Discussion 6

Mutability, Iterators, Generators

Antonio Kam
anto [at] berkeley [dot] edu


  • GL with the midterm 🥺
  • Y'all will do great!
  • Cats checkpoint due today
  • Cats due on next Tuesday
Slides by Antonio Kam (anto@)

Comments from last section

  • Cover more stuff on discussion worksheets 🥺
  • How do you reverse a tree
    • 💀
    • depends on what implementation of trees you're using (the 61a version is a bit harder, but still possible with the reverse function)
      • ask me over email, i'll send over something
    • Do you have a favorite cat breed?
      • I like all cat breeds (even the hairless ones are cute 🥺)
    • how tall are you?recursion and leap of faith and tuples!!!
      • im 180cm (i think that's 5'11")
      • we can get to the other stuff once we hit midterm review
Slides by Antonio Kam (anto@)

Comments from last section

  • how many pushups can u do
    • 💀
  • Definitely some extra review on trees
    • got some during lab, might get some more today!
  • Dr Pepper >> Coca cola
    • still haven't tried dr pepper (maybe)
  • In the game Enter the Gungeon, there is one NPC you can only understand if you have a specific item. He is very eleoquent
    • bullet hell games!! hard
Slides by Antonio Kam (anto@)

Temperature Check 🌡️

  • Iterators
  • Generators
Slides by Antonio Kam (anto@)


Slides by Antonio Kam (anto@)


  • Iterable
    • Old View: Something we can go through one at a time
    • New View: Something we can call iter on - this will return an iterator.
  • Iterator
    • A type of object that lets us keep track of which element is next in the sequence
Slides by Antonio Kam (anto@)

Functions to Interact with Iterators

  • iter(iterable)
    • Takes in an iterable and returns an iterator.
      • Calling iter on an iterable makes a brand new iterator
      • Calling iter on an iterator returns that same iterator (not a copy!) and does not change the state of that iterator
  • next(iterator)
    • Takes in an iterator and outputs the next element
      • Raises StopIteration when it has no more elements to go through
      • Cannot call next on an iterable
Slides by Antonio Kam (anto@)


  • Iterable = Book
  • Iterator = Bookmark
  • Calling iter on an iterable (book) will create a new bookmark
  • Calling iter on an iterator (bookmark) will just give you the same bookmark (no reason to mark where a bookmark is)
  • Calling next on an iterator moves the bookmark to the next chapter
Slides by Antonio Kam (anto@)

do some terminal example pls thx

Slides by Antonio Kam (anto@)


Slides by Antonio Kam (anto@)


Slides by Antonio Kam (anto@)


  • Generators are more specific versions of iterators that you can create yourself!
  • When a generator function is called, it returns a generator object!
  • The body of a generator function is not evaluated until next is called on the returned generator object.
  • Generator functions look like normal functions, but use yield instead of return. Python will automatically see yield and determine a function a generator function if necessary.
Slides by Antonio Kam (anto@)

Generators (continued)

  • When next is called on a generator object, the body of a function is executed until yield is reached.
  • From there, the yield statement will return a value, and then pauses that specific function at that moment (by saving the frame, and the line that it's on)
  • When next is called again, we continue where we left off, until yield is reached again.
  • StopIteration will be raised at the end of a generator function
Slides by Antonio Kam (anto@)


def countdown_generator(n):
    assert n >= 0
    while n >= 0:
        yield n
        n -= 1
Slides by Antonio Kam (anto@)

yield from

  • You can use yield from <iterable> if you want to yield every value from an iterable. These are equivalent:
lst = [1, 2, 3]
# Version 1
yield from lst

for item in lst:
    yield item

Slides by Antonio Kam (anto@)

Results from last section (links.rouxl.es/disc)

Slides by Antonio Kam (anto@)

Midterm Review

Slides by Antonio Kam (anto@)

Midterm tips

  • Studying
  • Question annotation
  • Example Question (Fall 2021 MT2 amounts)
Slides by Antonio Kam (anto@)

Mental Health Resources

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Slides by Antonio Kam (anto@)

Anonymous Feedback Form

Thanks for coming! 🥳

Please give me feedback on what to improve!

Slides by Antonio Kam (anto@)