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Dict comprehensions — Python Patterns

Day 12 · ~12 min●

Foundations introduced dict comprehensions briefly. Today's lesson is the deeper view — what shape they accept, common patterns, and when to use a for loop instead.

python

words = ["a", "bb", "ccc"]
lengths = {w: len(w) for w in words}
print(lengths)    # {'a': 1, 'bb': 2, 'ccc': 3}

The key: value form on the left, the for source on the right. Like a list comprehension but with a colon in the expression.

Right. Reads: "a dict of w: len(w) for each w in words." Both the key expression and the value expression can use the loop variable.

What if I want to build a dict from two parallel lists?

zip them, then comprehend:

python

keys = ["a", "b", "c"]
values = [1, 2, 3]
{k: v for k, v in zip(keys, values)}    # {'a': 1, 'b': 2, 'c': 3}

(Or directly: dict(zip(keys, values)) — same result, even shorter, but the comprehension form scales when you also want a transform.)

And filtering?

Same if clause as list comprehensions — at the end:

python

{k: v for k, v in pairs if v > 0}    # only positive values

Dict comprehension shape

python

{key_expr: value_expr for item in iterable}
{key_expr: value_expr for item in iterable if condition}

Build from a list — value is a transform of the item

python

words = ["a", "bb", "ccc"]
{w: len(w) for w in words}    # {'a': 1, 'bb': 2, 'ccc': 3}

Build from another dict — transform values

python

ages = {"Ada": 36, "Bob": 41}
{name: age + 1 for name, age in ages.items()}    # {'Ada': 37, 'Bob': 42}

Build from another dict — filter

python

ages = {"Ada": 36, "Bob": 41, "Cleo": 17}
{name: age for name, age in ages.items() if age >= 18}
# {'Ada': 36, 'Bob': 41}

Build from two parallel lists

python

keys = ["a", "b", "c"]
vals = [1, 2, 3]
{k: v for k, v in zip(keys, vals)}    # {'a': 1, 'b': 2, 'c': 3}

Invert a dict (keys become values)

python

ages = {"Ada": 36, "Bob": 41}
{age: name for name, age in ages.items()}
# {36: 'Ada', 41: 'Bob'}

Watch out: if two source values are equal, you'll lose one key — the second one wins.

Anti-pattern — comprehension with side effects

python

{k: print(k) for k in keys}    # builds a dict of None — use a for loop

If the per-item logic doesn't return a useful value, use a regular for loop.

Set comprehension — same shape, no key

Sets get a comprehension too — curly brackets, no colon:

python

{n * 2 for n in [1, 1, 2, 3]}    # {2, 4, 6}

When to use which

You want	Use
List of values	`[expr for x in xs]`
Set (no duplicates)	`{expr for x in xs}`
Dict (key→value)	`{k: v for x in xs}`
The same logic with side effects	regular `for` loop

Day 12 · ~12 min●

Foundations introduced dict comprehensions briefly. Today's lesson is the deeper view — what shape they accept, common patterns, and when to use a for loop instead.

python

words = ["a", "bb", "ccc"]
lengths = {w: len(w) for w in words}
print(lengths)    # {'a': 1, 'bb': 2, 'ccc': 3}

The key: value form on the left, the for source on the right. Like a list comprehension but with a colon in the expression.

Right. Reads: "a dict of w: len(w) for each w in words." Both the key expression and the value expression can use the loop variable.

What if I want to build a dict from two parallel lists?

zip them, then comprehend:

python

keys = ["a", "b", "c"]
values = [1, 2, 3]
{k: v for k, v in zip(keys, values)}    # {'a': 1, 'b': 2, 'c': 3}

(Or directly: dict(zip(keys, values)) — same result, even shorter, but the comprehension form scales when you also want a transform.)

And filtering?

Same if clause as list comprehensions — at the end:

python

{k: v for k, v in pairs if v > 0}    # only positive values

Dict comprehension shape

python

{key_expr: value_expr for item in iterable}
{key_expr: value_expr for item in iterable if condition}

Build from a list — value is a transform of the item

python

words = ["a", "bb", "ccc"]
{w: len(w) for w in words}    # {'a': 1, 'bb': 2, 'ccc': 3}

Build from another dict — transform values

python

ages = {"Ada": 36, "Bob": 41}
{name: age + 1 for name, age in ages.items()}    # {'Ada': 37, 'Bob': 42}

Build from another dict — filter

python

ages = {"Ada": 36, "Bob": 41, "Cleo": 17}
{name: age for name, age in ages.items() if age >= 18}
# {'Ada': 36, 'Bob': 41}

Build from two parallel lists

python

keys = ["a", "b", "c"]
vals = [1, 2, 3]
{k: v for k, v in zip(keys, vals)}    # {'a': 1, 'b': 2, 'c': 3}

Invert a dict (keys become values)

python

ages = {"Ada": 36, "Bob": 41}
{age: name for name, age in ages.items()}
# {36: 'Ada', 41: 'Bob'}

Watch out: if two source values are equal, you'll lose one key — the second one wins.

Anti-pattern — comprehension with side effects

python

{k: print(k) for k in keys}    # builds a dict of None — use a for loop

If the per-item logic doesn't return a useful value, use a regular for loop.

Set comprehension — same shape, no key

Sets get a comprehension too — curly brackets, no colon:

python

{n * 2 for n in [1, 1, 2, 3]}    # {2, 4, 6}

When to use which

You want	Use
List of values	`[expr for x in xs]`
Set (no duplicates)	`{expr for x in xs}`
Dict (key→value)	`{k: v for x in xs}`
The same logic with side effects	regular `for` loop

Dict comprehension shape

Build from a list — value is a transform of the item

Build from another dict — transform values

Build from another dict — filter

Build from two parallel lists

Invert a dict (keys become values)

Anti-pattern — comprehension with side effects

Set comprehension — same shape, no key

When to use which

Dict comprehension shape

Build from a list — value is a transform of the item

Build from another dict — transform values

Build from another dict — filter

Build from two parallel lists

Invert a dict (keys become values)

Anti-pattern — comprehension with side effects

Set comprehension — same shape, no key

When to use which

Dict comprehension shape

Build from a list — value is a transform of the item

Build from another dict — transform values

Build from another dict — filter

Build from two parallel lists

Invert a dict (keys become values)

Anti-pattern — comprehension with side effects

Set comprehension — same shape, no key

When to use which

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Dict comprehension shape

Build from a list — value is a transform of the item

Build from another dict — transform values

Build from another dict — filter

Build from two parallel lists

Invert a dict (keys become values)

Anti-pattern — comprehension with side effects

Set comprehension — same shape, no key

When to use which

Sign up to practice