BCA • Chapter 7

File Handling

Chapter 7 explores file I/O operations in Python, covering the different file modes, robust handling using the 'with' statement, reading/writing techniques, and cursor manipulation.

17 note blocks4 exam topics

🎯 Exam Focus Areas

Understand the destructive nature of the 'w' mode compared to 'a'.Explain why the `with` statement is vastly superior to `file = open(...)`.Identify the differences between `read()`, `readline()`, and `readlines()`.Understand how to use `seek()` to reset a file pointer.

Programs naturally lose their data when they terminate because variables are stored in Random Access Memory (RAM), which is volatile. To persist data permanently, it must be saved to a non-volatile storage medium, typically a hard drive, in the form of a file. File Handling in Python allows your scripts to create, read, update, and delete files. This is essential for logging, saving game states, reading configuration files, and processing large datasets.

7.1 Opening and Closing Files

Python uses the built-in `open()` function to interact with files. It requires two primary arguments: the file path and the mode. It returns a file object. It is critically important to close the file using `.close()` when you are done to free up system resources and ensure all data is written to the disk.

# Syntax: file_object = open('filename.txt', 'mode')

try:
    # Opening a file for writing
    file = open('data.txt', 'w')
    file.write("Hello, File!")
finally:
    # Always close the file, even if an error occurs above
    file.close()

7.2 File Modes

The mode dictates what you are allowed to do with the file and how the file is prepared when opened.

1'r' (Read): Default mode. Opens a file for reading. Raises an error if the file does not exist.
2'w' (Write): Opens a file for writing. If the file exists, it OVERWRITES it completely. If it doesn't exist, it creates a new file.
3'a' (Append): Opens a file for appending data to the end. Creates a new file if it doesn't exist.
4'x' (Exclusive Creation): Creates a new file. Fails if the file already exists.
5'r+' (Read and Write): Opens a file for both reading and writing. The file pointer is placed at the beginning.
6'b' (Binary): Appended to modes (like 'rb' or 'wb') to handle non-text files like images, audio, or compiled data.

7.3 The 'with' Statement (Context Managers)

Manually closing files using `.close()` is error-prone. If your program crashes before it reaches the close statement, the file remains open. Python provides the `with` statement, which automatically closes the file the moment the block of code exits, even if an exception is raised. This is the universally recommended way to handle files.

# Writing using 'with'. The file is automatically closed afterwards.
with open('log.txt', 'w') as file:
    file.write("System started.\n")
    file.write("Loading modules...\n")

# Appending to the same file
with open('log.txt', 'a') as file:
    file.write("System successfully booted.\n")

7.4 Reading Files

Python offers multiple ways to read data from a file, depending on how large the file is and how you want to process it.

1read(): Reads the entire file into a single massive string. Use with caution on large files to avoid running out of RAM.
2readline(): Reads the file line-by-line, moving the cursor forward after each call.
3readlines(): Reads all lines and returns them as a List of strings.

with open('log.txt', 'r') as file:
    # Reading the whole file
    content = file.read()
    print("--- Full Content ---")
    print(content)

with open('log.txt', 'r') as file:
    print("--- Reading Line by Line ---")
    # Iterating over the file object directly is highly memory efficient
    for line in file:
        # using end='' because the line already contains a \n character
        print(line, end='')

7.5 File Cursor Manipulation: seek() and tell()

When you read or write to a file, an internal 'cursor' (or file pointer) moves forward. `tell()` returns the current byte position of the cursor, and `seek(offset)` moves the cursor to a specific byte.

with open('log.txt', 'r') as file:
    print("Cursor at:", file.tell()) # Output: 0
    first_line = file.readline()
    print("Cursor now at:", file.tell())
    
    # Move cursor back to the absolute beginning
    file.seek(0)
    # Now reading again will start from the top
    data = file.read(5) # Read exactly 5 bytes/characters
    print(data)

📝 Quick Revision Points

1'w' overwrites, 'a' appends.
2`with open('file.txt', 'r') as f:` automatically handles closing.
3`readlines()` returns a list of strings.
4`f.tell()` gives cursor position; `f.seek(0)` rewinds the file.

← PreviousLists, Tuples, Sets, Dictionaries Next →Exception Handling

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# Syntax: file_object = open('filename.txt', 'mode') try: # Opening a file for writing file = open('data.txt', 'w') file.write("Hello, File!") finally: # Always close the file, even if an error occurs above file.close()

# Writing using 'with'. The file is automatically closed afterwards. with open('log.txt', 'w') as file: file.write("System started.\n") file.write("Loading modules...\n") # Appending to the same file with open('log.txt', 'a') as file: file.write("System successfully booted.\n")

with open('log.txt', 'r') as file: # Reading the whole file content = file.read() print("--- Full Content ---") print(content) with open('log.txt', 'r') as file: print("--- Reading Line by Line ---") # Iterating over the file object directly is highly memory efficient for line in file: # using end='' because the line already contains a \n character print(line, end='')

with open('log.txt', 'r') as file: print("Cursor at:", file.tell()) # Output: 0 first_line = file.readline() print("Cursor now at:", file.tell()) # Move cursor back to the absolute beginning file.seek(0) # Now reading again will start from the top data = file.read(5) # Read exactly 5 bytes/characters print(data)