Dictionary Loop Strategies

Overview

Iterating over dictionaries correctly is a daily Python task. Knowing when to use .items(), .keys(), and .values() — and how to build transformed dicts with dict comprehensions — separates junior from senior Python developers.

Syntax

python

d = {"a": 1, "b": 2, "c": 3}

# Iterate keys (default)
for key in d:
    print(key)

# Iterate key-value pairs
for key, value in d.items():
    print(f"{key}: {value}")

# Iterate values only
for value in d.values():
    print(value)

# Dict comprehension
squared = {k: v**2 for k, v in d.items()}
# {'a': 1, 'b': 4, 'c': 9}

# Filter with comprehension
filtered = {k: v for k, v in d.items() if v > 1}

Common Pitfalls

Iterating 'for key in dict' gives keys only — easy to forget to use .items() when you need both key and value.
Dict comprehension is Pythonic but can hurt readability with complex expressions — prefer explicit loops for clarity.
Interview tip: sorted(d.items(), key=lambda x: x[1]) sorts a dict by value — returns a list of tuples.

Real-World Example

Invert a dictionary (swap keys and values)

example

python

def invert_dict(d: dict) -> dict:
    if len(d) != len(set(d.values())):
        raise ValueError("Values must be unique to invert")
    return {v: k for k, v in d.items()}

code_map = {"A": 65, "B": 66, "C": 67}
reversed_map = invert_dict(code_map)
print(reversed_map)  # {65: 'A', 66: 'B', 67: 'C'}

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