Python: Working with Files

Working with Files

Definition : File is a named location on the disk to store information.

File is used to store the information permanently.

1. Types of Files

There are two types of files

1. Text file

2. Binary file

1) Text file :

•  The text ASCII file is a simple file containing a collection of characters that are readable to humans.

•  Various operations that can be performed on text files are ‒ Opening the file, reading the file, writing to the file, appending data to the file.

•  The text file deals with the text stream.

•  In text file each line contains any number of characters include one or more characters including a special character that denotes the end of file. Each line of the text have maximum of 255 characters.

•  When a data is written to the file, each newline character is converted to carriage return/ line feed character. Similarly when data is read from the file, each carriage return feed character is converted to newline character.

•  Each line of data in the text file ends with newline character and each file ends with special character called EOF (i.e. End of File) character.

2) Binary file :

•  Binary file is a file which contains data encoded in binary form.

•  This data is mainly used for computer storage or for processing purpose.

•  The binary data can be word processing documents, images, sound, spreadsheets, videos or any other executable programs.

•  We can read text files easily as the contents of text file are ordinary strings but we can not easily read the binary files as the contents are in encoded form.

•   The text file can be processed sequentially while binary files can be processed sequentially or randomly depending upon the need.

•  Like text files, binary files also put EOF as an endmarker.

Difference between text file and binary file

Text file

1.Data is present in the form of characters.

2. The plain text is present in the file.

3. It can not be corrupted easily.

4. It can be opened and read using simple text editor like notepad.

5. It have the extension such as .py or .txt.

Binary file

1. Data is present in the encoded form.

2. The image, audio or text data can be present in the file.

3. Even if single bit is changed then the file gets corrupted.

4. It can not read using the text editor like notepad.

5. It can have application defined extensions.

 

2. Text Files

•  The text files are types of files that store textual information.

•  The text files are considered as persistent storages. That means once you store data in a text file that remains in it even‒if you shutdown and restart the computer. One can be picked up where they left off.

•  Various operations that can be performed on text files are :

1. Open file

2. Close file

3. Writing to the file

4. Reading from file

Opening a file

In python there is a built‒in function open() to open a file.

Syntax

File_object=open(file_name,mode)

Where File_object is used as a handle to the file.

Example

inf=open("test.txt")

Here file named test.txt is opened and the file object is in variable inf

We can open the file in text mode or in binary mode. There are various modes of a file in which it is opened. These are enlisted in the following table ‒

Mode         Purpose

'r' ‒ Open file for reading.

'w' ‒ Open file for writing. If the file is not created, then create a new file and then write. If file is already existing then truncate it.

'x' ‒ Open a file for creation only. If the file already exists, the operation fails.

'a' ‒ Open the file for appending mode. Append mode is a mode in which the data is inserted at the end of existing text. A new file is created if it does not exist.

't' ‒ Opens the file in text mode.

'b' ‒ Opens the file in binary mode.

'+' ‒ Opens a file for updation i.e. reading and writing.

For example

fo=open("test.txt",w) #opens a file in write mode

fo=open("text.txt",rt) #Open a file for reading in text mode

Closing a file

• After performing all the file operations, it is necessary to close the file.

• Closing of a file is necessary because it frees all the resources that are associated with file.

Example

fo=open("test.txt",rt)

fo.close() #closing a file.

 

3. Reading from Files

For reading the file, we must open the file in r mode and we must specify the correct file name which is to be read.

The read() method is used for reading the file. For example ‒

Let us close this file and reopen it. Then call read statement inside the print statement, illustrated as follows –

Example:1

Write a Python program to read the contents of the file named 'test.txt.

Solution :

ReadFile.py

inf = open('D:\\test.txt','rt')

print(inf.read())

inf.close()

Output

In [10]: import ReadFile

Hello

How are you?

I am fine

Good Bye

Note that a blank line is returned when the file reaches the end of the file.

There are some other useful methods for reading the contents of the file. Let us discuss them with the help of necessary illustrations.

The readline() method

The readline() method allows us to read a single line from the file. When file reaches to the end, it returns an empty string.

Example:2

Write a Python program to read and display first two lines of the text file.

Solution :

ReadLineDemo.py

inf = open('D:\\test.txt','rt')

print(inf.readline())

print(inf.readline())

inf.close()

Output

Hello

How are you?

