Reading and Writing Files

Introduction

Working with files is a core skill for data roles. Python's built-in open() function handles reading from and writing to text files.

Opening a File

open(filepath, mode)

Common modes:

Mode	Meaning
"r"	Read (default) — file must exist
"w"	Write — creates or overwrites the file
"a"	Append — adds to the end without overwriting
"r+"	Read and write

The with Statement

Always use with when working with files. It automatically closes the file when the block exits, even if an error occurs.

with open("employees.txt", "r") as f:
    content = f.read()

print(content)

Reading Files

# Read the entire file as a single string
with open("employees.txt", "r") as f:
    content = f.read()

# Read one line at a time
with open("employees.txt", "r") as f:
    for line in f:
        print(line.strip())   # strip() removes the trailing newline

# Read all lines into a list
with open("employees.txt", "r") as f:
    lines = f.readlines()

Writing Files

# Write — creates or overwrites
with open("output.txt", "w") as f:
    f.write("Alice,Engineering,80000\n")
    f.write("Bob,Finance,65000\n")

# Append — adds to existing content
with open("output.txt", "a") as f:
    f.write("Charlie,HR,55000\n")

Writing Multiple Lines

employees = ["Alice", "Bob", "Charlie"]

with open("names.txt", "w") as f:
    for name in employees:
        f.write(name + "\n")

File Paths

# Relative path (relative to where you run the script from)
with open("data/employees.txt", "r") as f:
    ...

# Use pathlib for safer cross-platform paths (covered in Grade 5 modules section)
from pathlib import Path
path = Path("data") / "employees.txt"
with open(path, "r") as f:
    ...

Handling Missing Files

Trying to open a file that does not exist in "r" mode raises a FileNotFoundError. Error handling is covered in Grade 6 — for now, make sure the file exists before reading.

Practice Exercises

• Create a file called products.txt and write five product names to it, one per line.
• Read the file back and print each line without the trailing newline character.
• Append two more product names to the file and confirm they appear at the end.
• Read the file into a list and print only the lines that contain the letter "a".