PART 3 • LEVEL 10

📁 Modules, Exceptions & Files

Reuse trusted modules, protect programs from runtime failures and store structured information in text, CSV and JSON files.

⏱ 110–125 minutes📚 8 concepts💻 5 programs🧠 6-question quiz

Start Lesson ↓Jump to Problems

read_report.pyPython 3

try:
    with open("report.txt", "r") as file:
        text = file.read()
        print(text)
except FileNotFoundError:
    print("Report not found")

KEY IDEAOpen safely • close automatically

LEARNING OUTCOMES

By the End of This Level

You will be able to connect programs with reusable libraries and persistent data.

Import modules and call names through a clear namespace.

Handle expected runtime errors with focused exceptions.

Read and write files safely using a context manager.

Process common CSV and JSON data formats.

REUSE EXISTING CODE

Module Fundamentals

A module is a Python file that groups related variables, functions and classes for reuse.

import loads a module and gives your program access to its public names. Keeping the module name, as in math.sqrt(), makes the source of each tool visible.

Namespace

A mapping of names to objects. Module namespaces prevent unrelated files from accidentally using the same name.

1ImportLocate the module

→

2LoadExecute it once

→

3AccessUse module.name

→

4ReuseCall trusted logic

EXAMPLE 1

Use the Math Module

import math

radius = 4
area = math.pi * radius ** 2
print(round(area, 2))

Output

50.27

STANDARD LIBRARY

Import Styles and Useful Modules

Choose an import style that keeps names readable and avoids hidden collisions.

Style	Example	Best use
Import module	`import math`	Clear source: `math.sqrt(81)`
Import selected name	`from math import sqrt`	One frequently used name
Import with alias	`import statistics as stats`	Short conventional name
Avoid wildcard	`from math import *`	Usually unclear and collision-prone

📐

math

Numeric constants and functions.

🎲

random

Pseudo-random choices and numbers.

📅

datetime

Dates, times and durations.

📊

statistics

Mean, median and related measures.

RECOVER SAFELY

Exception Handling

An exception is an object that reports a runtime problem and interrupts normal execution unless it is handled.

1tryRun risky code

→

2exceptHandle a match

→

3elseRun if successful

→

4finallyAlways clean up

EXAMPLE 2

Safe Number Conversion

try:
    age = int(input("Age: "))
except ValueError:
    print("Enter a whole number")
else:
    print("Next year:", age + 1)
finally:
    print("Validation complete")

Possible output

Next year: 19
Validation complete

Catch narrowly

Handle the most specific expected exception. A broad except: can hide programming mistakes that should be fixed.

IDENTIFY THE FAILURE

Common Exceptions and Raising Errors

Different exception types communicate different causes and support precise recovery.

Exception	Typical cause	Example
`ValueError`	Right type of operation, invalid value	`int("ten")`
`ZeroDivisionError`	Division by zero	`8 / 0`
`FileNotFoundError`	Missing file in read mode	`open("missing.txt")`
`KeyError`	Missing dictionary key	`profile["age"]`
`TypeError`	Unsupported types or call shape	`"4" + 2`

VALIDATE

Reject Invalid Data

Use raise when a function cannot accept the supplied value.

if marks < 0:
    raise ValueError("marks cannot be negative")

MESSAGE

Explain the Cause

A useful error message states what was invalid and what the caller should supply.

raise ValueError("marks must be 0 to 100")

PERSISTENT DATA

File Paths and Modes

A file keeps data after the program finishes. A path identifies the file; a mode controls the permitted operation.

Mode	Meaning	If the file exists	If missing
`r`	Read text	Open from the beginning	Error
`w`	Write text	Replace existing content	Create
`a`	Append text	Add at the end	Create
`x`	Create exclusively	Error	Create
`b`	Binary modifier	Combine with another mode, such as `rb`

Path awareness

A relative path starts from the program’s current working directory. An absolute path begins from the filesystem root.

OPEN • USE • CLOSE

Reading and Writing Text Files

Use with open(...) so Python closes the file automatically, including when an exception occurs.

READ

Choose the needed method

read() returns all remaining text, readline() returns one line and iteration processes one line at a time.

with open("notes.txt", "r") as file:
    for line in file:
        print(line.strip())

WRITE

Write explicit text

write() does not add a newline automatically. Include \n when separate lines are required.

with open("result.txt", "w") as file:
    file.write("Passed\n")

Encoding

For predictable text handling across systems, production programs commonly pass encoding="utf-8".

