Dictionaries: Look Up Data by Key, Not Position

What You Might Be Wondering

"When do I use a list vs. a dict?"

If you access data by position, use a list. If you access data by field name (name/email/age), use a dict.

One-Line Definition

A dictionary is a key-value mapping structure, perfect for organizing structured data.

Real-Life Analogy

A dict is like a contact book: you look up someone's phone number (value) by their name (key), not by what row they're on.

Minimal Working Example

user = {
    "name": "Alice",
    "age": 25,
    "city": "Sydney",
}
print(user["name"])  # Alice

Reading & Updating

print(user.get("email"))  # None
user["age"] = 26
user["email"] = "a@example.com"

Deleting & Iterating

user.pop("city")
for k, v in user.items():
    print(k, v)

Real Example: Word Frequency Count

text = "python ai python"
counter = {}
for w in text.split():
    counter[w] = counter.get(w, 0) + 1
print(counter)  # {'python': 2, 'ai': 1}

Quick Quiz (5 min)

1. Build a student grades dict.
2. Print each student's name and score.
3. Find and output the top-scoring student.

Quiz Rubric & Grading Criteria

• Direction: write runnable code that covers the core requirements and edge cases from the prompt.
• Criterion 1 (Correctness): main flow produces correct results, key branches execute.
• Criterion 2 (Readability): clear variable names, no excessive nesting.
• Criterion 3 (Robustness): basic protection against empty values, type errors, or unexpected input.

Take-Home Task

Organize a "user registration form" as a dict, then validate that all required fields exist.

Acceptance Criteria

You can independently:

• Create, read, update, and delete dict entries
• Use get() for safe reads
• Use a dict to accumulate counts

Common Errors & Debugging Steps (Beginner Edition)

• Error message looks like gibberish: read the last line for the error type (TypeError, NameError, etc.), then trace back to the offending line.
• Not sure what a variable holds: drop a temporary print(variable, type(variable)) to check.
• Changed code but nothing happened: make sure you saved the file, you're running the right file, and your terminal environment (venv) is correct.

Common Misconceptions

• Misconception: dict[key] is always safe.

• Reality: if the key doesn't exist, it throws a KeyError. Use get() when you're not sure.

• Misconception: you can use lists or dicts as keys.

• Reality: keys must be hashable (typically str, int, or tuple).