Dict Comprehension — Squares and Word Lengths

What you will learn

• Dict syntax: {key: value for name in iterable}
• Mapping each number to its square
• Mapping each word to len(word)
• Looping with .items()
• Building the dict inline in a for header

This continues from Set comprehension. Dictionaries: dict data type.

The big idea

A dict comprehension needs a key and a value, separated by a colon, inside curly braces.

d = {int(val): int(val) ** 2 for val in raw.split(",")}

Without the colon you get a set, not a dict.

Squares

Classroom:

print("Enter List of Values separated comma:")
d = {int(val): int(val) ** 2 for val in input().split(",")}

Playground:

raw_nums = "2,3,4,5"
d = {int(val): int(val) ** 2 for val in raw_nums.split(",")}
print("Dict of Element=", d)
for k, v in d.items():
    print(f"\t{k}--->{v}")

Key	Expression	Value
2	2 ** 2	4
3	3 ** 2	9
4	4 ** 2	16

Word lengths

raw_words = "python,ai,lambda"
wordslengths = {word: len(word) for word in raw_words.split(",")}
for w, length in wordslengths.items():
    print(f"\t{w}--->{length}")

Word	len(word)
python	6
ai	2
lambda	6

You can skip the extra variable and iterate the comprehension directly:

for w, length in {word: len(word) for word in raw_words.split(",")}.items():
    print(f"\t{w}--->{length}")

Both forms appear in the teaching programs. The named variable is easier to debug; the one-liner is compact.

Optional if

Keep only longer words:

raw_words = "python,ai,lambda"
long_words = {w: len(w) for w in raw_words.split(",") if len(w) > 3}
print(long_words)

ai is dropped.

Common mistakes

Missing colon — {x for x in nums} is a set of keys with no values.

Unhashable key — keys must be hashable (int, str, tuple of hashables). Do not use a list as a key.

Reusing input() twice in a script — each call waits for a new line. In the playground, keep one string per example.

Interview cheat sheet

Question	Answer
Dict vs set braces?	Dict has key: value
Square mapping?	{n: n**2 for n in nums}
Word length mapping?	{w: len(w) for w in words}
Pair loop?	.items()

Practice in the playground

1. Change squares to cubes (** 3).
2. Add if len(word) > 2 to the word dict.
3. Print wordslengths.keys() and wordslengths.values().

Next: Tuple and generator.