Table of Contents

• File Modes
• Opening a File
• Reading a File
  • Reading the Entire File
  • Reading Line by Line
  • Reading a Specific Number of Characters
  • Reading as a List of Lines
• Writing to a File
  • Writing a New File
  • Appending to an Existing File
• Working with File Modes
  • "r+" Mode (Read and Write)
  • "w+" Mode (Write and Read)
  • "a+" Mode (Append and Read)
• Handling File Exceptions
  • File Not Found Errors
  • Permission Errors
  • Using finally to Ensure File Closure

File Modes

Mode Identifier	Mode Description
"r"	Read mode (default, file must exist).
"w"	Write mode (creates a new file or overwrites an existing file).
"a"	Append mode (adds data to an existing file).
"x"	Exclusive creation mode (fails if the files exists).
"r+"	Read and write mode (file must exist).
"w+"	Write and read mode (overwrites existing content).
"a+"	Append and read mode (adds new content while preserving existing content).

Opening a File

Use the open() function to open a file. The syntax is:

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

NOTE: Replace "mode" with a file mode (ex: "w").

Reading a File

Reading the Entire File

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

# Output:
Hello, this is an example file.

Reading Line by Line

This is especially useful for large files.

with open("example.txt", "r") as file:
    for line in file:
        print(line.strip())  # Removes extra newlines

# Output:
Hello, this is an example file.
This is the second line.

Reading a Specific Number of Characters

Reads only the first n characters from the file.

with open("example.txt", "r") as file:
    content = file.read(10)  # Reads first 10 characters
    print(content)

# Output:
Hello, thi

Reading as a List of Lines

Reads all lines and stores them in a list.

with open("example.txt", "r") as file:
    lines = file.readlines()
    print(lines)

# Output:
['Hello, this is an example file.\n', 'This is the second line.\n']

Writing to a File

Writing a New File

Creates a new file or overwrites an existing file.

with open("output.txt", "w") as file:
    file.write("Hello, this is a new file.\n")
    file.write("This file will be overwritten if it exists.")

# output.txt content:
Hello, this is a new file.
This file will be overwritten if it exists.

Appending to an Existing File

Adds data to an existing file without overwriting.

with open("output.txt", "a") as file:
    file.write("\nThis is an additional line.")

# output.txt content after running:
Hello, this is a new file.
This file will be overwritten if it exists.
This is an additional line.

Working with File Modes

"r+" Mode (Read and Write)

Reads and writes to an existing file (file must exist).

with open("example.txt", "r+") as file:
    content = file.read()
    file.write("\nAppending new content while reading.")

# example.txt content after running:
Hello, this is an example file.
This is the second line.
Appending new content while reading.

"w+" Mode (Write and Read)

Creates a new file or overwrites existing content.

with open("newfile.txt", "w+") as file:
    file.write("This file is new.")
    file.seek(0)  # Move cursor back to start
    print(file.read())

# Output:
This file is new.

"a+" Mode (Append and Read)

Appends data while preserving existing content.

with open("example.txt", "a+") as file:
    file.write("\nAdding a new line at the end.")
    file.seek(0)  # Move cursor back to start
    print(file.read())

# Output (example.txt content after running):
Hello, this is an example file.
This is the second line.
Adding a new line at the end.

Handling File Exceptions

File Not Found Errors

Handles cases where a file does not exist.

try:
    with open("missing_file.txt", "r") as file:
        content = file.read()
except FileNotFoundError:
    print("Error: File not found.")

# Output:
Error: File not found.

Permission Errors

Handles permission issues when trying to access restricted files.

try:
    with open("/root/protected.txt", "w") as file:
        file.write("Trying to write to a protected file.")
except PermissionError:
    print("Error: You do not have permission to write to this file.")

# Output:
Error: You do not have permission to write to this file.

Using finally to Ensure File Closure

Ensures that a file is properly closed even if an error occurs.

try:
    file = open("example.txt", "r")
    content = file.read()
except FileNotFoundError:
    print("File not found.")
finally:
    file.close()  # Ensures file is closed