TABULAR DATA

Working with CSV

CSV stores records as rows and fields. Python’s csv module handles quoting and delimiters safely.

EXAMPLE 3

Read Student Marks

import csv

with open("marks.csv", "r", newline="") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row[0], row[1])

marks.csv

Asha,88
Ravi,92

Rows are strings

csv.reader() returns each row as a list of strings. Convert numeric fields before calculating with them.

STRUCTURED DATA

Working with JSON

JSON represents objects, arrays, strings, numbers, booleans and null in a portable text format.

LOAD

JSON → Python

json.load(file) reads from an open file. json.loads(text) reads from a string.

with open("profile.json", "r") as file:
    profile = json.load(file)

DUMP

Python → JSON

json.dump(value, file) writes to a file. json.dumps(value) returns a string.

with open("profile.json", "w") as file:
    json.dump(profile, file, indent=2)

JSON objects become dictionaries, arrays become lists, true/false become True/False, and null becomes None.

COMMON MISTAKES

Applied Python Errors to Avoid

Using wildcard imports

They hide where names came from and can overwrite existing names.

Catching every exception

Handle only failures the program understands and can recover from.

Forgetting the file mode

w replaces existing content, while a preserves it and appends.

Parsing structured data manually

Use csv and json so quoting, escaping and types are handled correctly.

QUICK REVISION

📌 Modules, Exceptions & Files — Quick Revision

Review these ten rules before attempting the quiz and programming problems.

A module groups reusable Python code.

module.name keeps a tool’s source clear.

try contains code that may raise an expected exception.

except should name the most specific useful type.

else runs only when try succeeds.

finally runs whether an exception occurs or not.

with open(...) closes a file automatically.

Mode w replaces; mode a appends.

csv.reader() produces rows of strings.

json.load() converts JSON into Python values.

KNOWLEDGE CHECK

🧠 Level 10 Quiz

Select one answer for each question, then check your score.

1. Which statement keeps the module namespace visible?

from math import * import math include math

2. Which block runs only when the `try` block finishes without an exception?

except finally else

3. Which exception does `int("twelve")` raise?

ValueError KeyError FileNotFoundError

4. Which mode adds new text without deleting existing content?

r a w

5. Why is `with open(...)` preferred?

It encrypts the file. It converts every value to JSON. It closes the file automatically.

6. Which function reads JSON from an already-open file?

json.load(file) json.read(file) csv.reader(file)

PROGRAMMING PROBLEMS

💻 CodeBhavya Applied Python Challenges

Write and run each program in the browser. File problems use the displayed virtual files so nothing on your device is changed.

0 / 5Solved independently

0Completed with solution

0 / 500Best score

0%Progress

1. Circle Area with math

Import math, read a radius and display the circle area rounded to two decimal places.

Sample input: 4Expected output: Area = 50.27

💡 Use math.pi * radius ** 2, then pass the result and 2 to round().

Official Python Program

import math

radius = float(input())
area = math.pi * radius ** 2
print("Area =", round(area, 2))

2. Safe Integer Division

Read two integers. Display the quotient, or a clear message when the divisor is zero.

Sample inputs: 18, 0Expected output: Cannot divide by zero

💡 Put the division inside try and catch ZeroDivisionError.

Official Python Program

first = int(input())
second = int(input())

try:
    print("Quotient =", first // second)
except ZeroDivisionError:
    print("Cannot divide by zero")

3. Text File Report

Read notes.txt and display its number of lines and words.

Virtual file: Learn Python\nPractice daily\nBuild projectsExpected output: Lines = 3 and Words = 6

💡 Read the text once. Use splitlines() for lines and split() for words.

Official Python Program

with open("notes.txt", "r") as file:
    text = file.read()

print("Lines =", len(text.splitlines()))
print("Words =", len(text.split()))

4. CSV Marks Average

Read the three rows in marks.csv and display the whole-number average.

Virtual file: Asha,84 • Ravi,90 • Mina,96Expected output: Average = 90

💡 Convert row[1] to int, add every mark and divide by the row count.

Official Python Program

import csv

total = 0
count = 0
with open("marks.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        total += int(row[1])
        count += 1

print("Average =", total // count)

5. JSON Profile Summary

Load profile.json and display the learner’s name and number of skills.

Virtual file: {"name":"Bhavya","skills":["Python","C","DSA"]}Expected output: Name = Bhavya and Skills = 3

💡 Use json.load(file), then access the name and skills keys.

