Python - Reading and Writing Files.

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© Prof Mukesh N Tekwani, 2016
Unit I Chap 1 : Python ­ File Input and Output
I.1 Concept of a File:
In all the programs we have seen so far, the data provided to a Python program was through
the command line and it is not saved. If a program has to be run again, the user must again
input data. Similarly, the output of a program appears on the computer screen but is not
saved anywhere permanently.
In practical programming it is necessary to provide data to a program from a file, and save
the output data into a file. The output data may be required for further processing. A file is a
collection of data. This data may be, for example, students’ data (Roll No, Name, Address,
Tel No, Grades,...), employee data (EmpID, Name, Dept, Designation, Salary…), etc. A file
could also be used to store a picture (say the fingerprints of employees), audio, video, etc.
I.2 Types of Files:
There are many different types of files.
(i) Text files
(ii) audio files
(iii) video files
(iv) binary files
(v) presentations (Microsoft PowerPoint presentation .pptx files)
(vi) Word processor documents (Microsoft Word .docx files), etc
In this chapter we will study how Python handles text files. Text files contain only text
characters (alphabets, digits, mathematical symbols, and other readable characters.A text
file can be created and opened in any text editor such as Notepad. Text inside a text file
cannot be formatted such as bold, italic, underline, font colors, styles etc. These files donot
contain any metadata such as page numbers, header, footer, etc. But text files are very
useful as they can store data which can be imported into various types of programs. Text
files are small in size and therefore easier and faster to process. Some examples of text
files are: files created with text editor such as Notepad, programs in programming languages
such as Python, C, C++, etc, HTML files, CSV files, etc. CSV stands for ​C​omma ​S​eparated
V​alue. In a CSV file, data is separated by using a comma between different items. For
example, a CSV text file containing student data looks like this:
Name, Class, MajorSubject, Grade
Amit, FYBSc, Physics, 1
Nikhil, SYBCom, Accounts, 1
Prerana, TYBSc, Chemistry, 2
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In this chapter we will study how to write Python programs that use text files to input and
output data.
I.3 Opening a file:
For this exercise, create a folder called in C drive called: C:/37/Files. We will save all our
programs for this chapter in this folder.
In IDLE (​I​ntegrated ​D​evelopment and ​L​earning ​E​nvironment), select File → New File and
type the following three lines:
This is Line 1
This is Line 2
This is Line 3
Save this file as a text file with filename FileEx1.txt, in the folder you just created.
Create a new program file in IDLE and save it with the filename FileReader1.py. This file
should also be saved in the folder C:37Files
Program 1: FileReader1.py
file = open("C:/37/files/FileEx1.txt", "r")
contents = file.read()
print(contents)
file.close()
Run this program and the output is as shown below:
This is Line 1
This is Line 2
This is Line 3
Analysis of the program:
1. open() is a builtin function in Python. It opens a file; the file cursor is used to keep
information about the current location in the file, which part of the file has been read,
and which part has yet to be read. Initially, the file cursor points to the beginning of
the file but as we read the file, the file cursor begins to move forward.
2. The first argument in the function open() is "C:/37/files/FileEx1.txt". It is the name of
the file to open. The second argument is “r”, which means that the file has to be
opened in read­only mode. Here, “r” is called as the file mode. Other file modes are
“​w​” for ​writing​ to a file and “​a​” to ​append ​data to a file. If file mode is omitted in the
function call, it is assumed to be “read only”.
3. The second statement file.read() tells Python that you want to read the contents of
the entire file into a string called ​contents​.
4. The third statement print(contents), prints the string.
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5. The last statement is file.close(). It closes the file. Every file opened for
reading/writing must be closed.
The ​with​ statement:
Every open file must be closed in Python. So we have to use two statements in every
program like this:
file = open(filename, mode)
:
:
:
file.close()
In this code, the name ​file is a file handle. Python provides the ​with statement that
automatically closes a file when the end of the file is reached.
General syntax:
with open(filename, mode) as variable:
processing block
The complete program to read a text file is now :
Program 2: FileReader2.py
with open("C:/37/files/FileEx1.txt", "r") as file:
contents = file.read()
print(contents)
Output of the above program is the same as that for FileReader1.py
I.4 Techniques for Reading Files:
There are four techniques for reading a file and all these techniques work by starting at the
current file cursor.
