Iterator protocol in Python - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get an iterator from the list.

Python

my_list = [1, 2, 3]
my_iter = [1](my_list)

Drag options to blanks, or click blank then click option'

Alist

Bnext

Citer

Drange

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'next' instead of 'iter' to get the iterator.

Trying to convert the list to another list.

Using 'range' which creates a sequence, not an iterator from the list.

The iter() function returns an iterator object from an iterable like a list.

2fill in blank

medium

Complete the code to get the next item from the iterator.

Python

my_list = [10, 20, 30]
my_iter = iter(my_list)
item = [1](my_iter)

Drag options to blanks, or click blank then click option'

Aiter

Bnext

Clist

Dappend

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'iter' again instead of 'next'.

Trying to convert the iterator to a list.

Using 'append' which is a list method, not related to iterators.

The next() function returns the next item from an iterator.

3fill in blank

hard

Fix the error in the code to correctly iterate over the list using an iterator.

Python

my_list = [5, 6, 7]
my_iter = iter(my_list)
while True:
    try:
        item = [1](my_iter)
        print(item)
    except StopIteration:
        break

Drag options to blanks, or click blank then click option'

Anext

Biter

Clist

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'iter' inside the loop which resets the iterator.

Using 'list' which is not a function to get next item.

Using 'print' instead of fetching the next item.

The next() function is used inside the loop to get each item until StopIteration is raised.

4fill in blank

hard

Fill both blanks to create a dictionary of squares for numbers greater than 2.

Python

numbers = [1, 2, 3, 4, 5]
squares = {num: num[1]2 for num in numbers if num [2] 2}

Drag options to blanks, or click blank then click option'

A**

B>

C<

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication (*) instead of exponentiation (**).

Using less than (<) instead of greater than (>).

Confusing the operators for filtering.

The ** operator is used for exponentiation (square), and > filters numbers greater than 2.

5fill in blank

hard

Fill all three blanks to create a dictionary with uppercase keys and values greater than 10.

Python

data = {'a': 5, 'b': 15, 'c': 20}
result = { [1]: [2] for k, v in data.items() if v [3] 10}

Drag options to blanks, or click blank then click option'

Ak.upper()

Bv

C>

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using original keys without uppercase conversion.

Using wrong comparison operator like '<'.

Mixing keys and values in the dictionary comprehension.

Keys are converted to uppercase with k.upper(), values are v, and filter uses > to select values greater than 10.

Practice

1. What does the __iter__ method do in the iterator protocol?

easy

A. Returns the iterator object itself

B. Returns the next item in the sequence

C. Stops the iteration

D. Creates a list from the iterable

Solution

1. Step 1: Understand the role of __iter__

   The __iter__ method is called to get an iterator object from an iterable.

2. Step 2: Identify what __iter__ returns

   It returns the iterator object itself, which has the __next__ method to fetch items.

3. Final Answer:

   Returns the iterator object itself -> Option A

4. Quick Check:

   __iter__ returns iterator object [OK]

Hint: Remember: __iter__ returns the iterator itself [OK]

Common Mistakes:

• Confusing __iter__ with __next__
• Thinking __iter__ returns the next item
• Assuming __iter__ stops iteration

2. Which of the following is the correct way to define an iterator class in Python?

easy

A. class MyIter:
def __next__(self):
pass

B. class MyIter:
def next(self):
pass

C. class MyIter:
def __iter__(self):
return self
def __next__(self):
pass

D. class MyIter:
def iter(self):
return self

Solution

1. Step 1: Check required methods for iterator

   An iterator class must have __iter__ returning self and __next__ to get next item.

2. Step 2: Match methods with options

   class MyIter: def __iter__(self): return self def __next__(self): pass correctly defines both __iter__ and __next__ methods.

3. Final Answer:

   Defines both __iter__ and __next__ methods -> Option C

4. Quick Check:

   Iterator class needs __iter__ and __next__ [OK]

Hint: Iterator class must have __iter__ and __next__ methods [OK]

Common Mistakes:

• Using next() instead of __next__()
• Missing __iter__ method
• Defining iter() instead of __iter__()

3. What will be the output of this code?

class Count:
    def __init__(self, limit):
        self.limit = limit
        self.num = 0
    def __iter__(self):
        return self
    def __next__(self):
        if self.num < self.limit:
            self.num += 1
            return self.num
        else:
            raise StopIteration

for i in Count(3):
    print(i, end=' ')

medium

A. Error: StopIteration not handled

B. 0 1 2

C. 1 2 3 4

D. 1 2 3

Solution

1. Step 1: Understand the iterator behavior

   The Count class starts num at 0 and returns num+1 until it reaches limit 3.

2. Step 2: Trace the loop output

   Loop prints 1, 2, 3 then raises StopIteration to end loop.

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   Count(3) yields 1 to 3 [OK]

Hint: StopIteration ends loop; count from 1 to limit [OK]

Common Mistakes:

• Starting count from 0 instead of 1
• Expecting 4 as output
• Thinking StopIteration causes error

4. Identify the error in this iterator implementation:

class MyIter:
    def __init__(self):
        self.data = [1, 2, 3]
        self.index = 0
    def __iter__(self):
        return self
    def __next__(self):
        if self.index <= len(self.data):
            result = self.data[self.index]
            self.index += 1
            return result
        else:
            raise StopIteration

medium

A. The condition should be self.index < len(self.data)

B. Missing return self in __iter__

C. Should raise StopIteration before returning result

D. Index should start at 1, not 0

Solution

1. Step 1: Analyze the index condition

   Index goes from 0 to len(data)-1. Using <= allows index == len(data), causing IndexError.

2. Step 2: Correct the condition

   Change condition to self.index < len(self.data) to avoid out-of-range access.

3. Final Answer:

   The condition should be self.index < len(self.data) -> Option A

4. Quick Check:

   Index must be less than length to avoid error [OK]

Hint: Use < to avoid index out of range errors [OK]

Common Mistakes:

• Using <= instead of < in index check
• Forgetting to return self in __iter__
• Starting index at 1 instead of 0

5. You want to create a custom iterator that returns only even numbers from 0 up to a given limit (exclusive). Which implementation correctly follows the iterator protocol and filters evens?

class EvenIterator:
    def __init__(self, limit):
        self.limit = limit
        self.current = 0
    def __iter__(self):
        return self
    def __next__(self):
        while self.current < self.limit:
            val = self.current
            self.current += 1
            if val % 2 == 0:
                return val
        raise StopIteration

hard

A. Fails because __iter__ should return a new object each time

B. Correct implementation returning even numbers up to limit

C. Incorrect because it returns odd numbers instead

D. Raises StopIteration too early, missing some evens

Solution

1. Step 1: Check iterator protocol methods

   Class defines __iter__ returning self and __next__ with loop and StopIteration.

2. Step 2: Verify filtering logic

   Inside __next__, it loops until limit, returns only even values, skipping odds.

3. Final Answer:

   Correct implementation returning even numbers up to limit -> Option B

4. Quick Check:

   Iterator filters evens correctly [OK]

Hint: Use while loop inside __next__ to skip unwanted items [OK]

Common Mistakes:

• Returning odd numbers by mistake
• Not raising StopIteration when done
• Returning new iterator in __iter__ each time