File I/O & Reading Data

What is File I/O?

In computing, Input/Output (I/O) refers to all communication between a running program and the outside world — files on disk, network sockets, databases, or user input. File I/O specifically means reading data stored in files or writing results back to disk. Files are persistent: unlike variables in memory that disappear when the program ends, file data survives across runs.

As a data practitioner, virtually every real dataset you work with lives in a file — CSV, JSON, Excel, Parquet, or plain text. You must know how to read and write them reliably.

Reading Text Files

with open("data.txt", "r") as f:
    content = f.read()  # entire file as string
    lines = f.readlines()  # list of lines

The with statement: Automatically closes the file, even if an error occurs. Always use it!

Writing to Files

with open("output.txt", "w") as f:
    f.write("Hello, Data Science!\\n")
    f.writelines(["line1", "line2"])  # write multiple

Line-by-Line Processing (Memory Efficient)

with open("large_dataset.txt", "r") as f:
    for line in f:  # doesn't load entire file
        process(line.strip())