File Handling and Exceptions

Real programs need to interact with the outside world — reading configuration files, processing data, saving results, and handling errors gracefully. In this lesson, you will learn how to read and write files and how to handle exceptions so your programs do not crash unexpectedly.

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Reading Files

The open() Function

# Basic file reading
file = open("data.txt", "r")  # "r" = read mode
content = file.read()          # read entire file as a string
print(content)
file.close()                   # always close the file!

The with Statement (Recommended)

The with statement automatically closes the file, even if an error occurs:

# Much better — file is closed automatically
with open("data.txt", "r") as file:
    content = file.read()
    print(content)
# file is automatically closed here

Always use with for file operations.

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Reading Methods

# read() — read the entire file as one string
with open("data.txt", "r") as f:
    content = f.read()
    print(content)

# readline() — read one line at a time
with open("data.txt", "r") as f:
    first_line = f.readline()
    second_line = f.readline()
    print(first_line.strip())  # strip() removes the newline character

# readlines() — read all lines into a list
with open("data.txt", "r") as f:
    lines = f.readlines()
    print(lines)  # ['line 1\n', 'line 2\n', 'line 3\n']

# Iterate line by line (most memory-efficient)
with open("data.txt", "r") as f:
    for line in f:
        print(line.strip())

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Writing Files

# "w" — write mode (creates file or OVERWRITES existing content)
with open("output.txt", "w") as f:
    f.write("Hello, World!\n")
    f.write("This is line 2.\n")

# "a" — append mode (adds to the end of the file)
with open("output.txt", "a") as f:
    f.write("This line is appended.\n")

# writelines() — write a list of strings
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "w") as f:
    f.writelines(lines)

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File Modes

Mode	Description
"r"	Read (default) — file must exist
"w"	Write — creates or overwrites
"a"	Append — creates or adds to end
"x"	Exclusive create — fails if file exists
"r+"	Read and write
"b"	Binary mode (e.g., "rb", "wb")

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Working with CSV Files

import csv

# Writing CSV
with open("students.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerow(["Name", "Age", "Grade"])  # header
    writer.writerow(["Alice", 20, "A"])
    writer.writerow(["Bob", 22, "B"])
    writer.writerow(["Charlie", 21, "A"])

# Reading CSV
with open("students.csv", "r") as f:
    reader = csv.reader(f)
    header = next(reader)  # skip header row
    for row in reader:
        name, age, grade = row
        print(f"{name} (age {age}): grade {grade}")

# Using DictReader for named access
with open("students.csv", "r") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(f"{row['Name']} got {row['Grade']}")

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Working with JSON Files

import json

# Writing JSON
data = {
    "name": "Alice",
    "age": 20,
    "courses": ["Maths", "Physics"],
    "graduated": False
}

with open("student.json", "w") as f:
    json.dump(data, f, indent=2)  # indent for pretty printing

# Reading JSON
with open("student.json", "r") as f:
    loaded = json.load(f)
    print(loaded["name"])     # Alice
    print(loaded["courses"])  # ['Maths', 'Physics']

# JSON strings (no file)
json_string = json.dumps(data, indent=2)
print(json_string)

parsed = json.loads(json_string)
print(parsed["age"])  # 20

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File System Operations

import os
from pathlib import Path

# Check if a file exists
print(os.path.exists("data.txt"))     # True/False
print(Path("data.txt").exists())      # True/False (pathlib)

# Get file size
print(os.path.getsize("data.txt"))    # size in bytes