Tips and Examples

• Common Exam Mistake - Using Input in Functions
• Never Assume Your Code Works

Review

• Functions Returning Boolean Values
• Storing Functions in Modules

New Material

• Files
• Types of Files
• Identifying a File
• Storing Data on a Hard Disk
• Files and the Kernel
• Working with a File in Python
• Objects
• Accessing the Data in Files
• Opening a File
• Specifying the Location of a File
• Writing Data to a File
• Reading Data From a File
• Ethics and Technology
• The Spiderman Principle

I will also expect you to work on Class Exercise 18.

Graded Quiz

You can connect to Gradescope to take weekly graded quiz today during the last 15 minutes of the class.

Once you start the quiz you have 15 minutes to finish it.

You can only take this quiz today.

There is not makeup for the weekly quiz because Gradescope does not permit it.

Readings

If you have the textbook you should read Chapter 6, Files and Exceptions, from Starting Out with Python.

Solution to Homework 6

I have posted a solution to homework 6 here.

Let's take a look.

Homework 8

I have posted homework 8 here.

It is due this Sunday at 11:59 PM.

Midterm Scores

If you believe I have scored one of your Midterm answers incorrectly you can request a rescore for that question on Gradescope.

If your score on the Final is significantly better than your score on the Midterm, I will replace your Midterm grade with that of the Final when calculating your grade for the course.

If you want to discuss how you can improve your grade in this course, come see me during Office Hours.

You do not have to make an appointment to see me during Office Hours, but you may have to wait your turn if I am helping another student.

I want students to pass this course, and will do what I reasonably can to make that happen.

Your Current Standing in This Course

If you want to know your grade as of this moment, send me an email.

In performing this calculation, I will assume that your score on the final exam will be the same as that of you Midterm score.

I can do this only for your grade as of the Midterm.

I can't do this for your grade later in the semester.

Final Exam

The final for this course will take place in this room on Thursday, May 16th, from 11:30AM - 2:30PM

You will also find this information at the top of the class web page.

Questions

Are there any questions before I begin?

Tips and Examples

Common Exam Mistake - Using Input in Functions

• Functions usually get their values from their parameters
• You don't need an input statement to get values supplied by parameters
• Some students lose points on my exam making this mistake
• If a function has the following header

  def pos_neg_zero(num)

• You should NOT have an input statement like the one below

  num = int(input("Number: "))

Never Assume Your Code Works

• In the American justice system a defendant is presumed innocent ...
• until he or she is proven guilty
• In other words you are innocent until you have been shown to be guilty
• With code the opposite is true
• You should always assume your code is not working ...
• until you can test it to prove otherwise
• This means your work is not done once you have written the code
• It is only finished when you can prove it works

Review

Functions Returning Boolean Values

• We have seen that a function can return values that are
  • Integers
  • Floats
  • Strings
• They can also return boolean values
• Such functions are often very simple
• They often consist of one line
• A return statement with boolean expression

  # return true if value is between max and min
  def in_range(min, max, value):
      return min <= value <= max

Storing Functions in Modules

• A module is a file of Python functions and variables
• To use these functions in a script, you import the module
• The filename of the module must
  • End in .py
  • The module name cannot be a keyword

Attendance

New Material

Files

• Programs need data to do their work
• We can use the input function to get data from the user
• But manual data entry is slow and often inaccurate
• Some tasks involve processing a large amount of data
• When a large company does its payroll it needs data on every employee
• When processing large amount of data we need to use files
• A file is simply a linear arrangement of data ...
• on some long term storage device
• That storage device might be a
  • Hard disk
  • Flash drive
  • CD ROM
  • SSD card

Types of Files

• All files consist of binary numbers
• But those numbers can be interpreted in one of two ways
  • Text
  • Binary
• Consider the following text file

  $ cat fruit.txt 
  grapes
  pears
  oranges
  cranberries
  apples
  melons
  blueberries

• The Unix od (octal dump) command shows how this file is stored on disk

  $ od -b fruit.txt 
  0000000 147 162 141 160 145 163 012 160 145 141 162 163 012 157 162 141
  0000020 156 147 145 163 012 143 162 141 156 142 145 162 162 151 145 163
  0000040 012 141 160 160 154 145 163 012 155 145 154 157 156 163 012 142
  0000060 154 165 145 142 145 162 162 151 145 163 012
  0000073