Program explanation: In above program,

1) The file is opened using open statement.

2) The we call readline() statements inside two subsequent print statement. After reading from the file using the readline() method, the control automatically passes to the next line. Hence we call readline() inside the print statement again.

3) Finally we must not forget to close the file using close() method.

The readLines() method

The readLines() method is used to print all the lines in the program. Following program illustrates it ‒

ReadLines Demo.py

inf = open('D:\\test.txt','rt')

print(inf.readlines())

inf.close()

Output

Hello

How are you?

I am fine

Good Bye

India is Wonderful country

The list() method

The list method is also used to display the contents of the file as a list. The program is as follows‒

ListDemo.py

inf = open('D:\\test.txt','rt')

print(list(inf))

inf.close()

Output

In [11]: import ListDemo

['Hello\n', 'How are you?\n', 'I am fine\n', 'Good

Bye\n', 'India is Wonderful country']

Note that we passed the file object as an argument to the list method.

Displaying file using loop

This is the most commonly used method of reading the file. In this method the contents of the file are read line by line using for loop.

Example:3

Write a program to display the contents of the file using for loop.

Solution :

DisplayFile.py

inf = open('D:\\test.txt','rt')

for line in inf:

        print(line)

inf.close()

Output

Hello

How are you?

I am fine

Good Bye

India is Wonderful country

Opening a file using with

We can open the file using keyword with. The advantage of this is that the file gets closed properly after the read or write operations.

OpenWithDemo.py

with open('D:\\test.txt', 'rt') as inf:

for line in inf:

    print(line)

inf.close()

Output

Hello

How are you?

I am fine

Good Bye

India is Wonderful country

Example:4

Write a Python program to find the line that starts with the word "This" from the following text which is stored in a file.

Test.txt

This is a Python program

Python is superb.

This is third line of program

this Python program is nice

Solution :

FileDemo1.py

fh=open('d:\\test.txt')

i=0

for line in fh:

    line=line.rstrip()

    if line.find("This')= = ‒1:continue

    print(line)

Output

This is a python program

This is third line of program

Example:5

Write a program in Python to split the text line written in the file into words.

Solution :

with open('d:\\test.txt','rt') as inf:

    line=inf.readline()

    word_list=line.split()

    print(word_list)

Output

['This', 'is', 'a', 'python', 'program']

 

4. Writing to Files

For writing the contents to the file, we must open it in writing mode. Hence we use 'w' or 'a' or 'x' as a file mode. The write method is used to write the contents to the file,

Syntax

File_object.write(contents)

The write method returns number of bytes written. While using the 'w' mode in open, just be careful otherwise it will overwrite the already written contents. Hence in the next subsequent write commands we use "\n" so that the contents are written on the next lines in the file. For example :

Step 1: Create a python program in a file in which the file named output.txt is created and opened up for writing purposes. Write some contents to this file. Following screenshot illustrates this Python program ‒

Step 2: Run the above file

In [12]: import WriteFileDemo

Step 3: Open some text editor like notepad and check the contents of output.txt file

The writelines() method

The writelines() method is used to write a list of strings to the file. Following program illustrates this

Example:6

Write a Python program to write multiple lines to a text file using writelines() method.

Solution :

fo=open('d:\\test.txt', 'wt')

lines=["Welcome to the Python programming\n","It is fun\n",

    "Python is easy\n", "But it is powerful programming language"]

fo.writelines (lines)

fo.close()

Output

Now open the Notepad and open the test.txt file in it. It will be something like this ‒

Appending the file

Appending the file means inserting records at the end of the file.

For appending the file we must open the file in 'a' or 'ab' mode.

For example ‒

fo=open('d:\\test.txt','a')

fo.write("Python has a wide scope in future")

fo.close()

Output

Just open the existing d:\test.txt file to check the contents. It will be as follows ‒

Example:7

Write a Python program to write n number of lines to the file and display all these lines as output.

Solution :

print("How many lines you want to write")

n=int(input())

outFile=open("d:\\test.txt","wt")

for i in range(n):

    print("Enter line")

    line=input()

    outFile.write("\n"+line)

outFile.close()

print("The Contents of the file 'test.txt' are ...")

inFile=open("d:\\test.txt", 'rt')

print(inFile.read())

inFile.close()

Example:8

Write a Python program to write the contents in 'one.txt' file. Read these contents and write them to another file named 'two.txt'.

