Dictionaries — key-value pairs, access, methods (keys, values, items, get, update)

1.2.24 · Coding › Introduction to Programming (Python)

WHAT is a dictionary?

student = {"name": "Asha", "age": 20, "marks": [88, 91]}

Keys hashable kyun honi chahiye? Python keys ko ek hash table mein store karta hai. Wo key ko ek hash() function se run karta hai taaki kahan value store karni hai ye compute ho sake. Agar key change ho sakti (jaise ek list), toh uska hash bhi change ho jaata, aur Python value "kho" deta. Isliye mutable types keys ke roop mein forbidden hain.

HOW to access values

Action	Syntax	If key missing
Bracket access	`d["name"]`	❌ raises `KeyError`
Safe access	`d.get("name")`	returns `None`
Safe with default	`d.get("name", "?")`	returns `"?"`

Worked example Access in action

d = {"name": "Asha", "age": 20}
print(d["name"])          # Asha
print(d.get("city"))      # None  (no crash!)
print(d.get("city", "NA"))# NA
# print(d["city"])        # KeyError — would crash

.get() kyun use karein? Jab tumhe lagta hai ki key absent ho sakti hai, .get() program ko crash ki jagah continue karne deta hai. Yahi 80% real-world use hai.

Adding, updating, deleting

Worked example Mutation

d = {"name": "Asha"}
d["age"] = 20            # add new pair
d["age"] = 21            # overwrite existing key
d.update({"city": "Pune", "age": 22})  # add/overwrite many
del d["name"]            # remove a pair
val = d.pop("city")      # remove + return its value

Ye step kyun? d["age"] = 20 aur d["age"] = 21 syntactically ek jaise lagte hain, lekin pehla create karta hai, doosra overwrite karta hai — kyunki har key unique hoti hai, existing key ko assign karna replace karta hai, kabhi duplicate nahi.

The core methods (the 20% you use 80% of the time)

Worked example Iterating — the everyday pattern

scores = {"math": 90, "sci": 85, "eng": 78}
 
for k in scores:              # default loops over KEYS
    print(k)                  # math, sci, eng
 
for k, v in scores.items():   # unpack key AND value
    print(k, "->", v)         # math -> 90, ...
 
total = sum(scores.values())  # 253

.items() kyun? Ye tumhe dono cheezein ek saath deta hai, isliye tum loop ke andar slow pattern scores[k] se bachte ho.

Figure — Dictionaries — key-value pairs, access, methods (keys, values, items, get, update)

Derivation-from-scratch: build a tiny dict yourself

Worked example Reinventing the lookup

Socho koi dictionaries exist nahi karti thi. Tum pairs ko do parallel lists mein store karte:

keys   = ["a", "b", "c"]
values = [ 1 ,  2 ,  3 ]
def lookup(k):
    for i in range(len(keys)):   # O(n) scan
        if keys[i] == k:
            return values[i]

Ek dict is poore scan ko ek hash computation se replace kar deti hai. Isliye dict syntax d[k] exist karta hai: ye parallel-list pattern ko automatic aur fast bana deta hai.

Common mistakes

Recall Feynman: ek 12-saal ke bacche ko explain karo

Dictionary ek magic backpack hai jisme labeled pockets hain. Tum poora bag nahi khodते — tum ek label bol dete ho ("snacks") aur sahi pocket seedha khul jaati hai. Har label special hota hai (sirf ek "snacks" pocket hoti hai), aur tum andar koi bhi toy rakh sakte ho. .get() poochhne ka polite tarika hai — agar koi "homework" pocket nahi hai, toh bag sirf kehta hai "yahan kuch nahi" bajaye chillane ke.

Flashcards

Dictionary keys ka type kya hona chahiye?

Unique aur hashable (immutable: str, int, tuple)

Dict lookup list lookup se faster kyun hai?

Dict ek hash table use karta hai → average O(1); list element-by-element scan karti hai → O(n)

d["x"] aur d.get("x") mein kya fark hai?

d["x"] missing hone par KeyError raise karta hai; d.get("x") None (ya diya gaya default) return karta hai

d.items() kya return karta hai?

(key, value) tuples ka ek view

{"a":1, "a":2} ke saath kya hota hai?

Baad wali value jeetti hai → {"a": 2} (keys unique hoti hain)

for x in d iterate karne par kya milta hai?

Keys (values nahi)

Dict b ko dict a mein merge kaise karte ho?

a.update(b) (in place; shared keys overwrite ho jaati hain)

List dictionary key kyun nahi ban sakti?

Lists mutable/unhashable hoti hain; unka hash change ho sakta hai. Tuple use karo.

Key remove karke uski value wapas kaise paate ho?

d.pop("key")

Kya keys()/values()/items() copies hain?

Nahi — ye dynamic views hain jo dict mein changes reflect karti hain.

Connections

Lists — ordered mutable sequences
Tuples — immutable sequences (valid dict keys)
Sets — unique hashable elements (same hash-table machinery)
Hashing and Hash Tables
For loops and iteration
Big-O Notation — time complexity