How to read data from text file in Python using File Handling

How to read data from text file in Python using File Handling?

To read data from a text file in Python, you can use several methods provided by Python's file handling features. Here are the most commonly used methods:

1. read()
2. readline()
3. readlines()

Reading the Entire File with read()

The read() method reads the entire content of a file at once. This is useful when you want to read everything in the file as a single string.

Syntax:

file = open("filename.txt", "r")
content = file.read()
file.close()

Example:

file = open("example.txt", "r")  # Open the file in read mode
content = file.read()  # Read the entire file content
file.close()  # Close the file
print(content)

Output:

This is the first line.
This is the second line.

 

Reading One Line at a Time with readline()

The readline() method reads a single line from the file each time it's called. This is useful for reading large files line-by-line to avoid loading the entire file into memory.

Syntax:

file = open("filename.txt", "r")
line = file.readline()
file.close()

Example:

file = open("example.txt", "r")  # Open the file in read mode
line = file.readline()  # Read the first line
print(line)  # Print the first line

line = file.readline()  # Read the second line
print(line)  # Print the second line
file.close()  # Close the file

Output:

This is the first line.
This is the second line.

 

Reading All Lines as a List with readlines()

The readlines() method reads all lines in the file and returns them as a list, where each line is an item in the list.

This is helpful when you want to access lines individually and apply list operations.

Syntax:

file = open("filename.txt", "r")
lines = file.readlines()
file.close()

Example:

file = open("example.txt", "r")  # Open the file in read mode
lines = file.readlines()  # Read all lines as a list
file.close()  # Close the file

for line in lines:
    print(line.strip())  # Print each line

Output:

This is the first line.
This is the second line.

 

Summary of Reading Methods

• read(): Reads the entire file as a single string.
• readline(): Reads one line at a time.
• readlines(): Reads all lines and returns a list, where each item is a line in the file.

 

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Interview questions related to reading files in Python using file handling

Question 1. What is the purpose of using the read() method in Python file handling?

Answer: The read() method reads the entire content of a file at once and returns it as a single string. This method is useful when you need to process or display all of the file's content in one go.

 Example:

file = open("example.txt", "r")
content = file.read()
file.close()
print(content)

Output:

This is the first line.
This is the second line.

 

Question 2. How does the readline() method differ from read() in Python file handling?

Answer: readline() reads only one line from the file at a time, which is helpful when working with large files to avoid loading the entire content into memory at once.

 Example:

file = open("example.txt", "r")
print(file.readline())  # Reads the first line
print(file.readline())  # Reads the second line
file.close()

Output:

This is the first line.
This is the second line.

 

Question 3. What is the difference between readline() and readlines() methods?

Answer: readline() reads a single line from the file each time it is called, while readlines() reads all lines in the file and returns them as a list, where each line is an item in the list.

Example of readline():

file = open("example.txt", "r")
print(file.readline())  # First line
print(file.readline())  # Second line
file.close()

 Example of readlines():

file = open("example.txt", "r")
lines = file.readlines()  # Reads all lines as a list
file.close()
print(lines)

Output:

['This is the first line.\n', 'This is the second line.\n']

 

Question 4. Why is it important to close a file after reading it in Python?

Answer: Closing a file releases the resources associated with the file and ensures that any changes are saved. Failing to close a file can lead to memory leaks or file corruption.

 Example:

file = open("example.txt", "r")
content = file.read()
file.close()

 

Question 5. What happens if you try to read a file that does not exist?

Answer: Python raises a FileNotFoundError if the specified file does not exist. Exception handling.

 Example:

try:
    file = open("nonexistent.txt", "r")
    content = file.read()
    file.close()
except FileNotFoundError:
    print("File not found.")

Output:

File not found.

 

Question 6. How can you read the first 10 characters from a file?

Answer: You can pass an integer as an argument to read() to specify the number of characters to read.

 Example:

file = open("example.txt", "r")
content = file.read(10)  # Reads the first 10 characters
file.close()
print(content)

Output:

This is th

 

Question 7. Can you explain the difference between read() and readlines() in terms of output?

Answer: read() returns the entire content as a single string, while readlines() returns a list where each line is a separate item.

 Example:

file = open("example.txt", "r")
print(file.read())       # Single string
file.close()

file = open("example.txt", "r")
print(file.readlines())  # List of strings
file.close()

Output:

Single string output:
This is the first line.
This is the second line.

List output:
['This is the first line.\n', 'This is the second line.\n']

 

Question 8. What is the purpose of the strip() method when printing lines read by readlines()?

Answer: The strip() method removes any trailing newline characters or whitespace from each line, providing a cleaner output.

 Example:

file = open("example.txt", "r")
lines = file.readlines()
file.close()

for line in lines:
    print(line.strip())

Output:

This is the first line.
This is the second line.

 

Question 9. Is it possible to use a for loop to iterate over lines in a file? How?

Answer: Yes, you can iterate over the file object directly with a for loop, which reads each line one by one.

 Example:

file = open("example.txt", "r")
for line in file:
    print(line.strip())
file.close()

Output:

This is the first line.
This is the second line.

 

Question 10. How can you check if a file is open or closed?

Answer: You can check the closed attribute of the file object. It returns True if the file is closed, and False if it is open.

 Example:

file = open("example.txt", "r")
print(file.closed)  # False, because file is open
file.close()
print(file.closed)  # True, because file is closed

Output:

False
True