> numList3 = numList1[:] >>> numList1[0] = 0 >>> numList1 [0, 20, 30, 40] >>> numList2 [0, 20, 30, 40] >>> numList3 [10, 20, 30, 40]

 Functions and operators for lists: >>> range(1,5) [1, 2, 3, 4] >>> range(5) [0, 1, 2, 3, 4] >>> len(myList) 3 >>> False in myList True

 Useful list methods: >>> socialScience = ['sociology', 'chemistry'] >>> socialScience.append('political science') >>> socialScience ['sociology', 'chemistry', 'political science'] >>> socialScience.remove('chemistry') >>> socialScience ['sociology', 'political science']

>>> socialScience.insert(1, 'economics') >>> socialScience ['sociology', 'economics', 'political science'] >>> socialScience.count('political science') 1 >>> socialScience.index('political science') 2

 Strings, tuples, and lists use integers as indices.  Dictionaries map keys to values.  Create a dictionary >>> months = {} >>> months['Jan'] = 1 >>> months['Feb'] = 2 >>> months['Mar'] = 3 >>> months {'Feb': 2, 'Jan': 1, 'Mar': 3}

 Dictionary operators and functions >>> 'Jan' in months True >>> ‘Apr‘ not in months True >>> months['Feb'] 2 >>> len(months) 3 >>> months['Jan'] = 0 >>> months {'Feb': 2, 'Jan': 0, 'Mar': 3}

 Dictionary methods >>> months.items() [('Jan', 0), ('Mar', 3), ('Feb', 2)] >>> months.keys() ['Jan', 'Mar', 'Feb'] >>> months.values() [0, 3, 2] >>> months.pop('Jan') 0 >>> months {'Feb': 2, 'Mar': 3}

 Whenever a runtime error occurs, it creates and exception, and the program stops.

>>> 3/0 ZeroDivisionError: integer division or modulo by zero >>> tup = (1,2,3) >>> print tup[3] IndexError: tuple index out of range tup[0] = 0 TypeError: 'tuple' object does not support item assignment

>>> int(tup) TypeError: int() argument must be a string or a number, not 'tuple' >>> print mytup NameError: name 'mytup' is not defined >>> file = open('test.dat', 'r') IOError: [Errno 2] No such file or directory: 'test.dat'

def open_file(filename): try: f = open(filename, "r") print filename, 'now is open.' except: print 'There is no file named', filename

>>> open_file('test.txt') test.txt now is open. >>> open_file('test.dat') There is no file named test.dat

 Python is an object-oriented programming language.  Up to now we have been using procedural programming which focuses on writing functions that operate on data.  In object-oriented programming, the focus is on creating objects that contain both data and functions.

 A class defines a new data type.  Let’s define a new class called Point. >>> class Point: def __init__(self, x, y): self.x = x self.y = y def distanceFromOrigin(self): return (self.x**2 + self.y**2)**0.5

>>> p = Point(3,4) >>> p.x 3 >>> p.y 4 >>> p.distanceFromOrigin() 5.0

class Student(object): counter = 0 def __init__(self): Student.counter += 1 self.id = Student.counter def getType(self): return 'Student'

class Undergrad(Student): def __init__(self, year): Student.__init__(self) self.year = year def getType(self): return 'Undergraduate Student'

>>> student1 = Student() >>> student1.id 1 >>> student2 = Student() >>> student2.id 2 >>> student3 = Undergrad(2015) >>> student3.id 3

>>> student1.year AttributeError: 'Student' object has no attribute 'year' >>> student3.year 2015 >>> student1.getType() ‘Student’ >>> student3.getType() ‘Undergraduate Student’