Reading and Writing Files

Reading Files

Python provides several methods to read data from a file:

1. Reading the Entire File:

• read() method reads the entire contents of the file into a single string.

Python

with open("data.txt", "r") as file:
    content = file.read()
    print(content)

2. Reading Line by Line:

• readline() method reads a single line from the file.
• Iterating over the file object directly reads each line.

Python

with open("data.txt", "r") as file:
    for line in file:
        print(line, end="")  # Remove newline character

3. Reading All Lines into a List:

• readlines() method reads all lines into a list.

Python

with open("data.txt", "r") as file:
    lines = file.readlines()
    for line in lines:
        print(line, end="")

Writing to Files

To write data to a file, open it in write (“w”) or append (“a”) mode.

1. Writing to a File:

• write() method writes a string to the file.

Python

with open("output.txt", "w") as file:
    file.write("This is some text.")

2. Writing Multiple Lines:

• writelines() method writes a list of strings to the file.

Python

with open("output.txt", "w") as file:
    lines = ["Line 1\n", "Line 2\n"]
    file.writelines(lines)

Key Points

• Always close the file after use using close() or preferably with the with statement.
• Use try-except blocks to handle potential errors like FileNotFoundError or PermissionError.
• For large files, consider using iter() and next() for efficient line-by-line processing.
• For binary data, open the file in binary mode (“rb” for reading, “wb” for writing).

Example

Python

def read_file(filename):
  try:
    with open(filename, "r") as file:
      content = file.read()
      return content
  except FileNotFoundError:
    print(f"File '{filename}' not found.")
    return None

def write_to_file(filename, data):
  try:
    with open(filename, "w") as file:
      file.write(data)
  except PermissionError:
    print(f"Permission denied for file '{filename}'.")

# Example usage
data = read_file("data.txt")
if data:
  print(data)

write_to_file("output.txt", "Hello, world!")

By understanding these methods and concepts, you can effectively read and write files in your Python programs.

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