1. Techniques for Reading a File : “The Read Technique”​: This techniques is
used to:
(i) Read the contents of a file into a ​single string​, or
(ii) To specify exactly how many characters to read.
To read the contents of the file into a single string, we use the following syntax:
while open(filename, mode) as file:
contents = file.read()
print (contents)
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This reads the entire file from the current cursor location to the end of the file, and then
moves the cursor to the end of the file.
To read a specific number of characters, we specify the number of characters to read as the
argument to the read() function. For example, to read 10 characters from the beginning of
the file, the program is:
Program 3:
file = open("C:/37/files/FileEx1.txt", "r")
contents = file.read(10)
print(contents)
file.close()
The output of this program for the same input as shown below:
This is Li
>>>
The cursor now points to the character after the 10th character.
2. Techniques for Reading a File: “The Readlines Technique”​: This technique is
used to read ​each line ​of a text file and store it in a ​separate ​string. After reading the lines,
the cursor moves to the end of the file.
Program 4: FileReader5.py
# Read the lines of a file into separate strings
print ("This program reads the lines of a file into separate strings .")
with open("C:/37/files/FileEx1.txt", "r") as file:
lines = file.readlines()
print(lines)
In this program, the statement print(lines) prints all the strings. Output of this program is :
This program reads the lines of a file into separate strings .
['This is Line 1n', 'This is Line 2n', 'This is Line 3n']
>>>
Each line ends with the newline character ‘n’ . But the last line of a text file may not end with
a ‘n’ character.
To print only line no 2, we can modify the program as follows:
Program 5:
# Read the lines of a file into separate strings
print ("This program reads the lines of a file into separate strings and prints only the
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second line.")
with open("C:/37/files/FileEx1.txt", "r") as file:
lines = file.readlines()
print(lines[1])
file.close()
In the above program, the line print(lines[1]) prints only the 2nd line. Remember, indices start
at 0.
What happens when we give an index number more than the number of lines? Try this
statement: print(lines[10]). Now, we get an error as shown below
This program reads the lines of a file into separate strings and prints only the second line.
Traceback (most recent call last):
File "C:/37/files/FileReader5.py", line 6, in <module>
print(lines[10])
IndexError: list index out of range
>>>
Program 6: FileReader6.py
This program prints the lines of a text file in reverse order.
# Read the lines of a file into separate strings
# Then print the lines in reverse order
print ("This program reads the lines of a file into separate strings and prints them in
reverse order.")
with open("C:/37/files/FileEx2.txt", "r") as file:
lines = file.readlines()
print(lines)
for site in reversed(lines):
print(site.strip())
file.close()
Assume the file FileEx2.txt contains the following lines:
Google
Microsoft
IBM
Infosys
Wipro
Capgemini
This program reads the lines of a file into separate strings and prints them in reverse order.
['Googlen', 'Microsoftn', 'IBMn', 'Infosysn', 'Wipron', 'Capgeminin']
Capgemini
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Wipro
Infosys
IBM
Microsoft
Google
>>>
Analysis: We have used the built­in function reversed() to reverse the items in the list. The
strip() method is used to remove all whitespace characters from the left and right of a string.
We have used the strip() method without any arguments so it removes only whitespaces.
To remove whitespace characters only from the ​left of a string use the method ​lstrip()​. To
remove whitespace characters only from the ​right​ of a string use the method ​rstrip()​.
Run the following program and comment on the output:
Program 7: FileReader7.py
Using the strip() method.
# Read the lines of a file into separate strings
# Then print the lines in reverse order
print ("This program reads the lines of a file into separate strings and prints them in
reverse order. What is the output? What happened to the ‘C’? Why is there a blank line
between two lines in the o/p?")
with open("C:/37/files/FileEx2.txt", "r") as file:
lines = file.readlines()
print(lines)
for site in reversed(lines):
print(site.strip('C'))
file.close()
Why is there a blank line between two lines?
Program 8: FileReader8.py
Program to sort the lines of a text file.
# Read the lines of a file into separate strings
# Then print the lines in sorted order
print ("This program reads the lines of a file into separate strings and prints them in sorted
order.")
with open("C:/37/files/FileEx2.txt", "r") as file:
lines = file.readlines()
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print("Original lines:")
print(lines)
print("Lines in sorted order:")
for site in sorted(lines):
print(site.strip())
file.close()
The built­in function sorted() is used to sort the lines and display them. Note that, there is no
change made to the original text file.
