🚀 *Gen AI Engineer (Production Focused)* File Handling in Python
File handling is a crucial aspect of programming that allows you to read from and write to files. Python provides built-in functions and methods to work with files efficiently.
Basic File Operations
1. Opening a File
Use the open() function to open a file. It returns a file object.
python
# Syntax: open(filename, mode)
file = open("example.txt", "r") # Open for reading
2. File Modes
'r'– Read (default)'w'– Write (creates new file or truncates existing)'a'– Append'x'– Exclusive creation (fails if file exists)'b'– Binary mode't'– Text mode (default)'+'– Read and write
3. Reading Files
python
# Read entire file
with open("example.txt", "r") as file:
content = file.read()
print(content)
# Read line by line
with open("example.txt", "r") as file:
for line in file:
print(line.strip()) # strip() removes newline characters
# Read specific number of characters
with open("example.txt", "r") as file:
chunk = file.read(100) # Read first 100 characters
# Read all lines into a list
with open("example.txt", "r") as file:
lines = file.readlines()
4. Writing to Files
python
# Write to file (overwrites existing content)
with open("example.txt", "w") as file:
file.write("Hello, World!\n")
file.write("This is a new line.\n")
# Append to file
with open("example.txt", "a") as file:
file.write("This line is appended.\n")
5. The with Statement
The with statement automatically handles file closing, even if exceptions occur.
python
# Recommended way
with open("example.txt", "r") as file:
content = file.read()
# File is automatically closed here
# Traditional way (not recommended)
file = open("example.txt", "r")
try:
content = file.read()
finally:
file.close()
Practical Examples
Example 1: Reading and Processing Data
python
# Count lines in a file
with open("data.txt", "r") as file:
line_count = 0
for line in file:
line_count += 1
print(f"Total lines: {line_count}")
# Process CSV-like data
with open("data.csv", "r") as file:
headers = file.readline().strip().split(',')
for line in file:
values = line.strip().split(',')
# Process each row
print(dict(zip(headers, values)))
Example 2: Writing Structured Data
python
# Write multiple lines
data = ["Line 1", "Line 2", "Line 3"]
with open("output.txt", "w") as file:
for line in data:
file.write(line + "\n")
# Write formatted data
users = [
{"name": "Alice", "age": 25},
{"name": "Bob", "age": 30}
]
with open("users.txt", "w") as file:
for user in users:
file.write(f"{user['name']},{user['age']}\n")
Example 3: File Copy
python
def copy_file(source, destination):
try:
with open(source, "rb") as src, open(destination, "wb") as dest:
dest.write(src.read())
print("File copied successfully!")
except FileNotFoundError:
print("Source file not found!")
except Exception as e:
print(f"Error: {e}")
copy_file("source.txt", "copy.txt")
Error Handling
python
try:
with open("nonexistent.txt", "r") as file:
content = file.read()
except FileNotFoundError:
print("File not found!")
except PermissionError:
print("Permission denied!")
except Exception as e:
print(f"An error occurred: {e}")
Working with Binary Files
python
# Read binary file (like images)
with open("image.jpg", "rb") as file:
binary_data = file.read()
# Write binary data
with open("copy.jpg", "wb") as file:
file.write(binary_data)
File Position and Seeking
python
with open("example.txt", "r+") as file:
# Get current position
position = file.tell()
print(f"Current position: {position}")
# Read first 10 characters
content = file.read(10)
# Move to beginning
file.seek(0)
# Write at beginning
file.write("START")
Best Practices
- Always use the
withstatement for automatic resource management - Handle exceptions when working with files
- Use appropriate modes for your operations
- Close files explicitly if not using
with - Be careful with
'w'mode as it overwrites existing content - Use binary mode for non-text files
File handling in Python is straightforward and powerful, making it easy to work with various file formats and perform complex file operations.
Python open() Method and File Modes
The open() Method
The open() function is used to open files and returns a file object.
Syntax:
python
open(filename, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
File Modes with Examples
1. Read Mode ('r')
Opens a file for reading (text mode).
Example 1: Basic Reading
python
# Create a sample file first
with open("sample.txt", "w") as f:
f.write("Hello World!\nPython File Handling")
# Read the file
with open("sample.txt", "r") as file:
content = file.read()
print("File content:")
print(content)
Example 2: Reading Line by Line
python
with open("sample.txt", "r") as file:
print("Reading line by line:")
for i, line in enumerate(file, 1):
print(f"Line {i}: {line.strip()}")
2. Write Mode ('w')
Creates a new file or truncates existing file for writing.
Example 1: Creating New File
python
# Write to a new file (creates if doesn't exist)
with open("new_file.txt", "w") as file:
file.write("This is line 1\n")
file.write("This is line 2\n")
print("File created and written successfully!")
