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Day 26 · ~11 min●

List comprehensions build a list. Dicts have the same shape:

python

ages = {"Ada": 36, "Bob": 41, "Cleo": 28}
# Goal: build a new dict where each age is doubled

A dict comprehension has two expressions, separated by :, mapped from key: value:

python

doubled = {name: age * 2 for name, age in ages.items()}
print(doubled)    # {'Ada': 72, 'Bob': 82, 'Cleo': 56}

Read it: "a dict of name: age * 2 for each name, age pair in ages.items()."

And to get a dict from two parallel lists?

Use zip to pair them up:

python

names = ["Ada", "Bob", "Cleo"]
ages = [36, 41, 28]
lookup = {n: a for n, a in zip(names, ages)}
print(lookup)    # {'Ada': 36, 'Bob': 41, 'Cleo': 28}

zip produces pairs ("Ada", 36), ("Bob", 41), ... — the comprehension turns those into key-value pairs.

Dict comprehension shape

python

{key_expr: value_expr for item in iterable}

Read as: "a dict of key_expr: value_expr for each item in iterable."

Transform an existing dict

python

ages = {"Ada": 36, "Bob": 41, "Cleo": 28}

# Double every value
{n: a * 2 for n, a in ages.items()}
# {'Ada': 72, 'Bob': 82, 'Cleo': 56}

# Uppercase every key, keep the value
{n.upper(): a for n, a in ages.items()}
# {'ADA': 36, 'BOB': 41, 'CLEO': 28}

Build a dict from two lists

python

names = ["Ada", "Bob", "Cleo"]
ages = [36, 41, 28]

{n: a for n, a in zip(names, ages)}
# {'Ada': 36, 'Bob': 41, 'Cleo': 28}

zip turns (names, ages) into an iterable of pairs. The comprehension binds each pair to n, a and emits n: a.

Filter — `if` clause

python

# Only entries where age > 30
{n: a for n, a in ages.items() if a > 30}
# {'Ada': 36, 'Bob': 41}

When NOT to use it

Same rule as list comprehensions: if the per-item logic has more than one expression or has side effects, use a regular for loop. Comprehensions are for transforms, not for general iteration.

Set comprehensions

Sets follow the same shape as lists, just curly brackets without the key::

python

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

Three comprehension shapes — list [...], set {...}, dict {... : ...}.

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Day 26 · ~11 min●

List comprehensions build a list. Dicts have the same shape:

python

ages = {"Ada": 36, "Bob": 41, "Cleo": 28}
# Goal: build a new dict where each age is doubled

A dict comprehension has two expressions, separated by :, mapped from key: value:

python

doubled = {name: age * 2 for name, age in ages.items()}
print(doubled)    # {'Ada': 72, 'Bob': 82, 'Cleo': 56}

Read it: "a dict of name: age * 2 for each name, age pair in ages.items()."

And to get a dict from two parallel lists?

Use zip to pair them up:

python

names = ["Ada", "Bob", "Cleo"]
ages = [36, 41, 28]
lookup = {n: a for n, a in zip(names, ages)}
print(lookup)    # {'Ada': 36, 'Bob': 41, 'Cleo': 28}

zip produces pairs ("Ada", 36), ("Bob", 41), ... — the comprehension turns those into key-value pairs.

Dict comprehension shape

python

{key_expr: value_expr for item in iterable}

Read as: "a dict of key_expr: value_expr for each item in iterable."

Transform an existing dict

python

ages = {"Ada": 36, "Bob": 41, "Cleo": 28}

# Double every value
{n: a * 2 for n, a in ages.items()}
# {'Ada': 72, 'Bob': 82, 'Cleo': 56}

# Uppercase every key, keep the value
{n.upper(): a for n, a in ages.items()}
# {'ADA': 36, 'BOB': 41, 'CLEO': 28}

Build a dict from two lists

python

names = ["Ada", "Bob", "Cleo"]
ages = [36, 41, 28]

{n: a for n, a in zip(names, ages)}
# {'Ada': 36, 'Bob': 41, 'Cleo': 28}

zip turns (names, ages) into an iterable of pairs. The comprehension binds each pair to n, a and emits n: a.

Filter — `if` clause

python

# Only entries where age > 30
{n: a for n, a in ages.items() if a > 30}
# {'Ada': 36, 'Bob': 41}

When NOT to use it

Same rule as list comprehensions: if the per-item logic has more than one expression or has side effects, use a regular for loop. Comprehensions are for transforms, not for general iteration.

Set comprehensions

Sets follow the same shape as lists, just curly brackets without the key::

python

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

Three comprehension shapes — list [...], set {...}, dict {... : ...}.

Dict comprehension shape

Transform an existing dict

Build a dict from two lists

Filter — if clause

When NOT to use it

Set comprehensions

Sign up to practice

Dict comprehension shape

Transform an existing dict

Build a dict from two lists

Filter — if clause

When NOT to use it

Set comprehensions

Dict comprehension shape

Transform an existing dict

Build a dict from two lists

Filter — if clause

When NOT to use it

Set comprehensions

Sign up to practice

Dict comprehension shape

Transform an existing dict

Build a dict from two lists

Filter — if clause

When NOT to use it

Set comprehensions

Filter — `if` clause

Filter — `if` clause

Filter — `if` clause

Filter — `if` clause