3. Techniques for Reading a File : “The For Line In File” Technique​: This
techniques is used to carry out the same action on each line of a file. Suppose we want to
read a text file and print the length of each line i.e. number of characters in each line. Here is
the program
Program 9: FileReader9.py
Program to print the length of each line of a text file.
# Read the lines of a file order and print the length of each line
print ("This program reads the lines of a file and prints the length of each line.")
with open("C:/37/files/FileEx2.txt", "r") as file:
for line in file:
print(line, len(line))
file.close()
Output of this program is :
This program reads the lines of a file and prints the length of each line.
Google
7
Microsoft
10
IBM
4
Infosys
8
Wipro
6
Capgemini
10
>>>
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Observe that the length of each string is one more than the number of characters that can be
seen. But each string is terminated by the newline character ‘n’ so that increases the size by
one character.
Modification:
Make only this small change in the program and observe the output:
print(line.strip(), len(line))
What is the output and why is it like that?
Modification:
No modify the print line as follows and comment on the output:
print(line, len(line.strip()))
4. Techniques for Reading a File : “The Readline” Technique​: This techniques is
used to read one entire line from a file. This line can then be processed.
Program 10: FileReader10.py
Program to illustrate the use of readline() technique.
A text file contains the following data about number of students in various academic years in
FYBSc.
Filename: FileEx3.txt
Number of students in various academic years in FYBSc
#Year 2010­11 to 2015­16
#Semester 1
67
69
53
58
66
61
We have to count the total number of students in these academic years,
with open("C:/37/files/FileEx3.txt", "r") as inpfile:
#Read the description line
file.readline()
#Keep reading the comment lines until we read the first piece of data
data = inpfile.readline().strip()
while data.startswith("#"):
data = inpfile.readline().strip()
#Now we have the first actual data item. Add it to total number of students
numofstudents = int(data)
#Read the rest of the file
for data in inpfile:
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numofstudents = numofstudents + int(data)
print("Total number of students: ", numofstudents)
I.5 Techniques for Writing Files:
The built­in write() function is used to write data to a text file. The general syntax of write() is:
file.write(string)
The following program opens a file called FileEx4.txt and writes the words “I Love Python” to
the file and then closes the file.
Program 11: FileReader11.py
Program to write a string to a text file.
#Writing a file
with open("C:/37/files/FileEx4.txt", "w") as optfile:
optfile.write("I Love Python")
optfile.close()
The file is now opened in write mode by using “w” as the mode. If the file FileEx4.txt already
exists, it is overwritten with new data.If the filename does not exist, it is first created and then
data written to it.
Program 12: FileReader12.py
Program to write a the numbers 1 to 10, one per line to a text file.
#Writing the numbers 1 to 10 to a file
with open("C:/37/files/FileEx5.txt", ​"w"​) as optfile:
for i in range(1, 11):
optfile.write(str(i)+ "n")
optfile.close()
Check the output of the file FileEx5.txt by opening it in a text editor.
Program 13: FileReader13.py
Program to append data to a text file.
The following program appends numbers from 11 to 20 to the file FileEx5.txt that we created
earlier. Data already stored in the file is not lost, and new data is added to the end of the file
FileEx5.txt
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#Appending data to a file
with open("C:/37/files/FileEx5.txt", ​"a"​) as optfile:
for i in range(11, 21):
optfile.write(str(i)+ "n")
optfile.close()
Program 14: FileReader14.py
Program to read data from a file, process it and write it to another file.
A file FileEx6.txt contains the marks obtained by 5 students in two tests. These marks are
separated by a blank space. Write a program that reads the two marks for each student and
stores these marks and their total in another file called Results.txt
FileEx6.txt Results.txt
15 18
17 22
6 11
21 25
23 22
M1 M2 Total
15 18 33
17 22 39
6 11 17
21 25 46
23 22 45
​Program:
# Read the contents of a file, process each line and write results to a new file
print("Read the contents of a file, process the data and write result into another file")
optfile = open("C:/37/files/Results.txt", "w")
inpfile = open("C:/37/files/FileEx6.txt", "r")
optfile.write("M1 M2 Totaln")
for numpair in inpfile:
numpair = numpair.strip()
values = numpair.split()
total = int(values[0]) + int(values[1])
optline = '{0} {1}n'.format(numpair, total)
optfile.write(optline)
print("file written")
print("now closing all files")
optfile.close()
inpfile.close()
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Program 15: FileReader15.py
Program to prompt the user for a file and display the contents of the file.
