Lesson 03
Iterate list, tuple, str, set, and dict — Iterator Type Names
Map list, tuple, str, set, and dict to everyday stories—basket, queue, letters, lottery, keys—then match Python iterator type names.
 as turning a collection into a Next button that reveals one item at a time.
 → Iterator → next() → one value
What you will learn
- Five everyday stories mapped to list, tuple, str, set, and dict iterators
- Type names for list, tuple, str, set, and dict iterators
- Why
iter(123445)raisesTypeError - Dict iterators yield keys — use
x.get(key)for values - File lines as an iterator (
StringIOin the playground)
1. List = Shopping Basket
A basket already holds many items. You pick one product each time — you do not eat everything at once.
![shopping basket pick apple milk bread one at a time](data:image/svg+xml;charset=utf-8,%3Csvg%20font-family%3D%27ui-sans-serif%2C-apple-system%2Csystem-ui%2CArial%27%20width%3D%27100%25%27%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%20320%2064%27%3E%20%3Crect%20width%3D%27320%27%20height%3D%2764%27%20rx%3D%2710%27%20fill%3D%27%23EFF6FF%27%2F%3E%20%3Ccircle%20cx%3D%2754%27%20cy%3D%2732%27%20r%3D%2718%27%20fill%3D%27%23DBEAFE%27%20stroke%3D%27%232563EB%27%2F%3E%20%3Ctext%20x%3D%2754%27%20y%3D%2736%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%231D4ED8%27%3EAp%3C%2Ftext%3E%20%3Cpath%20d%3D%27M72%2032%20H142%27%20stroke%3D%27%2364748B%27%20stroke-dasharray%3D%273%203%27%2F%3E%20%3Cpolygon%20points%3D%27136%2C28%20144%2C32%20136%2C36%27%20fill%3D%27%2364748B%27%2F%3E%20%3Ccircle%20cx%3D%27160%27%20cy%3D%2732%27%20r%3D%2718%27%20fill%3D%27%23DBEAFE%27%20stroke%3D%27%232563EB%27%2F%3E%20%3Ctext%20x%3D%27160%27%20y%3D%2736%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%231D4ED8%27%3EMk%3C%2Ftext%3E%20%3Cpath%20d%3D%27M178%2032%20H248%27%20stroke%3D%27%2364748B%27%20stroke-dasharray%3D%273%203%27%2F%3E%20%3Cpolygon%20points%3D%27242%2C28%20250%2C32%20242%2C36%27%20fill%3D%27%2364748B%27%2F%3E%20%3Ccircle%20cx%3D%27266%27%20cy%3D%2732%27%20r%3D%2718%27%20fill%3D%27%23DBEAFE%27%20stroke%3D%27%232563EB%27%2F%3E%20%3Ctext%20x%3D%27266%27%20y%3D%2736%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%231D4ED8%27%3EBr%3C%2Ftext%3E%20%3C%2Fsvg%3E
Story example:
basket = ["Apple", "Milk", "Bread"]
it = iter(basket)
print(next(it)) # Apple
print(next(it)) # MilkIterator type: list_iterator
Mixed-type list — while + next:
x = [10, "RS", 23.45, 3+4j, True, False]
itr = iter(x)
print(type(itr)) # <class 'list_iterator'>
while True:
try:
print(next(itr))
except StopIteration:
break2. Tuple = People in a Train Queue
A queue has a fixed order. People do not swap places — you simply call the next passenger.
