What’s actually slowing this PC down?
Pick the symptom - the matching free tool is one click away.
A Python dictionary comprehension builds a new dictionary by evaluating a key and value expression for each item that passes its optional filters. Its basic form is {key_expression: value_expression for item in iterable if condition}. The colon separates the key from the value; the if clause is optional.
What is the syntax for a dictionary comprehension?
Put the key and value expressions before the for clause, inside braces:
As an Amazon Associate I earn from qualifying purchases.
{key_expression: value_expression for item in iterable if condition}For example, this maps each number from 0 through 4 to its square:
squares = {n: n * n for n in range(5)}The result is {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}. Each item that reaches the pair expression contributes one key/value pair to the new dictionary.
#1 Best Overall
The Python Language Reference describes the form as two expressions separated by a colon, followed by the usual for and if clauses. See Python 3.13.16 Language Reference: Expressions.
How do filters and multiple loops work?
An if clause after the iterable filters items: only those for which the condition is true produce a pair.
even_squares = {n: n * n for n in range(10) if n % 2 == 0}Dictionary comprehensions can also contain additional for and if clauses. Read them as nested loops and filters, in order: the key and value expressions are evaluated for each path that makes it through all the clauses.
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorscoordinates = {(x, y): x + y for x in range(2) for y in range(3)}Here, each value of x is paired with each value of y, so the comprehension produces six entries. Parenthesized tuples are hashable when their elements are hashable, making them valid keys in this example.
How do you transform existing data?
Use expressions that select the key and value you need from each input item. For instance, to map people by identifier:
names_by_id = {person.id: person.name for person in people}This is concise when each input should contribute one straightforward mapping. Check that identifiers are unique if every person’s name must remain in the result: a repeated identifier causes a later name to replace the earlier one.
What happens when keys repeat or are unhashable?
Dictionary keys must be hashable. If the key expression produces an unhashable value, such as a list, insertion fails with a TypeError.
Recommended Free Tools
If multiple iterations produce the same key, the dictionary keeps the value from the last one processed. The comprehension does not warn about duplicate keys. When every value must be preserved, group values into lists or use an explicit loop that handles duplicates deliberately.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How does a comprehension relate to a regular loop?
A dictionary comprehension is a compact way to build a dictionary from a mapping and optional filters. An explicit loop can make the same result while giving you room for multi-step transformations, error handling, or grouping.
| Use a comprehension when | Use an explicit loop when |
|---|---|
| The mapping is a clear key/value expression with a simple filter. | Each item needs several steps or conditional handling. |
| Each generated key is expected to be unique, or replacing earlier values is intended. | Repeated keys must be detected, reported, or grouped without losing values. |
A generator expression is different: it yields values lazily as it is iterated, whereas a dictionary comprehension constructs a dictionary immediately.
What should you know about scope and Python versions?
The comprehension’s iteration variables do not leak into the surrounding scope. Its body runs in an implicitly nested scope, although the iterable expression in the leftmost for is evaluated in the enclosing scope.
Free tools Windows power users keep installed
One-click scans. No signup required.
In Python 3.8 and later, the key expression is evaluated before the value expression. Before 3.8, that order was not specified; CPython evaluated the value first. This distinction matters mainly when expressions have side effects, so keep key and value expressions free of side effects where practical. The version change is documented in the language reference and discussed in PEP 572.
Quick Recap
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
