🐍 Python File I/O (Reading & Writing Files) 📝

File Input/Output (I/O) allows your Python programs to interact with external files, reading data from them or writing data to them. This is crucial for persistence, logging, and data processing.

🔹 1. Opening Files (open())

The open() function is used to open a file. It returns a file object.

Syntax:

file_object = open("filename.txt", "mode")

Common modes:
• "r": Read (default) - file must exist.
• "w": Write - creates a new file or overwrites an existing one.
• "a": Append - creates a new file or adds to the end of an existing one.
• "x": Exclusive creation - creates a new file, fails if it exists.

🔹 2. Reading from Files

Once opened in read mode, you can read content:
• .read(): Reads the entire file content as a single string.
• .readline(): Reads one line at a time.
• .readlines(): Reads all lines into a list of strings.

Example:

# Assuming 'data.txt' contains "Line 1\nLine 2\n"
file = open("data.txt", "r")
content = file.read()
print(content)
file.close() # Always close the file!

Output:
Line 1
Line 2

🔹 3. Writing to Files

Once opened in write ("w") or append ("a") mode, you can write content:
• .write(string): Writes a string to the file. Remember to add \n for new lines.

Example:

file = open("output.txt", "w")
file.write("Hello from Python!\n")
file.write("This is a new line.")
file.close()

(Creates/overwrites output.txt with the text.)

🔹 4. The with Statement (Best Practice!)

The with statement ensures files are automatically closed even if errors occur. This is the recommended way.

Syntax:

with open("filename.txt", "mode") as file_object:
    # perform file operations here
# File is automatically closed after this block
Example:python
with open("names.txt", "w") as f:
    f.write("Alice\nBob\n")

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

Output:
Alice
Bob

🔹 5. File Modes (Binary vs. Text)

• Text mode (default): "r", "w", "a" - handles text encoding/decoding.
• Binary mode: "rb", "wb", "ab" - handles raw bytes, useful for images, executables, etc.

Example (Binary Write):

with open("data.bin", "wb") as f:
    f.write(b'\x01\x02\x03') # Writing bytes

🎯 Today's Goal

✔️ Understand how to open files in different modes
✔️ Read and write data to files
✔️ Master the with statement for safe file handling