![train queue fixed order A B C next passenger](data:image/svg+xml;charset=utf-8,%3Csvg%20font-family%3D%27ui-sans-serif%2C-apple-system%2Csystem-ui%2CArial%27%20width%3D%27100%25%27%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%20320%2072%27%3E%20%3Crect%20width%3D%27320%27%20height%3D%2772%27%20rx%3D%2712%27%20fill%3D%27%23ECFDF5%27%2F%3E%20%3Ctext%20x%3D%2718%27%20y%3D%2720%27%20font-size%3D%278%27%20font-weight%3D%27bold%27%20fill%3D%27%23047857%27%3ENext%3C%2Ftext%3E%20%3Ccircle%20cx%3D%2778%27%20cy%3D%2740%27%20r%3D%2716%27%20fill%3D%27%23A7F3D0%27%20stroke%3D%27%23059669%27%2F%3E%20%3Ctext%20x%3D%2778%27%20y%3D%2744%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%23065F46%27%3EA%3C%2Ftext%3E%20%3Ccircle%20cx%3D%27148%27%20cy%3D%2740%27%20r%3D%2716%27%20fill%3D%27%23A7F3D0%27%20stroke%3D%27%23059669%27%2F%3E%20%3Ctext%20x%3D%27148%27%20y%3D%2744%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%23065F46%27%3EB%3C%2Ftext%3E%20%3Ccircle%20cx%3D%27218%27%20cy%3D%2740%27%20r%3D%2716%27%20fill%3D%27%23A7F3D0%27%20stroke%3D%27%23059669%27%2F%3E%20%3Ctext%20x%3D%27218%27%20y%3D%2744%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%23065F46%27%3EC%3C%2Ftext%3E%20%3C%2Fsvg%3E
Story example:
people = ("A", "B", "C")
it = iter(people)
print(next(it)) # AIterator type: tuple_iterator
Tuple example:
x = (10, "RS", True)
itr = iter(x)
print(type(itr)) # <class 'tuple_iterator'>
print(next(itr), next(itr), next(itr))3. String = Reading a Word
When you read PYTHON, you naturally take one letter at a time.
![PYTHON letters one character at a time](data:image/svg+xml;charset=utf-8,%3Csvg%20font-family%3D%27ui-sans-serif%2C-apple-system%2Csystem-ui%2CArial%27%20width%3D%27100%25%27%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%20320%2072%27%3E%20%3Crect%20width%3D%27320%27%20height%3D%2772%27%20rx%3D%2712%27%20fill%3D%27%23F5F3FF%27%2F%3E%20%3Crect%20x%3D%2710%27%20y%3D%2720%27%20width%3D%2734%27%20height%3D%2732%27%20rx%3D%278%27%20fill%3D%27%23DDD6FE%27%20stroke%3D%27%237C3AED%27%2F%3E%20%3Ctext%20x%3D%2727%27%20y%3D%2740%27%20font-size%3D%2712%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%235B21B6%27%3EP%3C%2Ftext%3E%20%3Crect%20x%3D%2760%27%20y%3D%2720%27%20width%3D%2734%27%20height%3D%2732%27%20rx%3D%278%27%20fill%3D%27%23DDD6FE%27%20stroke%3D%27%237C3AED%27%2F%3E%20%3Ctext%20x%3D%2777%27%20y%3D%2740%27%20font-size%3D%2712%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%235B21B6%27%3EY%3C%2Ftext%3E%20%3Crect%20x%3D%27110%27%20y%3D%2720%27%20width%3D%2734%27%20height%3D%2732%27%20rx%3D%278%27%20fill%3D%27%23DDD6FE%27%20stroke%3D%27%237C3AED%27%2F%3E%20%3Ctext%20x%3D%27127%27%20y%3D%2740%27%20font-size%3D%2712%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%235B21B6%27%3ET%3C%2Ftext%3E%20%3Crect%20x%3D%27160%27%20y%3D%2720%27%20width%3D%2734%27%20height%3D%2732%27%20rx%3D%278%27%20fill%3D%27%23DDD6FE%27%20stroke%3D%27%237C3AED%27%2F%3E%20%3Ctext%20x%3D%27177%27%20y%3D%2740%27%20font-size%3D%2712%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%235B21B6%27%3EH%3C%2Ftext%3E%20%3Crect%20x%3D%27210%27%20y%3D%2720%27%20width%3D%2734%27%20height%3D%2732%27%20rx%3D%278%27%20fill%3D%27%23DDD6FE%27%20stroke%3D%27%237C3AED%27%2F%3E%20%3Ctext%20x%3D%27227%27%20y%3D%2740%27%20font-size%3D%2712%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%235B21B6%27%3EO%3C%2Ftext%3E%20%3Crect%20x%3D%27260%27%20y%3D%2720%27%20width%3D%2734%27%20height%3D%2732%27%20rx%3D%278%27%20fill%3D%27%23DDD6FE%27%20stroke%3D%27%237C3AED%27%2F%3E%20%3Ctext%20x%3D%27277%27%20y%3D%2740%27%20font-size%3D%2712%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%235B21B6%27%3EN%3C%2Ftext%3E%20%3C%2Fsvg%3E
Story example:
word = "PYTHON"
for ch in iter(word):
print(ch)Iterator type: str_iterator (exact name may vary slightly by Python version)
The same for val in itr pattern works on "PYTHON".
