[PDF] CME 193: Introduction to Scientific Python Lecture 4: File I/O and





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CME 193: Introduction to Scientific Python

Lecture 4: File I/O and Classes

Sven Schmit

stanford.edu/~schmit/cme193

4: File I/O and Classes4-1

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4: File I/O and Classes4-2

Contents

File I/O

Classes

Exercises

4: File I/O and Classes4-3

File I/O

How to read from and write to disk.

4: File I/O and Classes4-4

The file object

Interaction with the file system is pretty straightforward in Python. Done usingfile objectsWe can instantiate a file object usingopenorfile4: File I/O and Classes4-5

Opening a file

f = open(filename, option)filename: path and filename option: "r" read file "w" write to file "a" app endto file We need to close a file after we are done:f.close()4: File I/O and Classes4-6 with open() as f Very useful way to open, read/write and close file:withopen ("data/text_file.txt"," r")asf: printf.read()4: File I/O and Classes4-7

Reading files

read() Read entire line (o rfirst ncharacters, if supplied) readline()

Reads a single line p ercall

readlines()

Returns a list with lines (splits at newline)

Another fast option to read a filewithopen ("f.txt"," r")asf: forlineinf: printline4: File I/O and Classes4-8

Reading files

read() Read entire line (o rfirst ncharacters, if supplied) readline()

Reads a single line p ercall

readlines() Returns a list with lines (splits at newline) Another fast option to read a file with open ("f.txt"," r")asf: forlineinf: printline4: File I/O and Classes4-9

Writing to file

Usewrite()to write to a filewithopen (filename," w")asf: f.write( Hello n .format(name))4: File I/O and Classes4-10

More writing examples

write elements of list to file with open (filename," w")asf: forxinxs: f.write( n .format(x)) write elements of dictionary to file with open (filename," w")asf: fork, vind.iteritems(): f.write( n .format(k, v))4: File I/O and Classes4-11

Contents

File I/O

Classes

Exercises

4: File I/O and Classes4-12

Defining our own objects

So far, we have seen many objects in the course that come standard with Python.Integers

Strings

Lists

Dictionaries

etc But often one wants to build (much) more complicated structures.

4: File I/O and Classes4-13

Defining our own objects

So far, we have seen many objects in the course that come standard with Python.Integers

Strings

Lists

Dictionaries

etc But often one wants to build (much) more complicated structures.

4: File I/O and Classes4-14

Hangman example

Objects:Game

Agents (different versions)

4: File I/O and Classes4-15

Object Oriented Programming

Express computation in terms of objects, which are instances of classes Class

Bluep rint(only one)

Object

Instance (many)

Classes specify attributes (data) and methods to interact with the attributes.

4: File I/O and Classes4-16

Object Oriented Programming

Express computation in terms of objects, which are instances of classes Class

Bluep rint(only one)

Object

Instance (many) Classes specify attributes (data) and methods to interact with the attributes.

4: File I/O and Classes4-17

Python"s way

In languages such as C++ and Java: data protection with private and public attributes and methods.

Not in Python: only basics such as inheritance.

Don"t abuse power: works well in practice and leads to simple code.

4: File I/O and Classes4-18

Simplest example

define class classLeaf: pass instantiate object leaf = Leaf() printleaf __main__ Leaf instance at 0 x10049df80 >4: File I/O and Classes4-19

Initializing an object

Define how a class is instantiated by defining the__init__method. Seasoned programmer: in Python only one constructor method.

4: File I/O and Classes4-20

Initializing an object

The init orconstructor method.classLeaf:

def__init__(self, color): self.color = color# private attribute redleaf = Leaf( red blueleaf = Leaf( blue printredleaf.color red Note how weaccessobjectattributes.4: File I/O and Classes4-21 Self

Theselfparameter seems strange at first sight.

It refers to the the object (instance) itself.

