Lecture 10

• The type of a file is usually understood from its extension
• The extension of a file is the substring after the last ‘.’
• There are two types of files on your harddisk
  • Binary files: contains characters unreadable by humans (only meaningful for a computer)
    • Image files : .JPG, .GIF, .PNG, .BMP
    • Audio files : .OGG, .MP3, .M4A, .WAV
    • Video files : .MP4, .MPEG, .AVI, .MKV
    • Data files : .SAV, .DAT, .BIN
    • Executable files : EXE
  • Text files: contain multiple lines of strings
    • Usually ends with .TXT

A text file can be thought of as a sequence of lines

MUH101 Introduction to Programming
A Course By Burkay Genc
2019-2020 Spring 

Topics:
1 - Introduction
2 - Branching and Iteration
3 - Iterations
...

You can download this file from here

• Download this file and place it into your work folder
• If you are working in Colab, then you must upload the file to Colab

• Before we can read the contents of a file, we must tell Python which file we are going to work with and what we will be doing with the file
• This is done with the open() function
• open() returns a file handle - a variable used to perform operations on the file
• Similar to “File -> Open” in a Word Processor

handle = open(filename, mode)

returns a handle used to manipulate the file

• filename is a string
  • it must clearly show where the file is located on the computer
• mode is optional and should be
  • ‘r’ if we are planning to read from the file
  • ‘w’ if we are going to write to the file

muh101_file = open("muh101.txt", "r")
muh101_file

## <_io.TextIOWrapper name='muh101.txt' mode='r' encoding='cp65001'>

muh101_file.close()

• Never forget to close a file when you are done
  • handle.close()

• If you try to open a handle to a non-existent file, you will get an error:

muh101_file = open("muh100.txt", "r")

FileNotFoundError: [Errno 2] No such file or directory: 'muh100.txt'

A text file can be thought of as a sequence of lines

MUH101 Introduction to Programming
A Course By Burkay Genc
2019-2020 Spring 

Topics:
1 - Introduction
2 - Branching and Iteration
3 - Iterations
...

• A text file has newlines at the end of each line
  • But they are not normally visible in a text editor

MUH101 Introduction to Programming\n
A Course By Burkay Genc\n
2019-2020 Spring\n
\n
Topics:\n
1 - Introduction\n
2 - Branching and Iteration\n
3 - Iterations\n
...

We can put an if statement in our for loop to only print lines that meet some criteria

fhandle = open('muh101.txt')
for line in fhandle:
  if line.startswith('3') or line.startswith('5'):
    print(line)

## 3 - Iterations
## 
## 5 - Functions and Recursion

fhandle.close()

• When you print a string read from a file you get extra newlines printed
  • This is because print() adds a newline by default
  • So it becomes two newlines
• We can avoid this by using rstrip() function to remove the newlines from each line

fhandle = open('muh101.txt')
for line in fhandle:
  line = line.rstrip()
  if line.startswith('3') or line.startswith('5'):
    print(line)

## 3 - Iterations
## 5 - Functions and Recursion

fhandle.close()

• rstrip removes extra whitespaces from strings
  • including the newline character at the end

We can conveniently skip a line by using the continue statement

fhandle = open('muh101.txt')
i = 0
for line in fhandle:
  i += 1
  line = line.rstrip()
  if i < 5:                # Skip the first 4 lines
    continue
  print(line)

## Topics:
## 1 - Introduction
## 2 - Branching and Iteration
## 3 - Iterations
## 4 - Functions and Scope
## 5 - Functions and Recursion
## 6 - Tuples and Lists
## 7 - Dictionaries
## 8 - Testing and Debugging
## 9 - Exceptions and Assertions
## 10 - File IO

fhandle.close()

We can look for a string anywhere in a line as our selection criteria

fhandle = open('muh101.txt')
for line in fhandle:
  line = line.rstrip()
  if 'and' in line:          # Print the line only if it contains 'and'
    print(line)

## 2 - Branching and Iteration
## 4 - Functions and Scope
## 5 - Functions and Recursion
## 6 - Tuples and Lists
## 8 - Testing and Debugging
## 9 - Exceptions and Assertions

fhandle.close()

• It is very likely to get IO Exceptions when dealing with file input and output
• So, always surround file operations with try/except blocks

try:
  fhandle = open("muh100.txt")
  print("File contains", len(fhandle.read()), "characters.")
except:
  print("File not found!")

## File not found!

• Writing is similar to reading
  • Use ‘w’ instead of ‘r’

fhandle = open("muh102.txt", "w")
fhandle.write("test string")        # Outputs number of written characters

## 11

fhandle.close()

We can now read from newly created muh102.txt:

fhandle = open("muh102.txt", "r")
print(fhandle.read())

## test string

fhandle.close()

• If you write on an existing file, it gets overwritten:

fhandle = open("muh102.txt", "w")
fhandle.write("another string")

## 14

fhandle.close()
fhandle = open("muh102.txt", "r")
print(fhandle.read())

## another string

fhandle.close()

• You can use the append, 'a', mode to avoid this behaviour.

• If you write on an existing file, it gets overwritten:

fhandle = open("muh102.txt", "a")
fhandle.write("appended string")

## 15

fhandle.close()
fhandle = open("muh102.txt", "r")
print(fhandle.read())

## another stringappended string

fhandle.close()

• Use \n to write to a new line
  • fhandle.write("\nappended string")

There are many other modes besides ‘r’, ‘w’ and ‘a’

• Create a file containing squares of the first 10 positive integers, line by line

• Create a file containing squares of the first 10 positive integers, line by line

fhandle = open("squares.txt", "w")
for i in range(1, 11):
  fhandle.write(str(i**2) + "\n")

## 2
## 2
## 2
## 3
## 3
## 3
## 3
## 3
## 3
## 4

fhandle.close()

Check the file:

fhandle = open("squares.txt", "r")
print(fhandle.read())

## 1
## 4
## 9
## 16
## 25
## 36
## 49
## 64
## 81
## 100

fhandle.close()

The with statement works with the open() function to open a file. Unlike open() where you have to close the file with the close() method, the with statement closes the file for you.

with open("squares.txt", "r") as fhandle:
  lines = fhandle.readlines()
print(lines)

## ['1\n', '4\n', '9\n', '16\n', '25\n', '36\n', '49\n', '64\n', '81\n', '100\n']

• readlines() returns all lines in the file, as a list where each line is an item in the list object.

These slides are a direct adaptation of the slides used on the Py4e Webpage.

Original work by:

Dr. Charles R. Severance

Adapted by and for:

Asst. Prof. Dr. Burkay Genç. MUH101 Introduction to Programming, Spring 2020. Hacettepe University, Computer Engineering Department.