#To prompt the user for a filename and display the contents of that file
name = input("Enter the file name ")
fname = open(name, "r")
print("Contents of the file are : ")
contents = fname.read()
print(contents)
fname.close()
Program 16: FileReader16.py
Program to prompt the user for a filename and backup the file.
# Write a program that makes a backup copy of your file
#import the module os so that we can use functions from this module
import os
orgfile = input('Which file would you like to back­up? ')
base, ext = os.path.splitext(orgfile)
#print (base)
#print(ext)
new_filename = base + '.bak'
backup = open(new_filename, 'w')
orgfile = open(orgfile, "r")
for line in orgfile:
backup.write(line)
orgfile.close()
backup.close()
I.6 Manipulating Directories:
Files are arranged in folders or directories. Every running program has a “current directory”.
Python searches for files in the default or current directory.
Python has a module named ​os module which contains many functions for working with files
and directories. We now study some of these built­in functions of the ​os​ module.
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Program 17: FileReader17.py
Program to study directory manipulation.
#Import the module os
import os
#Get the current working directory or relative path
cwd = os.getcwd()
print("Current working directory: ", cwd)
#To get the absolute path names
print("Absolute path: ", os.path.abspath('Results.txt'))
#To check if a file or directory exists
print("Does file exist: ", os.path.exists("Results.txt"))
print("Does file exist: ", os.path.exists("Results.xls"))
#To check if it is a directory
print("Is it a directry: ", os.path.isdir("Results.txt"))
print("Is it a directry: ", os.path.isdir("C:37files"))
#To check if it is a file
print("Is it a file: ", os.path.isfile("Results.txt"))
#To return a list of files and directories in a directory
print("List of files and sub directories: ")
print(os.listdir(cwd))
Practical Exercises
1. Write a program to ignore the first 10 characters of a text file and print the remaining
characters.
2. Write a program that prompts the user for a file to display and then show the contents of
the file.
3. Write a program that makes a backup of a file. Your program should prompt the user for
a file to copy and then write a new file with the same contents but with .bak as the file
extension.
4. Write a program that prints the length of each line in a text file.
5. Develop a Python program that reads in any given text file and displays the number of
lines, words and total number of characters in the file including spaces and special
characters but not the newline character ‘n’.
6. Develop a Python program that reads messages contained in a text file, and encodes the
messages saved in a new file. For encoding messages, a simple substitution key should
be used as shown : A­­>C, B → D, C → E, D → F, … etc. The unencrypted text file has
the filename msgs.txt and the encrypted file has the same name but filename extension
is .enc
7. Develop a Python program that reads encrypted messages contained in a text file, and
decodes the messages saved in a new file. For decoding messages, a simple
substitution key should be used as shown : C­­A, D → B, E→ C, F→ D, … etc. The
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encrypted text file has the filename msgs.enc and the decrypted file has the same name
but filename extension is .txt
8. Write a Python program that reads a text file and prints each word on a separate line.
9. A text file called marks contains the marks obtained by 20 students. Assume marks are
not repeated. Write a Python program to find and display the highest mark.
10. Write a program that reads the first 10 characters of a text file but does not display them.
The rest of the characters should be displayed,
11. Write a program that prints the length of each line. The output should have this format:
Line No Length
1
2
:
12. What is the output of the following program?
f = None
for i in range (5):
with open("data1.txt", "w") as f:
if i > 2:
break
print (f.closed)
g = open("data2.txt", "w")
for i in range (5):
if i > 2:
break
print (g.closed)
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Miscellaneous Questions
1. Give an instruction in Python that opens a file named “Markss.txt” for reading and
assigns the identifier inpfile to the file object created.
2. Give an instruction in Python that opens a file named “Results.txt” for writing and assigns
the identifier optfile to the file object created.
3. Multiple Choice Questions:
3.1. Which of the following statements are true? (multiple answers allowed)
a) ​A. When you open a file for reading, if the file does not exist, an error occurs.
b) When you open a file for reading, if the file does not exist, the program will open
an empty file.
c) When you open a file for writing, if the file does not exist, a new file is created.
d) When you open a file for writing, if the file exists, the existing file is overwritten with
the new file.
Ans a, c, d
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