• The first column is the offset
• It is how far the data on the right is from the start of the file
• Every column after the first shows the octal value of a byte in the file
• Each byte of the file represents a character in Unicode
• To see the characters, I can run od with the -c option

  od -c fruit.txt 
  0000000   g   r   a   p   e   s  \n   p   e   a   r   s  \n   o   r   a
  0000020   n   g   e   s  \n   c   r   a   n   b   e   r   r   i   e   s
  0000040  \n   a   p   p   l   e   s  \n   m   e   l   o   n   s  \n   b
  0000060   l   u   e   b   e   r   r   i   e   s  \n
  0000073

• A binary file, such as a JPEG image also consist of numbers
• Consider the JPEG file that holds the following image

• When we look at this with od we get

  $ od -b square.jpg
  0000000   377 330 377 340 000 020 112 106 111 106 000 001 001 000 000 110
  0000020   000 110 000 000 377 341 000 200 105 170 151 146 000 000 115 115
  0000040   000 052 000 000 000 010 000 004 001 032 000 005 000 000 000 001
  ...

• Trying to interpret it as text gives us garbage

  $ od -c square.jpg 
  0000000  377 330 377 340  \0 020   J   F   I   F  \0 001 001  \0  \0   H
  0000020   \0   H  \0  \0 377 341  \0 200   E   x   i   f  \0  \0   M   M
  0000040   \0   *  \0  \0  \0  \b  \0 004 001 032  \0 005  \0  \0  \0 001
  ...

• Both files contain binary numbers
• But the numbers in the text file represent Unicode characters
• They make sense when we view them as text
• But binary in the JPEG file make no sense when viewed as text
• To use a file in Python we have to specify if it is text or binary
• Why?
• Because text files have a simple structure
• Each text file is a collection of lines
• A line is a series of characters followed by the newline character, \n
• Binary files do not have this structure
• We will only be using text files in this course

Identifying a File

• You need two pieces of information to identify a file
  • Name
  • Location
• Operating systems have different rules about the characters in a filename
• Linux and Unix are case sensitive
• This means that memo.txt and MEMO.txt are different file names
• Windows is not case sensitive
• You cannot have two files with the same name ...
• inside this same directory
• The location of a file is given by a path
• A path is a list of directories
• The path tells what directory holds the file

Storing Data on a Hard Disk

• Every type of storage device has its own way of storing data
• Hard disks consists of rapidly spinning disks
• The disks contain information in the form of magnetic fields
• The disk is made up of material that can be magnetized
• This material consists of millions of small regions
• In each regions the magnetic field points in some direction
• Either up or down
• Each of these regions represents a binary digit called a bit
• One direction means 0 and the other means 1
• The surface of the disk is broken up into rings called tracks
• And each track is broken up into sectors

• Most files consist of many sectors
• The sectors do not have to be one right after the other
• The sectors are usually scattered all over the disk

Files and the Kernel

• A file can be scattered over different sectors of the disk
• So how does the operating system know where to get the different parts?
• It doesn't have to
• That's the job of the device driver
• The device driver is software that allows a device to talk to the computer
• Device drivers must be provided by the manufacturer
• If you install a new printer on your machine
• You must select the right device driver to talk to it
• At the core of every operating system is a program called the kernel
• The kernel handles all interactions with hardware
• When a program needs a file it must ask the kernel ...
• to set up a connection to the file
• The kernel does not have to worry about where the file is stored on the disk
• That's the job of the device driver
• This division of labor makes everything work better
• It also means that you can work with files on many different devices

Working with a File in Python

• All operations inside a computer take place in RAM
• RAM is working memory
• A file lives on a some external storage device ...
• like a hard disk or thumb drive
• But any interactions with the file takes place in RAM
• RAM is fast and expensive
• But external storage is slower and cheap
• RAM space is limited ...
• and files can be very large
• So you usually only have a small part of a file in RAM ...
• at any one time
• The kernel sets aside a section of RAM to work with the file
• This section holds the part of the file that has been read into RAM ...
• and various other pieces of bookkeeping information
• Like the location of the last line read from disk

Objects

• Sometimes you need several pieces of information about one thing
• If we were writing software for a car dealer we would need to know
  • Manufacturer
  • Model
  • Year
  • Color
• You would need many variables and functions to work with this data
• These variables and functions might be scattered over different parts of the code
• This could become unwieldy ...
• and make the code hard to understand and maintain
• Objects were created to deal with this problem
• An object is region of RAM ...
• that contains all the data about some thing
• It also has all the functions that deal with this data
• The data is contained in variables inside the object
• The functions that work on this data are called methods
• Objects have no name
• They only have a location in RAM
• We create an object by using a special function
• This function returns a pointer to the object in RAM
• We store this value of this pointer in a variable
• And we use this variable to work with the object