# Verify content
with open("new_file.txt", "r") as file:
print("Content:", file.read())
Example 2: Overwriting Existing File
python
# Original content
with open("data.txt", "w") as file:
file.write("Original content")
# Overwrite the file
with open("data.txt", "w") as file:
file.write("New content - old content is gone!")
# Check result
with open("data.txt", "r") as file:
print("After overwrite:", file.read())
3. Append Mode ('a')
Opens file for appending, creates if doesn’t exist.
Example 1: Appending to File
python
# Initial content
with open("log.txt", "w") as file:
file.write("Log started\n")
# Append new entries
with open("log.txt", "a") as file:
file.write("Entry 1: User logged in\n")
file.write("Entry 2: File uploaded\n")
# Check final content
with open("log.txt", "r") as file:
print("Log content:")
print(file.read())
Example 2: Appending Multiple Times
python
# Append data at different times
events = ["Event 1: Started", "Event 2: Processing", "Event 3: Completed"]
for event in events:
with open("events.log", "a") as file:
file.write(f"{event}\n")
# Read all events
with open("events.log", "r") as file:
print("All events:")
print(file.read())
4. Exclusive Creation Mode ('x')
Creates file exclusively, fails if file exists.
Example 1: Successful Creation
python
try:
with open("unique_file.txt", "x") as file:
file.write("This file was created exclusively!")
print("File created successfully with 'x' mode")
except FileExistsError:
print("File already exists!")
Example 2: Handling Existing File
python
# First create a file
with open("test_x.txt", "w") as file:
file.write("Existing content")
# Try to create with 'x' mode
try:
with open("test_x.txt", "x") as file:
file.write("This won't work")
except FileExistsError as e:
print(f"Error: {e}")
print("File already exists, cannot create exclusively")
5. Binary Mode ('b')
Used for non-text files (images, executables, etc.).
Example 1: Reading Binary File
python
# First create a simple binary file
with open("binary_data.bin", "wb") as file:
file.write(b'\x48\x65\x6C\x6C\x6F') # Hello in bytes
# Read binary file
with open("binary_data.bin", "rb") as file:
binary_content = file.read()
print("Binary content:", binary_content)
print("As string:", binary_content.decode('utf-8'))
Example 2: Copying Binary File
python
# Create source binary file
with open("source.bin", "wb") as file:
file.write(bytes(range(10))) # Write bytes 0-9
# Copy binary file
with open("source.bin", "rb") as src, open("copy.bin", "wb") as dest:
dest.write(src.read())
print("Binary file copied successfully!")
6. Read/Write Mode ('+')
Opens file for both reading and writing.
Example 1: Read and Write ('r+')
python
# Create file with initial content
with open("data_rw.txt", "w") as file:
file.write("Line 1\nLine 2\nLine 3")
# Read and modify
with open("data_rw.txt", "r+") as file:
content = file.read()
print("Original:", repr(content))
file.seek(0) # Go to beginning
file.write("Modified Line 1\n")
file.seek(0)
print("After modification:", file.read())
Example 2: Write and Read ('w+')
python
with open("data_wr.txt", "w+") as file:
# Write first
file.write("Initial content\n")
file.write("Second line\n")
# Then read
file.seek(0)
content = file.read()
print("Content after write+read:")
print(content)
7. Append and Read ('a+')
Opens file for appending and reading.
Example 1: Append and Read
python
with open("data_ar.txt", "a+") as file:
# Append new content
file.write("New appended line\n")
# Read what was written
file.seek(0)
content = file.read()
print("All content after append:")
print(content)
Example 2: Multiple Operations
python
with open("log_ar.txt", "a+") as file:
# Read existing content first
file.seek(0)
old_content = file.read()
print("Existing content:", repr(old_content))
# Append new content
file.write(f"New entry at position {file.tell()}\n")
# Read everything again
file.seek(0)
print("Final content:")
print(file.read())
Mode Combinations Table
| Mode | Description | File Position |
|---|---|---|
'r' | Read (default) | Beginning |
'r+' | Read + Write | Beginning |
'w' | Write (truncate) | Beginning |
'w+' | Write + Read (truncate) | Beginning |
'a' | Append | End |
'a+' | Append + Read | End |
'x' | Exclusive creation | Beginning |
'x+' | Exclusive creation + Read/Write | Beginning |
'rb' | Read binary | Beginning |
'wb' | Write binary | Beginning |
Important Notes
- Always use
withstatement for automatic file closing - Text mode (
't') is default – handles line endings automatically - Binary mode (
'b') preserves exact byte content - File position matters when using read/write modes together
- Use
seek()to navigate within the file - Handle exceptions for file operations
These examples demonstrate the power and flexibility of Python’s file handling capabilities!
