3.5 File I/O

File I/O is the process of reading data from a file or writing data to a file. This is crucial for handling persistent data, configuration, and data sets.

Opening and closing files

To interact with any file, you must first open it. The best practice is to use the with statement, which automatically handles closing the file, even if errors occur. The open() function requires a file path, and a mode (what you intend to do with the file).

Mode	Purpose
`r`	Read (default). File pointer is at the beginning.
`w`	Write. Overwrites the file if it exists. Creates it if it doesn’t.
`a`	Append. Adds new content to the end of the file. Creates it if it doesn’t.
`b`	Binary mode (used with r, w, or a).
`t`	Text mode (default, used with r, w, or a).

# Open for reading in text mode ('rt' is the default for 'r')
with open("data.txt", 'r') as file:
    content = file.read()
    # The file is automatically closed when the 'with' block ends

Files are fundamentally stored as sequences of bytes, but how those bytes are interpreted defines the format.

In ASCII/Text format, bytes are interpreted as human-readable characters (letters, numbers, symbols) according to an encoding standard (like UTF-8 🔥 or ASCII). This is what happens when using the default text mode (t). This is common for files such as source code, configuration files, text logs, and simple data tables (CSV).

In binary format, bytes are read directly as raw data (e.g., machine code, image pixels, compressed files) without character encoding. This is what happens when using binary mode (b). This is common for images (PNG, JPG), audio files, executables, and serialized Python objects (using the pickle module).

Reading from text files

Text files are the simplest format, typically used for configuration or raw log data. Three commonly used methods are: file.read() which reads the entire file content into a single string, file.readline() which reads just one line from the file, and file.readlines() which reads all lines into a list of strings, where each element is one line.

# Example: Reading line by line (efficient for large files)
with open("data.txt", 'r') as f:
    for line in f:
        print(line.strip()) # the strip() function removes the newline character "\n"

Writing to text files

Use the write() method to place strings into the file. Remember the need for explicit newline characters (\n).

with open("output.txt", 'w') as f:
    f.write("First line of data.\n")
    f.write("Second line of data.")
# "output.txt" now contains the two lines.

Reading from CSV-files

CSV (Comma Separated Values) files are the standard format for exchanging tabular data. Python’s built-in csv module simplifies working with them. The csv.reader object treats each row as a list of strings.

import csv

with open('grades.csv', 'r', newline='') as f:
    reader = csv.reader(f)
    header = next(reader) # Read the header row
    for row in reader:
        # Each 'row' is a list: ['Name', 'Score']
        print(f"Name: {row[0]}, Score: {row[1]}")

Writing to CSV-files

The csv.writer object formats your data (lists or tuples) into proper CSV syntax.

import csv

data_rows = [
    ['Product', 'Price'],
    ['Laptop', 1200],
    ['Monitor', 300]
]

with open('products.csv', 'w', newline='') as f:
    writer = csv.writer(f)
    writer.writerows(data_rows) # Writes all rows at once

Note: The newline='' argument in open() prevents extra blank rows in the output CSV.