File I/O (Input/Output) in Python

File I/O is a fundamental operation for working with data.
Python makes it easy to read from and write to text files.

The built-in open() function is used to open a file, and you can then use file object methods to read or write data.
When done, the file should be closed to free resources—best handled using a with statement for automatic cleanup.

Opening a File

To open a file, use:

open(filename, mode)

Mode options:

• 'r' → Reading (default mode, if not specified)

• 'w' → Writing (creates a new file or truncates an existing one)

• 'a' → Appending (adds to end of file without truncating)

• 'x' → Exclusive creation (fails if file exists — less common in basics)

Example:

file = open("notes.txt", 'r')  # open for reading
# ... use file.read() or other methods ...
file.close()

Using with for Automatic Closing

with open("notes.txt", 'r') as f:
    content = f.read()
    # file is automatically closed after the indented block

• Ensures proper closing even if an error occurs.

• Recommended pattern in most cases.

Reading from a Text File

Python file objects provide multiple methods:

• f.read() → Entire file as a single string.

• f.readline() → One line (up to n).

• f.readlines() → All lines in a list of strings.

You can also iterate over the file object directly:

Example: Read Entire File

with open("example.txt", 'r') as f:
    data = f.read()
print(data)

Example: Read Line by Line

with open("example.txt", 'r') as f:
    for line in f:
        print(line.strip())  # removes newline

Example: Using readline() in a Loop

with open("example.txt", 'r') as f:
    while True:
        line = f.readline()
        if not line:
            break
        print(line.strip())

Writing to a Text File

Modes for writing:

• 'w' → Overwrite or create new file

• 'a' → Append to file

Example: Write New File

new_content = "Hello, world!nThis is a new file."
with open("output.txt", 'w') as f:
    f.write(new_content)

Example: Write Multiple Lines

with open("numbers.txt", 'w') as f:
    for i in range(5):
        f.write(f"Line {i}n")

Example: Append to File

with open("output.txt", 'a') as f:
    f.write("Appending a new line.n")

Flushing & Closing

• Exiting a with block automatically flushes and closes the file.

• If not using with (not recommended), call:

  f.flush()
  f.close()
  

Encoding Note

• Default encoding is UTF-8 (works for most text).

• To use another encoding:

  open("file.txt", 'r', encoding='utf-16')