Is the Counter, defaultdict, namedtuple lesson free?

Yes. The first 5 lessons on PyRun are free forever — no signup, no card. Counter, defaultdict, namedtuple runs real Python in your browser via Pyodide.

How long does the Counter, defaultdict, namedtuple lesson take?

About 7 minutes end-to-end. It's a hands-on lesson — you read a paragraph, edit the code, and run it in the same tab.

Do I need to install Python to try the Counter, defaultdict, namedtuple lesson?

No install required. PyRun runs real CPython in your browser via Pyodide (Python compiled to WebAssembly). Works on Chrome, Firefox, Safari, and Edge — desktop or mobile.

What level is the Counter, defaultdict, namedtuple lesson?

Intermediate level. Best after finishing the Beginner track (syntax, control flow, functions, data types).

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Intermediate·7 min·intermediate · data structures

Counter, defaultdict, namedtuple

You’ll be able to

Use Counter to count occurrences in a sequence
Use defaultdict to avoid KeyError boilerplate
Use deque for O(1) appends and pops on both ends
Use namedtuple for lightweight structured records

Why this matters

Counter, defaultdict, and deque are the difference between clean code and reinventing wheels in interviews and production. deque is O(1) at both ends — list.pop(0) is O(n), a bug that quietly slows queue workers.

Common pitfalls

Using dict.get(k, []).append(v) instead of defaultdict(list) — the .get version never mutates the dict.
Using list as a FIFO queue — pop(0) is O(n); reach for collections.deque for O(1) popleft.
Sorting a Counter manually when .most_common(n) already returns the top-n by count.

The collections module saves you boilerplate.

`Counter`

A dict subclass that counts hashable items. .most_common(n) is the killer feature.

`defaultdict`

A dict where missing keys auto-create a default value. defaultdict(list) is the cleanest way to group things.

`namedtuple`

A tuple with named fields. Tiny, fast, and lets you write p.x instead of p[0]. Modern alternative: dataclasses.dataclass (we'll meet it later).

Try it

Use Counter to find the 3 most-common letters in "abracadabra".
Use defaultdict(int) to count votes from ["a","b","a","c","b","a"].

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Use collections.Counter to count the letter frequencies in "mississippi". Print the two most common letters as a list of (letter, count) tuples using .most_common(2). Expected: [('i', 4), ('s', 4)].
Exercise 2
Use collections.defaultdict(list) to group words = ["apple", "ant", "bear", "kite", "kiwi", "bee"] by their first letter into groups. Then print dict(groups). Expected includes 'a': ['apple', 'ant'].
Exercise 3
Use collections.deque to implement a sliding window of size 3. Feed the numbers [1, 2, 3, 4, 5] one at a time; after each push, print the current window as a list. Expected 5 lines, last one [3, 4, 5].

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Counter, defaultdict, namedtuple

You’ll be able to

Use Counter to count occurrences in a sequence
Use defaultdict to avoid KeyError boilerplate
Use deque for O(1) appends and pops on both ends
Use namedtuple for lightweight structured records

Why this matters

Common pitfalls

Using dict.get(k, []).append(v) instead of defaultdict(list) — the .get version never mutates the dict.
Using list as a FIFO queue — pop(0) is O(n); reach for collections.deque for O(1) popleft.
Sorting a Counter manually when .most_common(n) already returns the top-n by count.

The collections module saves you boilerplate.

`Counter`

A dict subclass that counts hashable items. .most_common(n) is the killer feature.

`defaultdict`

A dict where missing keys auto-create a default value. defaultdict(list) is the cleanest way to group things.

`namedtuple`

A tuple with named fields. Tiny, fast, and lets you write p.x instead of p[0]. Modern alternative: dataclasses.dataclass (we'll meet it later).

Try it

Use Counter to find the 3 most-common letters in "abracadabra".
Use defaultdict(int) to count votes from ["a","b","a","c","b","a"].

Practice

3 graded exercises

Write the code, click Check. We’ll tell you exactly what to fix.

Progress0 / 3

Exercise 1
Use collections.Counter to count the letter frequencies in "mississippi". Print the two most common letters as a list of (letter, count) tuples using .most_common(2). Expected: [('i', 4), ('s', 4)].
Exercise 2
Use collections.defaultdict(list) to group words = ["apple", "ant", "bear", "kite", "kiwi", "bee"] by their first letter into groups. Then print dict(groups). Expected includes 'a': ['apple', 'ant'].
Exercise 3
Use collections.deque to implement a sliding window of size 3. Feed the numbers [1, 2, 3, 4, 5] one at a time; after each push, print the current window as a list. Expected 5 lines, last one [3, 4, 5].