Dictionary Key-

Overview

Dictionaries are Python's most powerful built-in data structure — O(1) average key lookup via hash tables. Understanding dict creation, valid key types, and basic operations is essential for virtually every real Python project.

Syntax

python

# Creation
person = {"name": "Alice", "age": 30}
empty = {}
built = dict(name="Alice", age=30)

# Access
person["name"]          # "Alice"
person.get("email")     # None (safe, no KeyError)
person.get("email", "")  # "" (default value)

# Modification
person["age"] = 31      # update
person["city"] = "NYC"  # add new key
del person["city"]      # delete

# Check membership
"name" in person        # True

Common Pitfalls

dict[key] raises KeyError if key doesn't exist — use dict.get(key, default) for safe access.
Dict keys must be hashable — lists and dicts cannot be keys, but tuples can.
Interview tip: Since Python 3.7+, dicts maintain insertion order — this is now part of the language spec, not just an implementation detail.

Real-World Example

Frequency counter using a dictionary

example

python

def word_count(text: str) -> dict[str, int]:
    counts: dict[str, int] = {}
    for word in text.lower().split():
        word = word.strip(".,!?")
        counts[word] = counts.get(word, 0) + 1
    return dict(sorted(counts.items(), key=lambda x: -x[1]))

result = word_count("the quick brown fox jumps over the lazy dog")
print(result)  # {'the': 2, 'quick': 1, ...}

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