Lists, dicts and sets

What you'll learn

How to use a list, a dictionary and a set, and which one to choose for the job.

Why it matters

Real programs work with groups of things: names, scores, settings. These three collections cover most cases, and choosing the right one makes code simpler and faster.

Explanation

List: an ordered, changeable sequence in square brackets, indexed from 0.

Dictionary (dict): key-value pairs in curly braces. Look a value up by its key.

Set: an unordered collection of unique values. Great for removing duplicates and checking membership quickly.

A list comprehension builds a new list in one line: [n * 2 for n in nums if n > 1].

Example

Step-by-step walkthrough

1. max, min and sum / len give 95 60 82.0.
2. sorted(scores) returns a new sorted list: [60, 72, 88, 95, 95]. The original is unchanged.
3. The list comprehension keeps scores of 80 or more: [95, 88, 95].
4. set(scores) removes the duplicate 95, leaving [60, 72, 88, 95] once sorted.
5. We add Zoya to the dict and loop over .items(), which gives each key and value: Asha 30, Ravi 25, Zoya 28.
6. .get("Maya", "unknown") avoids an error for a missing key and prints unknown.

Try it yourself

Count how many times each word appears in ["a", "b", "a", "c", "a"] using a dictionary.

CollectionsPython

Editable codescores = [72, 95, 88, 95, 60] print(sorted(set(scores))) counts = {} for word in ["a", "b", "a", "c", "a"]: counts[word] = counts.get(word, 0) + 1 print(counts)

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Common mistakes

• Using dict["key"] on a missing key, which raises KeyError. Use .get() when unsure.
• Expecting sets to keep order or allow indexing.
• Modifying a list while looping over it. Build a new list instead.
• Using {} for an empty set. It creates an empty dict. Use set().

Quick quiz

Summary

• Lists keep order, dicts map keys to values, sets keep unique items.
• Use .get() for safe dict lookups.
• List comprehensions build lists concisely.

Next: reading and writing files.

tags = {"python", "code", "python"}   # duplicates removed
"code" in tags                         # True

nums = [3, 1, 2]
nums.append(4)       # add to the end
nums.sort()          # [1, 2, 3, 4]
nums[0]              # 1
len(nums)            # 4

user = {"name": "Asha", "age": 30}
user["city"] = "Pune"
user.get("phone", "none")   # safe lookup with a default

scores = [72, 95, 88, 95, 60]

print(max(scores), min(scores), sum(scores) / len(scores))
print(sorted(scores))

high = [s for s in scores if s >= 80]
print(high)

unique = set(scores)
print(sorted(unique))

ages = {"Asha": 30, "Ravi": 25}
ages["Zoya"] = 28
for name, age in ages.items():
    print(name, age)
print(ages.get("Maya", "unknown"))