Accessing the Data in Files

• There are two ways that you can get the data in a file
  • Sequential access
  • Direct access
• In sequential access the script starts at the beginning of the file
• And travels through the file ...
• until it gets to the end
• Sequential access is how a tape recorder works
• In direct access you go directly to the part of the file that holds the data
• This is like getting the number of a book from the library card index ...
• and going directly to the shelf that holds the book
• We will only be using sequential access in this class

Opening a File

• To work with a file we have create a file object
• We do this using the built-in function open
• The open function does two things
  • It creates a file object
  • It returns a reference to the object
• The reference is a memory location that is stored in a variable
• We use the variable to work with the object
• The picture in memory looks like this

• Here is the format for using the open function

  FILE_VARIABLE = open(FILENAME, MODE)

• FILE_VARIABLE is the name of the variable that holds the reference
• FILENAME is a string giving the name and location of the file
• MODE is a letter specifying what we want to do with the file
• The three most commonly used access modes are

  Mode	Description
  "r"	Open a file for reading
  "w"	Open a file for writing
  "a"	Open a file for writing to the end of the file

• The a in the last mode stands for "append"
• Opening an existing file in this mode does not destroy its content
• Instead, new lines will be added to the bottom of the file
• If a file is open for reading it cannot be changed
• If you try to open a file for reading ...
• and the file does not exist ...
• you will get an error
• If you try to open a file for writing or appending ...
• that does not exists ...
• the kernel will create the file for you
• If you open a file for writing that already exists ...
• its contents will be replaced by whatever you write to it
• The mode argument to the open command must be given as a string
• To open the file memo.txt for reading, I would write

  file = open("memo.txt", "r")

• Not

  file = open("memo.txt", r)

Specifying the Location of a File

• A file is identified by two pieces of information
  • Its name
  • Its location
• If the file is in your current directory things are simple
• All you need is the name of the file
• If I open a file for writing with the following statement

  test_file = open("test.txt", "w")

• The Python interpreter will ask the kernel ...
• to look for the file test.txt in my current directory
• To open the file test.txt in my /home/ghoffman directory on pe15 I would write

  test_file = open("/home/ghoffman/tmp/test.txt", "w")

• To open the file in C:\Users\ghoffman of a Windows machine I would write

  test_file = open(r"C:\Users\ghoffman\test.txt", "w")

• Note the r before the pathname
• The r stands for raw
• The problem is that Windows uses the backslash character, \, ...
• to mark the end if a directory name
• Python uses this character in an escape sequence
• The r before the pathname string ...
• tells Python to ignore the special meaning of  \
• We will only use relative pathnames in this class

Writing Data to a File

• Objects contain values ...
• and the functions that work with those values
• The functions inside objects are called methods
• To call a method you must use dot notation
• Dot notation for methods has the following format

  OBJECT_VARIABLE.METHOD_NAME

• To write data to a file we must first open it for writing

  teams_file = open("teams.txt", "w")

• This will give you a file object referenced by teams_file
• Now we can use the write method of this file object ...
• to send some text to the file

  $ cat write_al_east.py
  # writes the names of the teams in the American League East
  
  teams_file = open("teams.txt", "w")
  
  teams_file.write("Boston Red Sox\n")
  teams_file.write("Baltimore Orioles\n")
  teams_file.write("Toronto Blue Jays\n")
  teams_file.write("Tampa Bay Rays\n")
  teams_file.write("New York Yankees\n")
  
  $ python3 write_al_east.py 
  
  $ cat teams.txt 
  Boston Red Sox
  Baltimore Orioles
  Toronto Blue Jays
  Tampa Bay Rays
  New York Yankees

• Notice that I had to add the newline character \n at the end of every line
• Otherwise all the text would be on a single line

Reading Data From a File

• There is more than one method that can read a line
• The simplest is read
• Which returns a string containing everything in the file

  $ cat read_teams.py
  # reads the contents of the file teams.txt
  
  teams_file = open("teams.txt", "r")
  
  teams = teams_file.read()
  
  print(teams)
  
  teams_file.close()
  
  $ python3 read_teams.py 
  Boston Red Sox
  Baltimore Orioles
  Toronto Blue Jays
  Tampa Bay Rays
  New York Yankees

• It turns out that read is not very useful
• Normally we want to read a file line by line
• I'll show you how to do this in the next class

Class Exercise

Class Quiz