4. Set = Lottery Ball Machine
Lottery balls come out one by one — but the order is unpredictable.
:
print(n)Important: never depend on the printed order.
Iterator type: set_iterator
Set example:
x = {10, "RS", 23.45, 3+4j, True, False}
itr = iter(x)
print(type(itr)) # set_iterator
for val in itr:
print(val)5. Dictionary = Hotel Room Keys
The receptionist hands you room numbers first. You open each room to see the guest name.
![hotel keys 101 102 103 map to guest names](data:image/svg+xml;charset=utf-8,%3Csvg%20font-family%3D%27ui-sans-serif%2C-apple-system%2Csystem-ui%2CArial%27%20width%3D%27100%25%27%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%20320%20120%27%3E%20%3Crect%20width%3D%27320%27%20height%3D%27120%27%20rx%3D%2714%27%20fill%3D%27%23FEF2F2%27%2F%3E%20%3Crect%20x%3D%2716%27%20y%3D%2714%27%20width%3D%2756%27%20height%3D%2728%27%20rx%3D%276%27%20fill%3D%27%23FCA5A5%27%20stroke%3D%27%23DC2626%27%2F%3E%20%3Ctext%20x%3D%2744%27%20y%3D%2732%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%237F1D1D%27%3E101%3C%2Ftext%3E%20%3Crect%20x%3D%278%27%20y%3D%2758%27%20width%3D%2772%27%20height%3D%2734%27%20rx%3D%276%27%20fill%3D%27%23FEE2E2%27%2F%3E%20%3Ctext%20x%3D%2744%27%20y%3D%2778%27%20font-size%3D%278%27%20text-anchor%3D%27middle%27%20fill%3D%27%23991B1B%27%3ERahul%3C%2Ftext%3E%20%3Crect%20x%3D%27116%27%20y%3D%2714%27%20width%3D%2756%27%20height%3D%2728%27%20rx%3D%276%27%20fill%3D%27%23FCA5A5%27%20stroke%3D%27%23DC2626%27%2F%3E%20%3Ctext%20x%3D%27144%27%20y%3D%2732%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%237F1D1D%27%3E102%3C%2Ftext%3E%20%3Crect%20x%3D%27108%27%20y%3D%2758%27%20width%3D%2772%27%20height%3D%2734%27%20rx%3D%276%27%20fill%3D%27%23FEE2E2%27%2F%3E%20%3Ctext%20x%3D%27144%27%20y%3D%2778%27%20font-size%3D%278%27%20text-anchor%3D%27middle%27%20fill%3D%27%23991B1B%27%3EPriya%3C%2Ftext%3E%20%3Crect%20x%3D%27216%27%20y%3D%2714%27%20width%3D%2756%27%20height%3D%2728%27%20rx%3D%276%27%20fill%3D%27%23FCA5A5%27%20stroke%3D%27%23DC2626%27%2F%3E%20%3Ctext%20x%3D%27244%27%20y%3D%2732%27%20font-size%3D%279%27%20font-weight%3D%27bold%27%20text-anchor%3D%27middle%27%20fill%3D%27%237F1D1D%27%3E103%3C%2Ftext%3E%20%3Crect%20x%3D%27208%27%20y%3D%2758%27%20width%3D%2772%27%20height%3D%2734%27%20rx%3D%276%27%20fill%3D%27%23FEE2E2%27%2F%3E%20%3Ctext%20x%3D%27244%27%20y%3D%2778%27%20font-size%3D%278%27%20text-anchor%3D%27middle%27%20fill%3D%27%23991B1B%27%3ESwathi%3C%2Ftext%3E%20%3Ctext%20x%3D%27160%27%20y%3D%27108%27%20font-size%3D%278%27%20text-anchor%3D%27middle%27%20fill%3D%27%23991B1B%27%3EKey%20first%2C%20guest%20name%20with%20get%28%29%3C%2Ftext%3E%20%3C%2Fsvg%3E
Story example:
rooms = {101: "Rahul", 102: "Priya", 103: "Swathi"}
for m in iter(rooms):
print(m, rooms.get(m))Key ideas:
iter(dict)→ gives keysrooms.get(key)→ gives values
Iterator type: dict_keyiterator (name may vary by version)
Dict example:
x = {10: "Python", 20: "C", 30: "Java", 40: "C++"}
itr = iter(x)
for val in itr:
print("{} {}".format(val, x.get(val)))Why does iter(12345) fail?
