Python Dictionaries

Python Dictionaries: Key-Value Lookups and Safe Access

A Python dictionary (dict) stores data as key: value pairs inside curly braces {}, giving you instant lookups by key name. Since Python 3.7, dictionaries preserve the exact order in which keys were inserted.

Dictionary keys must be unique and hashable (such as strings, numbers, or tuples), while values can be any Python object including nested lists or other dictionaries.

Reading, Updating, and Merging Dictionaries

Using user["role"] raises KeyError when "role" is missing. Calling user.get("role", "guest") safely returns a fallback value instead.

user = {"id": 101, "name": "Farhan", "plan": "starter"}
extra = {"plan": "pro", "verified": True}

# Safe lookup and Python 3.9+ merge operator
role = user.get("role", "member")
merged = user | extra

for key, value in merged.items():
    print(f"{key}: {value}")

Dictionary Comprehensions

You can construct or transform dictionaries concisely with a dictionary comprehension instead of filling an empty dict inside a multi-line for loop. When keys might be missing, .setdefault() or collections.defaultdict also avoids repetitive membership checks.

scores = {"alice": 88, "bob": 52, "carol": 94}
passed = {name: score for name, score in scores.items() if score >= 60}
print(passed)  # {'alice': 88, 'carol': 94}

Common Mistakes to Avoid

• Accessing missing keys with d[key]: Prefer d.get(key) when a key might not exist, or handle KeyError explicitly.
• Modifying a dictionary while iterating over it: Adding or deleting keys during for k in d: raises RuntimeError; iterate over list(d.keys()) instead.
• Using mutable lists as keys: Lists are unhashable and raise TypeError; convert composite keys to tuples first.

Frequently Asked Questions

What is the difference between .keys(), .values(), and .items()?

.keys() iterates over keys, .values() iterates over values, and .items() yields (key, value) pairs for unpacking in loops.

How do I count items cleanly with a dictionary?

Use collections.Counter(items) or counts[key] = counts.get(key, 0) + 1 to avoid manual if key in counts checks.

Does merged = a | b modify dictionary a?

No. The | operator returns a new dictionary, while a |= b updates a in place.