Henceself.color = colorsets the color of the objectself.color equal to the variablecolor.4: File I/O and Classes4-22

Another example

Classes havemethods(similar to functions)classStock(): def__init__(self, name, symbol, prices=[]): self.name = name self.symbol = symbol self.prices = prices defhigh_price(self): if len (self.prices) == 0: return"MISSINGPRICES " return max (self.prices) apple = Stock( Apple APPL , [500.43, 570.60])

printapple.high_price()Recall:list.append()ordict.items(). These are simply class methods!4: File I/O and Classes4-23

Another example

Classes havemethods(similar to functions)classStock(): def__init__(self, name, symbol, prices=[]): self.name = name self.symbol = symbol self.prices = prices defhigh_price(self): if len (self.prices) == 0: return"MISSINGPRICES " return max (self.prices) apple = Stock( Apple APPL , [500.43, 570.60])

printapple.high_price()Recall:list.append()ordict.items(). These are simply class methods!4: File I/O and Classes4-24

Class attributes

classLeaf: n_leafs = 0 class attribute shared def__init__(self, color): self.color = color# object attribute

Leaf.n_leafs += 1

redleaf = Leaf( red blueleaf = Leaf( blue printredleaf.color red printLeaf.n_leafs

2 Class attributes are shared among all objects of that class.

4: File I/O and Classes4-25

Class hierarchy through inheritance

It can be useful (especially in larger projects) to have a hierarchy of classes.

ExampleAnimal

Bird Hawk

Seagull

Pet Dog Cat

4: File I/O and Classes4-26

Inheritance

Suppose we first define an abstract classclassAnimal: def__init__(self, n_legs, color): self.n_legs = n_legs self.color = color defmake_noise(self): print"noise"4: File I/O and Classes4-27

Inheritance

We can define sub classes and inherit from another class.classDog(Animal): def__init__(self, color, name):

Animal.__init__(self, 4, color)

self.name = name defmake_noise(self): print self .name +" :" + " woof" bird = Animal(2, white bird.make_noise() noise brutus = Dog( black

Brutus

brutus.make_noise()

Brutus

woof shelly = Dog( white

Shelly

shelly.make_noise()

Shelly

woof 4: File I/O and Classes4-28

Base methods

Some methods to override__init__: Constructor__repr__: Represent the object (machine)__str__: Represent the object (human)__cmp__: Compare4: File I/O and Classes4-29

Example

Implementing Rational numbersclassRational:

pass4: File I/O and Classes4-30 Setup

What information should the class hold?Numerator

Denominator

4: File I/O and Classes4-31

Setup

What information should the class hold?Numerator

Denominator

4: File I/O and Classes4-32

Init

Implement the__init__methodclassRational:

def__init__(self, p, q=1): self.p = p self.q = q4: File I/O and Classes4-33 Init

Implement the__init__methodclassRational:

def__init__(self, p, q=1): self.p = p self.q = q4: File I/O and Classes4-34

Issues

Issues?classRational:

def__init__(self, p, q=1): self.p = p self.q = qIgnore the division by 0 for now, more on that later.

4: File I/O and Classes4-35

Issues

Issues?classRational:

def__init__(self, p, q=1): self.p = p self.q = qIgnore the division by 0 for now, more on that later.

4: File I/O and Classes4-36

Greatest common divisor

1020
and12 are the same rational. Implement agcd(a, b)function that computes the greatest common divisor ofaandb.defgcd(a, b): ifb == 0: returna else: returngcd(b, a%b)Exercise: Verify Euclidean Algorithm

4: File I/O and Classes4-37

Greatest common divisor

classRational: def__init__(self, p, q=1): g = gcd(p, q) self.p = p / g self.q = q / gWhy is this awesome?

4: File I/O and Classes4-38

Representing your class: Operator overloading

Implement__repr__or__str__early toprint

Debugging

4: File I/O and Classes4-39

Operator overloading: adding two Rationals

Add Rationals just like Ints and Doubles?

Rational(10,2) + Rational(4,3)

To use+, we implement the__add__methodclassRational: def__add__(self, other): p =self.p * other.q + other.p *self.q q =self.q * other.q returnRational(p, q) ... 4: File I/O and Classes4-40

Operator overloading: Comparing

__cmp__compares objectsIfselfis smaller thanother, return a negative valueIfselfandotherare equal, return 0Ifselfis larger thanother, return a positive value4: File I/O and Classes4-41

More on Operator Overloading

To learn more:

Google 'Python operator overloading".

4: File I/O and Classes4-42

Contents

File I/O

Classes

Exercises

4: File I/O and Classes4-43

Exercises

See course website for exercises for this week.

Get to know the person next to you and do them in pairs!

Let me know if you have any question

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