DSA Interview Prep · Lesson

Dictionaries and Sets in Python

Explore dict and set construction, membership testing, and common patterns like counting frequencies with collections.Counter.

Python Dictionaries: Key-Value Stores

A Python dict maps keys to values with O(1) average lookups, insert, and delete. It is the engine behind two-sum, anagram checks, and frequency counting. The code shows it.

d = {'apple': 3, 'banana': 5}
print(d['apple'])   # 3
d['cherry'] = 7
print(len(d))       # 3
print('banana' in d)  # True
del d['apple']
print(d)            # {'banana': 5, 'cherry': 7}

Safe Lookup with .get()

Reading a missing key with d[key] crashes with a KeyError. Use d.get(key, default) to return a fallback instead — a safe habit that avoids surprise runtime errors.

freq = {}
words = ['the', 'cat', 'sat', 'on', 'the', 'mat']
for w in words:
    freq[w] = freq.get(w, 0) + 1
print(freq)
# {'the': 2, 'cat': 1, 'sat': 1, 'on': 1, 'mat': 1}

print(freq.get('dog', 0))  # 0  (no KeyError)

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