How do I build a dictionary with a comprehension in Python?

The short version

A dictionary comprehension builds a dict in one expression, with the shape {key: value for item in iterable}. Add an if to keep only some items. It reads clearly for simple transformations and is the idiomatic way to invert a dictionary, filter its entries, or build a lookup from a pair of sequences.

The basic shape

squares = {n: n * n for n in range(4)}
print(squares)   # {0: 0, 1: 1, 2: 4, 3: 9}

The part before the colon is the key, the part after is the value, and the for clause supplies the items. It mirrors a list comprehension but produces key-value pairs.

Filtering entries

An if at the end keeps only matching items, which is the clean way to make a smaller dictionary from a larger one.

prices = {"pen": 2, "mug": 5, "cup": 4}
dear = {name: p for name, p in prices.items() if p > 3}
print(dear)   # {'mug': 5, 'cup': 4}

Iterating prices.items() gives the key and value together, and the condition drops the cheap ones.

Inverting a dictionary

Swapping keys and values is a classic use, turning a name-to-code map into a code-to-name map.

codes = {"red": "R", "blue": "B"}
inverse = {v: k for k, v in codes.items()}
print(inverse)   # {'R': 'red', 'B': 'blue'}

Note the catch: if two keys shared a value, inverting collapses them, because a dictionary cannot hold duplicate keys and the last one wins. Only invert when the values are unique.

Building from two sequences

Pairing two lists into a dictionary is neatly done with zip inside a comprehension, or directly with dict(zip(...)).

names = ["pen", "mug"]
prices = [2, 5]
{name: price for name, price in zip(names, prices)}   # {'pen': 2, 'mug': 5}

When to prefer a plain loop

As with list comprehensions, keep them simple. If the key or value needs several steps to compute, or you are nesting conditions, a normal loop is easier to read and debug. The comprehension is for concise, single-expression construction, not for cramming complex logic onto one line.

The related set comprehension

The same brace syntax without a colon builds a set instead, which is handy for collecting the distinct results of a transformation.

{len(word) for word in ["pen", "mug", "cup"]}   # {3}