A number is like a single brick. You cannot walk through it because it is not a sequence.
# TypeError: 'int' object is not iterableIf you meant digits as characters, cast first:
for d in str(123445):
print(d)Remember: int is not iterable. The playground demonstrates iter(123445) raising TypeError.
File iterator = Reading a Notebook
A text file behaves like turning notebook pages — you read one line at a time. The playground uses StringIO instead of a file path on disk.
![notebook one line per page Swathi Rahul Priya](data:image/svg+xml;charset=utf-8,%3Csvg%20font-family%3D%27ui-sans-serif%2C-apple-system%2Csystem-ui%2CArial%27%20width%3D%27100%25%27%20xmlns%3D%27http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%27%20viewBox%3D%270%200%20320%20100%27%3E%20%3Crect%20width%3D%27320%27%20height%3D%27100%27%20rx%3D%2712%27%20fill%3D%27%23FFFBEB%27%20stroke%3D%27%23E5E7EB%27%2F%3E%20%3Crect%20x%3D%2740%27%20y%3D%2716%27%20width%3D%27200%27%20height%3D%2768%27%20rx%3D%278%27%20fill%3D%27%23FFFFFF%27%20stroke%3D%27%23D97706%27%2F%3E%20%3Cline%20x1%3D%2770%27%20y1%3D%2716%27%20x2%3D%2770%27%20y2%3D%2784%27%20stroke%3D%27%23FDE68A%27%20stroke-width%3D%273%27%2F%3E%20%3Ctext%20x%3D%27120%27%20y%3D%2738%27%20font-size%3D%279%27%20text-anchor%3D%27middle%27%20fill%3D%27%23374151%27%3ESwathi%3C%2Ftext%3E%20%3Cline%20x1%3D%2780%27%20y1%3D%2748%27%20x2%3D%27200%27%20y2%3D%2748%27%20stroke%3D%27%23E5E7EB%27%2F%3E%20%3Ctext%20x%3D%27120%27%20y%3D%2762%27%20font-size%3D%279%27%20text-anchor%3D%27middle%27%20fill%3D%27%23374151%27%3ERahul%3C%2Ftext%3E%20%3Cline%20x1%3D%2780%27%20y1%3D%2772%27%20x2%3D%27200%27%20y2%3D%2772%27%20stroke%3D%27%23E5E7EB%27%2F%3E%20%3Ctext%20x%3D%27120%27%20y%3D%2786%27%20font-size%3D%279%27%20text-anchor%3D%27middle%27%20fill%3D%27%23374151%27%3EPriya%3C%2Ftext%3E%20%3Ctext%20x%3D%27260%27%20y%3D%2750%27%20font-size%3D%278%27%20fill%3D%27%2392400E%27%3Eone%20line%3C%2Ftext%3E%20%3Ctext%20x%3D%27260%27%20y%3D%2762%27%20font-size%3D%278%27%20fill%3D%27%2392400E%27%3Eper%20page%3C%2Ftext%3E%20%3C%2Fsvg%3E
from io import StringIO
file = StringIO("Swathi\nRahul\nPriya\n")
for line in file:
print(line.strip())Output:
Swathi
Rahul
PriyaRun the full mix — list, str, tuple, set, dict, and iter(123445) — in the playground below.
Interview memory table
| Collection | Think of... | Iterator type |
|---|---|---|
| List | Shopping basket | list_iterator |
| Tuple | Train queue | tuple_iterator |
| String | Reading letters | str_iterator |
| Set | Lottery balls | set_iterator |
| Dictionary | Hotel room keys | dict_keyiterator |
| Integer | Single brick | Not iterable |
One-minute visual recap
 works only on iterables — list, tuple, string, set, dictionary, file. It does not work on an int; that raises TypeError.
Next: Generators and yield.
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Check your understanding
Collection iterators quiz
For a list, iter(lst) gives type(itr) as:
1 / 10