Fibonacci generator

Write a generator that yields Fibonacci numbers, and take the first 10.

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Approach

1. An infinite generator plus itertools.islice.

Solution

from itertools import islice def fibonacci():    a, b = 0, 1    while True:        yield a        a, b = b, a + b print(list(islice(fibonacci(), 10)))

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

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Countdown generator

Write a generator that counts down from n to 1.

[5, 4, 3, 2, 1]

Approach

1. yield inside a while loop.

Solution

def countdown(n):    while n > 0:        yield n        n -= 1 print(list(countdown(5)))

[5, 4, 3, 2, 1]

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Even numbers generator

Write a generator that yields even numbers up to a limit.

[0, 2, 4, 6, 8, 10, 12]

Approach

1. Loop with a step of 2 and yield each value.

Solution

def evens(limit):    for n in range(0, limit + 1, 2):        yield n print(list(evens(12)))

[0, 2, 4, 6, 8, 10, 12]

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A for loop by hand

Write a Python program that rebuilds a for loop using while, iter() and next().

10
20
30

Approach

1. Call next() until StopIteration.

Solution

items = [10, 20, 30]it = iter(items)while True:    try:        value = next(it)    except StopIteration:        break    print(value)

10
20
30

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Use iter() and next()

Write a Python program that walks through a list with iter() and next().

login
search
logout
no more steps

Approach

1. next() returns items one by one; a default avoids StopIteration.

Solution

it = iter(["login", "search", "logout"])print(next(it))print(next(it))print(next(it))print(next(it, "no more steps"))

login
search
logout
no more steps

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Python Exception Handling Programs with Solutions

20 exception handling programs: catching specific errors, else and finally, raising and re-raising, custom exceptions, retries and safe input parsing. Each solution runs both the success path and the failure path so you can see every branch.

All programs

• #1Handle division by zero
  Write a Python program that divides two numbers and prints a friendly message when the divisor is 0.Easy
• #2Handle invalid integer input
  Write a Python program that converts text to an integer and reports invalid values.Easy
• #3Handle a missing dictionary key
  Write a Python program that reads a key from a dictionary and handles a missing key.Easy
• #4Handle an index out of range
  Write a Python program that reads a list item by index and handles invalid indexes.Easy
• #5Several except blocks
  Write a Python function that handles TypeError and ZeroDivisionError separately.Easy
• #6Catch several errors in one block
  Write a Python program that catches ValueError and TypeError in one except block.Easy
• #7Use the else block
  Write a Python program that uses else to run code only when no exception occurred.Easy
• #8Use the finally block
  Write a Python program that shows finally always runs.Easy
• #9Raise an exception
  Write a Python function that raises ValueError for an invalid age.Easy
• #10Custom exception class
  Write a Python program with a custom InsufficientBalanceError.Medium
• #11Log and re-raise
  Write a Python program that logs an error and then re-raises it to the caller.Easy
• #12Chain exceptions with raise from
  Write a Python program that converts a low-level error into a clearer one and keeps the cause.Medium
• #13Handle a missing file
  Write a Python program that opens a file and handles the case where it does not exist.Easy
• #14Retry on failure
  Write a Python program that retries an unreliable operation up to 3 times.Medium
• #15Use assert for checks
  Write a Python program that uses assert to check a condition and handles the AssertionError.Easy
• #16Ask until the input is valid
  Write a Python program that keeps asking for a number until a valid one is entered.Easy
• #17Print exception details
  Write a Python program that prints the type, message and line of a caught exception.Easy
• #18Clean-up with a context manager
  Write a Python program showing that a with block closes a file even when an error happens inside it.Easy
• #19Collect several errors
  Write a Python program that validates several fields and reports every error at once.Medium
• #20Exception hierarchy order
  Write a Python program showing why specific exceptions must be caught before general ones.Easy

Exception hierarchy order

Write a Python program showing why specific exceptions must be caught before general ones.

True True
specific handler: file not found

Approach

1. FileNotFoundError is a subclass of OSError, so it must come first.

Solution

print(issubclass(FileNotFoundError, OSError), issubclass(OSError, Exception))try:    open("nope.txt")except FileNotFoundError:    print("specific handler: file not found")except OSError:    print("general handler: some OS error")

True True
specific handler: file not found

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Clean-up with a context manager

Write a Python program showing that a with block closes a file even when an error happens inside it.

caught: something failed
file closed: True

Approach

1. Raise inside the with and check f.closed afterwards.

Solution

try:    with open("demo.txt", "w") as f:        f.write("data")        raise RuntimeError("something failed")except RuntimeError as e:    print("caught:", e)print("file closed:", f.closed)

caught: something failed
file closed: True

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Collect several errors

Write a Python program that validates several fields and reports every error at once.

valid
3 errors: name is required; email is invalid; age must be 18-120

Approach

1. Collect messages in a list and raise one exception at the end.

Solution

class ValidationError(Exception):    def __init__(self, errors):        super().__init__("; ".join(errors))        self.errors = errors def validate(user):    errors = []    if not user.get("name"):        errors.append("name is required")    if "@" not in user.get("email", ""):        errors.append("email is invalid")    if not 18 <= user.get("age", 0) <= 120:        errors.append("age must be 18-120")    if errors:        raise ValidationError(errors)    return "valid" for user in [{"name": "Asha", "email": "asha@x.com", "age": 29}, {"name": "", "email": "bad", "age": 5}]:    try:        print(validate(user))    except ValidationError as e:        print(len(e.errors), "errors:", e)

valid
3 errors: name is required; email is invalid; age must be 18-120

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Print exception details

Write a Python program that prints the type, message and line of a caught exception.

type: IndexError
message: list index out of range
line: 3

Approach

1. type(e).__name__, str(e) and the traceback object.

Solution

try:    numbers = [1, 2, 3]    numbers[5]except IndexError as e:    tb = e.__traceback__    print("type:", type(e).__name__)    print("message:", e)    print("line:", tb.tb_lineno)

type: IndexError
message: list index out of range
line: 3

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