Working with File Operations in Python

In this chapter, we’ll explore how to work with files in Python, including reading, writing, and managing files efficiently. Files are often used to store inputs or outputs during script execution, and Python’s built-in support for file handling makes this process easy. We’ll go over the most commonly used file handling methods like open(), read(), write(), and seek(). You’ll also learn about file modes, binary files, and how Python manages buffering when handling large amounts of data. By the end of this chapter, you’ll be equipped to handle text and binary files in Python with confidence.

Commonly Used File Handling Functions :

File Function	Description
open()	Used to create or open a file.
fid.read()	Reads the entire content or specific bytes.
fid.readline()	Reads a file line by line.
fid.write()	Writes data to a file.
fid.writelines()	Writes multiple lines to a file.
fid.seek()	Moves the file object to a specific position.
fid.tell()	Returns the current position of the file object.
fid.readable()	Checks if the file is readable.
fid.writable()	Checks if the file is writable.

Opening a File Using open()

The open() function is used to open a file and returns a file object. This file object can be used to read, write, or manipulate the file using methods like read(), write(), and close().

Syntax:

fid = open("abcd.txt", "r")

The first argument is the file name. The second argument ("r") is the mode, indicating how the file should be opened.

File Modes:

Mode	Description
r	Opens the file for reading. The file must exist.
r+	Opens the file for both reading and writing. The file must exist.
w	Opens the file for writing. If the file doesn’t exist, it will be created.
w+	Opens the file for reading and writing. If the file doesn’t exist, it is created.
a	Opens the file for appending. If the file doesn’t exist, it is created.
a+	Opens the file for both appending and reading
x	Creates a new file for writing. Raises an exception if the file exists.

Reading from a File

We can read the contents of a file in a couple of ways. Make sure the file is present before using the ‘read’ mode. If the file is not present, an ‘OSError’ will be raised.Before trying the following code, create a file named ‘abcd.txt’ with the following lines in the same folder where you are executing your script.

File name : abcd. txt

ethernet0 status: up
ethernet1 status: down
ethernet2 status: up

Method 1. Using read() : The read() function reads the entire content of a file as a string.

fid = open("abcd.txt", "r")
tmp = fid.read()
print(tmp)

#Output:
ethernet0 status: up
ethernet1 status: down
ethernet2 status: up

Explanation:
The read() function reads the entire file and returns it as a string. You can also specify a number of bytes to read.

Method 2: Using readline()

fid = open("abcd.txt", "r")
print(fid.readline())

#Output :
ethernet0 status: up

Explanation: The readline() function reads one line at a time from the file. This is useful when you want to process files line by line.

Method 3: Using readlines()

fid = open("abcd.txt", "r")
print(fid.readlines())

#Output :
['ethernet0 status: up\n', 'ethernet1 status: down\n', 'ethernet2 status: up\n']

Explanation:
The readlines() function reads the entire file and returns it as a list where each element is a line from the file.

Writing to a File Using write()

The write() function is used to write data to a file. If the file is opened in "w" mode, any existing content will be deleted before writing new content.

fid = open("abcd.txt", "w")
fid.write("hello world")
fid.close()

Explanation:
This code opens the file abcd.txt in write mode ("w"), writes the string "hello world", and then closes the file. Writing in "w" mode overwrites the existing content.

Appending to a File Using writelines()

The writelines() function writes a list of lines to a file without adding newlines unless specified in the data.

a = ['ethernet0 status: up\n', 'ethernet1 status: down\n', 'ethernet2 status: up\n']
fid = open("xyz.txt", "w")
fid.writelines(a)
fid.close()

Explanation:
This code writes multiple lines to the file xyz.txt using the writelines() method.

Moving the Cursor with seek() and Getting the Position with tell()

Using seek(): The seek() function moves the cursor to a specified position in the file. The first argument is the number of bytes, and the second argument is the reference position (0 for the start, 1 for the current position, and 2 for the end).

fid.seek(5, 0)  # Moves cursor to 6 bytes from the start
fid.seek(-5, 2) # Moves cursor 5 bytes before the end

Using tell(): The tell() function returns the current position of the cursor.

position = fid.tell()
print(position)

Explanation:
This method is useful for tracking where the cursor is located in the file during reading or writing operations.

Differences Between r+ and w+ Modes

r+ allows both reading and writing without overwriting the content. If you read the file first, the cursor moves to the end, and any writing will append at the end.

w+ allows reading and writing, but it first clears the file content, so there’s nothing to read unless you write something first.

Text Files vs Binary Files

Text Files: Store data in the form of text strings with specific encoding like UTF-8 or ASCII. End of line (EOL) characters vary by platform (\r\n for Windows, \n for Linux).

Binary Files: Store data like images, audio, or executables. No encoding or EOL conversion is performed by Python for binary files. For binary operations, use modes like "rb", "wb", "ab".

Buffering in File Operations

When reading or writing to a file, Python stores data in a buffer to improve performance. The buffer is a temporary storage that holds chunks of data, making operations faster.

import io
print(io.DEFAULT_BUFFER_SIZE)

#Output :
8192

Explanation:
The default buffer size is 8192 bytes. You can control buffering with the buffering parameter in open(). Set buffering=0 to disable buffering, buffering=1 for line buffering, or provide a specific buffer size in bytes.

Some Additional File Attributes:

• fid.encoding: Returns the file encoding (e.g., ‘UTF-8’).
• fid.mode: Shows the mode in which the file is opened (e.g., ‘r’).
• fid.readable(): Checks if the file is readable.
• fid.writable(): Checks if the file is writable.
• fid.seekable(): Checks if the file supports seeking.
• fid.name: Returns the file name.

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Programming Exercises:

1. Write a Python program to count the number of words in a file.
2. Write a Python program to count the number of lines in a file.
3. Write a Python program to copy the contents of one file to another.
4. Write a Python program to print lines 2 to 5 from a file (assuming the file has more than 5 lines).
5. Write a Python program to insert a new line at the beginning of a file.
6. Write a Python program to replace a specific line in a file with another line.
7. Write a Python program to move the contents of a file into an array (list).
8. Write a Python program to print the last two lines of a file.

Router# show ip interface brief
Interface     IP-Address     Status    Protocol
Ethernet0     10.1.1.1       up        up      
Ethernet1     unassigned     down      down    
Loopback0     20.1.1.1       up        up      
Serial0       30.1.1.1       up        up

9. Store the above lines in a file and:
   1. Write a Python program to check if a given IP address is present in the file.
   2. Write a Python program to find the status of a given interface.
   3. Write a Python program to count how many interfaces are “UP.”
   4. Write a Python program to print the names of all interfaces that are “UP.”

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