Solution :

print("How many lines you want to write")

n=int(input())

outFile=open("d:\\one.txt", 'wt')

for i in range(n):

    print("Enter line")

    line=input()

    outFile.write("\n"+line)

outFile.close()

inFile=open("d:\\one.txt", 'rt')

outFile=open("d:\\two.txt","wt")

i=0

for i in range(n+1):

    line‒inFile.readline()

    outFile.write("\n"+line)

inFile.close()

outFile.close()

print("The Contents of the file 'two.txt' are...")

inFile=open("d:\\two.txt","rt')

print(inFile.read())

inFile.close()

Example:9

How to merge multiple files into a new file using Python.

Solution :

one = two ="'

fp=open('d:\\first.txt','rt')

one = fp.read()

fp=open('d:\\second.txt','rt')

second = fp.read()

one+= "\n"

one+= second

fp = open('d:\\third.txt','wt')

fp.write(one)

fp.close()

Step 1: Create a first.txt file using some text‒editor like Notepad.

Step 2: Create a second.txt file using some text‒editor like Notepad.

Step 3: Now open the third.txt file using some text‒editor. It will be as follows ‒

 

5. File Positions

The seek and tell method: The seek method is used to change the file position. Similarly the tell method returns the current position.

The method seek() sets the file's current position at the offset.

Syntax

fileObject.seek(offset[, whence])

Where

•  offset ‒ This is the position of the read/write pointer within the file.

•  whence ‒ This is optional and defaults to 0 which means absolute file positioning, other values are 1 which means seek relative to the current position and 2 means seek relative to the file's end.

The method tell() returns the current position of the file read/write pointer within the file.

Syntax

fileObject.tell()

Programming example

inf=open('d:\\test.txt','rt')

print("\tThe contents of the file are...")

print(inf.read())

print("\tThe current position is...")

print(inf.tell())#get current position of file

inf.seek(0) #moves the file cursor to initial position

print("\tThe current position is...")

print(inf.tell())#get current position of file

Output

In [13]: import FilePosDemo

The contents of the file are...

Hello

How are you?

I am fine

Good Bye

India is Wonderful country

The current position is...

68

The current position is...

0

6. Command Line Arguments

In Python the sys module is used to use the command line arguments. There are three important steps to be followed while accessing the command line arguments.

1. Import the sys module.

2. We can use sys.argv for getting the list of command line arguments.

3. The len(sys.argv) gives a total number of command line arguments.

The Python program illustrating the access to command line arguments is as given below.

Step 1: Write a Python script as follows. Here the name of the script file is CmdLine.py

CmdLine.py

import sys

print("Number of arguments: ",len(sys.argv))

print("Argument List: ",str(sys.argv))

print("The name of this program is: ",str(sys.argv[0]))

Step 2: The command to run the command line argument program use the %run command as follows ‒

In [17]: %run CmdLine.py 11 22 33

Number of arguments: 4

Argument List: ['CmdLine.py', '11', '22', '33']

The name of this program is:

CmdLine.py

7. Binary File Handling

• The binary files store the data in the binary form, that means in the form of 0's and 1's. These files are machine readable files. Typically image files, audio files are the binary files.

1. Reading Binary Data Format

1) Open the binary file using open() method in binary mode(rb).

2) After opening the binary file in binary mode, we can use the read() method to read its content into a variable. The" read()" method will return a sequence of bytes, which represents the binary data.

3) Process the data based on the requirements.

4) Finally close the file using close() method.

Python code

# Opening the binary file in binary mode as rb(read binary)

f = open("test.zip", mode="rb")

# Reading file data with read() method

data = f.read()

# Finding the Type of data

print(type(data))

# Printing our byte sequenced data

print(data)

# Closing the opened file

f.close()

2. Writing to Binary File

• We can write the bytes to the file using write() method. Following code illustrates the write operation

Python code

data = b'\x21'

# Open file in binary write mode

binary_file = open("test.txt", "wb")

# Write bytes to file

binary_file.write(data)

# Close file

binary_file.close()

 

Review Question

1. How to use command line arguments in Python?