11-dictionaries

11 · Dictionaries

Part 2 — Data Structures · Estimated time: 30–40 min · Prerequisite: 09 · Lists

A dictionary stores data as key → value pairs, like a real dictionary maps a word to its definition. Where a list finds things by position, a dictionary finds them by a meaningful name (the key). It's the perfect tool for "look something up": a contact's phone number, a product's price, a player's score.

What you'll learn

• Creating dictionaries and reading values by key
• Adding, updating, and the safe .get() lookup
• Checking for keys with in, and removing with .pop()
• Looping over keys, values, and .items() (both at once)

---

1. Creating and accessing

person = {"name": "Ada", "age": 36}

print(person["name"])    # Ada    look up by key
print(len(person))       # 2      number of pairs

person["age"] = 37       # update an existing key
person["city"] = "London"  # add a new key
print(person)            # {'name': 'Ada', 'age': 37, 'city': 'London'}

Looking up a key that doesn't exist with [] raises a KeyError. Use .get() to ask safely:

print(person.get("email"))         # None  (no crash)
print(person.get("email", "n/a"))  # n/a   (your own default)

▶️ Run it: python examples/01_creating_and_accessing.py

---

2. Looping over a dictionary

scores = {"math": 90, "art": 75}

for subject in scores:            # loops over the KEYS by default
    print(subject)

for score in scores.values():     # just the values
    print(score)

for subject, score in scores.items():   # key AND value together
    print(f"{subject}: {score}")

.items() is the one you'll reach for most — it hands you both pieces at once.

▶️ Run it: python examples/02_looping.py

💡 Try it yourself: loop over scores.items() and print only the subjects where the score is 80 or above.

---

3. Checking and removing

inv = {"apples": 5, "pears": 2}
print("apples" in inv)   # True   (in checks the KEYS)
print("milk" in inv)     # False

removed = inv.pop("pears")   # remove a key and get its value back
print(removed)               # 2
print(inv)                   # {'apples': 5}

inv.update({"apples": 10, "bread": 3})   # update existing + add new
print(inv)                   # {'apples': 10, 'bread': 3}

▶️ Run it: python examples/03_methods.py

---

Common mistakes

Mistake	What happens	Fix
person["email"] when missing	KeyError	Use person.get("email") or check "email" in person.
Expecting in to check values	in checks keys	Use value in person.values() for values.
Using a list as a key	TypeError (unhashable)	Keys must be immutable — use a string, number, or tuple.
Two pairs with the same key	the later one wins	Keys are unique; assigning again overwrites.

---

Recap / cheat-sheet

d = {"a": 1, "b": 2}
d["a"]                 # 1   look up by key
d["c"] = 3             # add / update
d.get("x", default)    # safe lookup
"a" in d               # key membership
d.pop("a")             # remove key, return value
len(d)                 # number of pairs

for k in d: ...            # keys
for v in d.values(): ...   # values
for k, v in d.items(): ... # both

---

Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_lookup.py — look up a value by its key.
2. exercises/02_inventory.py — add to and update a stock dictionary.
3. exercises/03_total_prices.py — total all the values in a dictionary.

Try each yourself before opening the solution.

---

Next → 12 · Comprehensions (coming soon)