Perfect.
DAY 11 is arguably the most important day in Python.

If you master dictionaries at this level, you’ll understand:

• Why Python is fast
• Why sets exist
• Why Pandas / Spark / ORM / JSON all rely on dicts
• Why Python won the language wars

🧠 DAY 11 — Dictionaries

(CPython Hash Table Internals & Order Preservation)

🔑 CORE IDEA OF DAY 11

Python dictionaries are highly optimized hash tables with insertion-order preservation (Python 3.7+).

Almost everything in Python uses dicts:

• Objects (__dict__)
• Modules
• Classes
• Function locals / globals
• Keyword arguments
• JSON

1️⃣ What a Python Dict REALLY Is

A dictionary is:

• A hash table
• Mapping: key → value
• Keys must be hashable
• Values can be anything

d = {"a": 1, "b": 2}

Internally (conceptual):

dict
 ├── indices table
 ├── entries array (key, value, hash)
 └── probing logic

2️⃣ Dict Internal Structure (Modern CPython)

Key design:

• Split table design
• Keys & values stored efficiently
• Memory optimized for common cases

This design makes:

• Lookups fast
• Iteration fast
• Order preservation cheap

3️⃣ Why Dict Lookups Are O(1) Average

When you do:

value = d[key]

Python:

1. Computes hash(key)
2. Maps hash → index
3. Probes until key found
4. Compares equality only if hash matches

👉 No scanning like lists.

4️⃣ Hashing + Equality Contract (ABSOLUTE RULE)

For dictionary keys:

if a == b → hash(a) == hash(b)

But:

hash(a) == hash(b) ↛ a == b

Collisions are allowed.
Python resolves them internally.

5️⃣ The Legendary Dict Trap 🔥🔥🔥

d = {True: "yes", 1: "no"}
print(d)

✔ Output:

{True: 'no'}

Why?

• True == 1
• hash(True) == hash(1)
• Second key overwrites first

🧠 Dicts care about equality, not type.

6️⃣ Why Dicts Preserve Order (Python 3.7+)

Since Python 3.7:

Insertion order is a language guarantee

Example:

d = {}
d["a"] = 1
d["b"] = 2
d["c"] = 3

print(list(d.keys()))

✔ ['a', 'b', 'c']

Why Python did this:

• Common developer expectation
• Zero performance penalty (with new design)
• Simplifies many algorithms

⚠️ This was an implementation detail in 3.6
⚠️ Guaranteed from 3.7+

7️⃣ Why Dicts Are Faster Than You Expect

Reasons:

• Implemented in C
• Open addressing
• Sparse resizing
• Aggressive caching

Dicts are often faster than:

• Trees
• Custom objects
• Linear scans

8️⃣ Mutability of Dicts (Danger Zone)

d = {"x": 1}
e = d

d["y"] = 2
print(e)

✔ {'x': 1, 'y': 2}

Because:

• Dict mutated in place
• e points to same object

9️⃣ Why Dict Keys Must Be Immutable

Bad:

d = {[1, 2]: "fail"}

Why forbidden?

• List can change
• Hash would change
• Dict invariants break

Good:

d = {(1, 2): "ok"}

🔟 Dict Views Are LIVE (INTERVIEW FAVORITE)

d = {"a": 1, "b": 2}
keys = d.keys()

d["c"] = 3
print(list(keys))

✔ ['a', 'b', 'c']

Dict views:

• Are not copies
• Reflect real-time changes
• Very efficient

11️⃣ get() vs [] (Subtle but Important)

d["x"]      # KeyError if missing
d.get("x")  # None (or default)

Use get() when:

• Key may not exist
• Defaults make sense

12️⃣ Dictionary Comprehensions

{x: x*x for x in range(5)}

Why good:

• Faster than loops
• Cleaner
• Fewer temporary objects

🔥 INTERVIEW TRAPS (DAY 11)

Q1

d = {}
d[1] = "a"
d[True] = "b"

print(d)

✔ {1: 'b'}

Q2

d = {"a": 1}
d2 = d.copy()
d["a"] = 100

print(d2)

✔ {'a': 1}
(Shallow copy of dict structure)

Q3

d = {"a": 1}
print("a" in d)
print(1 in d)

✔ True
✔ False
(in checks keys, not values)

🧠 DAY 11 MENTAL CHECKLIST

Before using a dict:

1. Are keys immutable?
2. Is equality well-defined?
3. Do I rely on order?
4. Am I mutating shared dicts?

📝 DAY 11 ASSIGNMENT (MANDATORY)

1️⃣ Predict output (NO RUNNING):

d = {}
d[(1, 2)] = "a"
d[(1, 2.0)] = "b"

print(d)

2️⃣ Explain clearly:

• Why dict lookup is O(1) average
• Why Python dict order preservation is not “free magic”

3️⃣ Design question:

Why didn’t Python choose tree-based maps (like Java’s TreeMap) as default?

🔜 DAY 12 PREVIEW

DAY 12 — Control Flow Deep Dive (if / for / while / bytecode)

You’ll learn:

• How if actually works
• for vs while at bytecode level
• Why for is preferred in Python
• Hidden costs of control flow

When ready, say 👉 “START DAY 12”