File Handling

When you create a program, you often need to write to or read from files. In this lesson, I will cover some the basic concepts of working with files.

Note: this will NOT work on the online editor. Feel free to read or follow along locally on your computer.

File Basics

In Python, working with files follows a simple pattern:

1. Open the file
2. Perform operations (read or write)
3. Close the file

While you can do this manually, Python provides a better way using the with statement, which automatically closes the file for you when you're done:

# The standard pattern for file operations
with open("<filename>", "<mode>") as file:
    # Perform operations with the file
    # The file automatically closes when the block ends

File Modes

When opening a file, you specify a mode that determines what operations you can perform. Note that in the previous example, <mode> will be replaced by one of the following:

Mode	Description
"r"	Read (default) - open for reading
"w"	Write - open for writing (creates a new file or truncates an existing file)
"a"	Append - open for writing, appending to the end of the file
"r+"	Read/Write - open for both reading and writing
"b"	Binary mode (added to other modes, e.g., "rb", "wb")
"t"	Text mode (default, added to other modes, e.g., "rt", "wt")

Writing to Files

This is how you create a file and write to it:

# Writing a single string to a file
with open("greeting.txt", "w") as file:
    file.write("Hello, World!")

You can also chain multiple write operations in order to append more lines to the file:

# Writing multiple lines
with open("sample.txt", "w") as file:
    # "\n" represents a newline
    # this is equivalent to pressing the Enter key
    file.write("Line 1\n")
    file.write("Line 2\n")
    file.write("Line 3\n")
    
    # You can also write with a list
    lines = ["Line 4\n", "Line 5\n", "Line 6\n"]
    file.writelines(lines)

# read and return the new content
with open("sample.txt", "r") as file:
    content = file.read()
    print(content)

Appending to Files

You can also append content to a file without erasing the original content.

# Creating a sample file
with open("log.txt", "w") as file:
    file.write("Original entry: 2023-01-01\n")

# Appending to the file; the content will remain the same
with open("log.txt", "a") as file:
    file.write("Second entry: 2023-01-02\n")
    file.write("Third entry: 2023-01-03\n")

# Read the file and print its content
with open("log.txt", "r") as file:
    content = file.read()
    print(content)

Reading from Files

Similarly to writing to a file, you can also read from it:

Reading the Entire File

You can chose to read the entire file at once or read it line by line.

with open("story.txt", "w") as file:
    file.write("Once upon a time, there was a Python programmer.\n")
    file.write("They loved working with files.\n")
    file.write("The end.")

# This will read the entire content and return it at once!
with open("story.txt", "r") as file:
    content = file.read()
    print(content)

Reading Line by Line

It may be better to read line by line for larger files.

# Create a file with numbered lines
with open("numbered.txt", "w") as file:
    for i in range(1, 11):
        file.write(f"Line {i}: This is line number {i}\n")

with open("numbered.txt", "r") as file:
    for line in file:
        # strip() removes the trailing newline character
        print(line.strip())

Reading into a List of Lines

# Read all lines into a list
with open("numbered.txt", "r") as file:
    lines = file.readlines()
    
print(f"The file has {len(lines)} lines")
print("First 3 lines:")
for line in lines[:3]:
    print(line.strip())

Reading Specific Amount of Content

You can also chose to read a specific amount of characters to load from a file.

# Create a longer file
with open("long_text.txt", "w") as file:
    for i in range(1, 101):
        file.write(f"This is sentence {i} in the long text file.\n")

# Read first 50 characters
with open("long_text.txt", "r") as file:
    beginning = file.read(50)
    print("First 50 characters:")
    print(beginning)
    
    # Continue reading the next 50 characters
    next_part = file.read(50)
    print("\nNext 50 characters:")
    print(next_part)

File Positions

When reading or writing, Python keeps track of your position in the file. You can control this position using seek() and check it with tell():

# Create a simple file
with open("positions.txt", "w") as file:
    file.write("ABCDEFGHIJKLMNOPQRSTUVWXYZ")

# Demonstrate file positions
with open("positions.txt", "r") as file:
    # Read the first 5 characters
    print(file.read(5))  # ABCDE
    
    # Check our current position
    position = file.tell()
    print(f"Current position: {position}")  # 5
    
    # Read 5 more characters
    print(file.read(5))  # FGHIJ
    
    # Move back to position 0 (beginning of file)
    file.seek(0)
    print(f"After seek(0), next 5 chars: {file.read(5)}")  # ABCDE
    
    # Jump to position 20
    file.seek(20)
    print(f"After seek(20), remaining text: {file.read()}")  # UVWXYZ

Working with File Paths

We have only handled files in the current directory; however, you can also use more complex paths to open files from different directories.

Using the OS Module

The traditional way to work with file paths is to use the os module. You can use os.getcwd() to get the current working directory and os.path.join() to create file paths that work across different operating systems.

import os

current_directory = os.getcwd()
print(f"Current working directory: {current_directory}")

# Join paths to create a file path
data_dir = os.path.join("data", "user_files") # "data/user_files"
user_file = os.path.join(data_dir, "config.txt") # "data/user_files/config.txt"
print(f"File path using os.path: {user_file}")

Using the Pathlib Module

The pathlib module provides a more Pythonic way to work with file paths.

from pathlib import Path

# Define paths using pathlib
data_path = Path("data") / "user_files" # "data/user_files" 
user_file_path = data_path / "config.txt" # "data/user_files/config.txt"
print(f"File path using pathlib: {user_file_path}")

Working with Binary Files

So far, we've worked with text files. For binary files (like images), you need to use binary mode:

# Let's create a simple binary file
with open("binary_data.bin", "wb") as file:  # Note the 'b' in 'wb'
    # Write some bytes
    file.write(b'\x00\x01\x02\x03\x04\xFF\xFE\xFD\xFC\xFB')

# Read from a binary file
with open("binary_data.bin", "rb") as file:  # Note the 'b' in 'rb'
    binary_data = file.read()
    print(f"Type of data: {type(binary_data)}")
    print(f"Bytes read: {binary_data}")
    print(f"Length: {len(binary_data)} bytes")
    
    # Display as hex values
    hex_values = ' '.join(f'{b:02X}' for b in binary_data)
    print(f"Hex values: {hex_values}")