Official Python Program

import json

with open("profile.json", "r") as file:
    profile = json.load(file)

print("Name =", profile["name"])
print("Skills =", len(profile["skills"]))

KEY TAKEAWAY

🎯 Build Programs That Connect and Recover

1Import Clearly

Reuse tools through namespaces.

→

2Validate Risk

Expect realistic failures.

→

3Manage Files

Open safely with with.

→

4Parse Formats

Use CSV and JSON libraries.

Applied Python programs depend on more than calculations. They reuse existing code, communicate failures deliberately and transform persistent data without leaving resources open or formats half-parsed.

INTERACTIVE LEARNING

🎬 Safe File Processing — Visual Flow

Follow one file-processing task from opening the resource to a safe, parsed result.

PROGRAM TRACING

🔎 Program Tracing — Safe Division

Trace how try transfers control to the matching except block when the divisor is zero.

INTERVIEW PREPARATION

🎤 Modules, Exceptions & Files — Interview Questions

Answer aloud before opening each explanation.

What is the difference between a module and a package?

Why is import math often clearer than a wildcard import?

Calls such as math.sqrt() reveal the source of a name and avoid silently overwriting names from other modules.

How do else and finally differ in exception handling?

else runs only when the try block succeeds. finally runs whether execution succeeds or raises an exception.

Why should exceptions be caught specifically?

What problem does a context manager solve for files?

What is the difference between json.load() and json.loads()?

json.load() reads JSON from a file object, while json.loads() parses JSON from a string.

EXTRA TIPS

💡 Modules, Exceptions & Files — Extra Tips

01
Prefer explicit imports so readers can locate each external name quickly.
02
Keep try blocks small; include only statements expected to fail together.
03
Log or display an actionable error message instead of silently ignoring failure.
04
Use with for every file unless a longer resource lifetime is intentionally required.
05
Convert CSV fields to numeric types before totals, comparisons or averages.
06
Validate JSON keys and types when data comes from outside your program.

EXTRA PRACTICE

✍️ Modules, Exceptions & Files — Extra Practice Questions

Use math.sqrt() to display the square root of a valid positive number.
Import statistics with an alias and calculate a list’s mean.
Catch invalid integer input and ask the user to try again.
Raise ValueError when marks fall outside 0 to 100.
Count uppercase letters stored in a text file.
Append a dated study note without replacing earlier notes.
Copy only non-empty lines from one text file to another.
Read a CSV expense file and display its largest amount.
Write a dictionary to JSON using an indentation of two spaces.
Handle both missing-file and invalid-JSON errors with separate messages.

LEVEL 10 CHECKPOINT

Modules, Exceptions & Files Complete

When you can import clearly, recover from expected failures and process text, CSV and JSON data safely, mark this level complete and continue.

Continue to Level 11 →

📁 Modules, Exceptions & Files

By the End of This Level

Module Fundamentals

Use the Math Module

Import Styles and Useful Modules

math

random

datetime

statistics

Exception Handling

Safe Number Conversion

Common Exceptions and Raising Errors

Reject Invalid Data

Explain the Cause

File Paths and Modes

Reading and Writing Text Files

Choose the needed method

Write explicit text

Working with CSV

Read Student Marks

Working with JSON

JSON → Python

Python → JSON

Applied Python Errors to Avoid

📌 Modules, Exceptions & Files — Quick Revision

🧠 Level 10 Quiz

1. Which statement keeps the module namespace visible?

2. Which block runs only when the try block finishes without an exception?

3. Which exception does int("twelve") raise?

4. Which mode adds new text without deleting existing content?

5. Why is with open(...) preferred?

6. Which function reads JSON from an already-open file?

💻 CodeBhavya Applied Python Challenges

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

Python Code Editor

Program Output

Test Case

🎯 Build Programs That Connect and Recover

🎬 Safe File Processing — Visual Flow

📁 How Safe File Processing Works

1. Choose the Path and Mode

🔎 Program Tracing — Safe Division

🎤 Modules, Exceptions & Files — Interview Questions

💡 Modules, Exceptions & Files — Extra Tips

✍️ Modules, Exceptions & Files — Extra Practice Questions

Modules, Exceptions & Files Complete

2. Which block runs only when the `try` block finishes without an exception?

3. Which exception does `int("twelve")` raise?

5. Why is `with open(...)` preferred?