Building Dicts the Short Way

The idea

Same concept as list and set comprehensions — but now you're building a dict.

Every element produces a key-value pair instead of just a value.

The syntax

{key_expr: value_expr for item in iterable}

Curly braces, a colon between key and value, the rest the same.

squares = {n: n ** 2 for n in range(1, 6)}
print(squares)

Output → {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

"For each n in range — key is n, value is n squared."

From a list — build a lookup dict

Turn a list of names into a dict with name lengths as values:

names = ["Raven", "Wolf", "Ghost", "Viper"]
lengths = {name: len(name) for name in names}
print(lengths)

Output → {'Raven': 5, 'Wolf': 4, 'Ghost': 5, 'Viper': 5}

Transform values in an existing dict

You have scores. You want to know if each soldier passed:

scores = {"Raven": 85, "Wolf": 74, "Ghost": 91}
status = {name: "passed" if score >= 80 else "failed"
          for name, score in scores.items()}
print(status)

Output → {'Raven': 'passed', 'Wolf': 'failed', 'Ghost': 'passed'}

Filter key-value pairs

Keep only the passing scores:

scores = {"Raven": 85, "Wolf": 74, "Ghost": 91, "Viper": 63}
passing = {name: score for name, score in scores.items() if score >= 80}
print(passing)

Output → {'Raven': 85, 'Ghost': 91}

Swap keys and values

original = {"Raven": 85, "Wolf": 74, "Ghost": 91}
swapped  = {score: name for name, score in original.items()}
print(swapped)

Output → {85: 'Raven', 74: 'Wolf', 91: 'Ghost'}

Only works cleanly when values are unique — duplicate values would overwrite each other.

From two lists

names  = ["Raven", "Wolf", "Ghost"]
scores = [85, 74, 91]
result = {name: score for name, score in zip(names, scores)}
print(result)

Output → {'Raven': 85, 'Wolf': 74, 'Ghost': 91}

You saw dict(zip()) earlier — this is the comprehension version. Same result, more flexible.

Heads up!

• Keys must be unique — if the comprehension produces duplicate keys, last one wins
• Use .items() when iterating over an existing dict
• The if goes at the end — after the for
• Swapping keys and values only works when all values are unique

What you should understand now

• Dict comprehension syntax: {key: value for item in iterable}
• Add a filter: {key: value for item in iterable if condition}
• Use .items() to transform or filter an existing dict
• Swap keys and values with {v: k for k, v in d.items()}
• Build from two lists with zip()