Exam 22026-08-13

Dict and File Practical Exam

60 minutes · 60 points · dict lookup, nested dict, counting, file modes, debugging, and a save/load pair

Instructions

Scope: This exam covers Tutorial 4 only: dict, dict inside dict, and reading and writing text files. You may use for, if, in, not in, del, len, int, str, split, open, with, and try / except. Write the loops yourself: .get(), .items(), .values() and .keys() are not allowed. No class is needed anywhere.

Time

60 min

Total

60 pts

Language

Python

No shortcuts

no .get()/.items()

Teacher note: Sections A to D reward reading code carefully. A student who understood the tutorial but writes slowly can still pass on those alone.

Section A — Code Tracing

3 questions, 4 points each. Write the exact output and one short reason.

Add, change, delete

4 pts

Two of these lines look identical but do different jobs. Say which is which.

python

bag = {"rope": 2, "lamp": 1}bag["lamp"] = 5bag["torch"] = 1del bag["rope"]print(len(bag))print(bag)print("rope" in bag)

A dict inside a dict

4 pts

python

rooms = {    "hall": {"name": "Great Hall", "exits": {"north": "library"}},    "library": {"name": "Library", "exits": {"south": "hall"}},}print(rooms["library"]["name"])print(rooms["hall"]["exits"]["north"])rooms["hall"]["exits"]["east"] = "kitchen"print(rooms["hall"]["exits"])

Counting, then looping

4 pts

Six printed lines in total. The last loop is the one students most often get wrong.

python

items = ["bolt", "rope", "bolt", "bolt", "rope"]counts = {}for item in items:    if item not in counts:        counts[item] = 0    counts[item] = counts[item] + 1print(counts)print(counts["bolt"])for key in counts:    print(key)

Section B — Fill And Complete

4 questions, 14 points total. Fill every blank so the comment on the right comes true.

Look up a room safely

3 pts

A room id that is not a key must not crash the program.

python

ROOMS = {    "hall": {"name": "Great Hall"},    "library": {"name": "Library"},}def room_name(room_id):    if room_id ____ ROOMS:        return "Unknown Room"    return ROOMS[room_id]____print(room_name("library"))   # Libraryprint(room_name("attic"))     # Unknown Room

Two lookups deep

3 pts

Return the room you arrive at, or None when that direction has no exit.

python

ROOMS = {    "hall": {"name": "Great Hall", "exits": {"north": "library"}},    "library": {"name": "Library", "exits": {"south": "hall"}},}def next_room(room_id, direction):    exits = ROOMS[room_id]["exits"]    if direction not in ____:        return ____    return exits[____]print(next_room("hall", "north"))   # libraryprint(next_room("hall", "east"))    # None

A bag that counts

4 pts

A brand new item must end up at 1, and a repeat must count up.

python

def add_item(bag, item):    if item not in bag:        bag[item] = ____    bag[item] = ____ + 1bag = {}add_item(bag, "rope")add_item(bag, "rope")add_item(bag, "lamp")print(bag)     # {'rope': 2, 'lamp': 1}

Use one up and tidy the bag

4 pts

Once a count reaches 0, the key must disappear completely.

python

def remove_item(bag, item):    if item not in bag:        return ____    bag[item] = bag[item] - 1    if bag[item] == 0:        ____ bag[item]    return Truebag = {"rope": 2, "lamp": 1}print(remove_item(bag, "lamp"))    # Trueprint(bag)                         # {'rope': 2}print(remove_item(bag, "torch"))   # False

Section C — Choices

5 questions, 12 points total. Q8-Q10 are single choice. Q11-Q12 are multiple choice.

Single choice: which line adds a pair?

2 pts

Starting from bag = {"rope": 2}, which line ADDS a new pair rather than touching an existing one?

A. bag["rope"] = 5
B. bag["lamp"] = 1
C. del bag["rope"]
D. print(bag["rope"])

Single choice: what does the loop give you?

2 pts

python

bag = {"rope": 2, "lamp": 1}for x in bag:    print(x)

A. The values 2 and 1
B. The keys "rope" and "lamp"
C. Pairs such as ("rope", 2)
D. Nothing, a dict cannot be looped

Q10

Single choice: exact output

2 pts

python

stock = {"a": 1, "b": 2}stock["a"] = stock["a"] + stock["b"]print(stock)

A. {'a': 1, 'b': 2}
B. {'a': 3, 'b': 2}
C. {'a': 2, 'b': 2}
D. 3

Q11

Multiple choice: which lines crash?

3 pts

Each line runs on its own, starting from bag = {"rope": 2}. Which ones raise KeyError?

A. print(bag["rope"])
B. print(bag["lamp"])
C. bag["lamp"] = 1
D. del bag["lamp"]

Q12

Multiple choice: files

3 pts

Which statements are TRUE?

A. open(path, "w") empties the file even before you write anything.
B. open(path, "a") keeps the old lines and writes at the end.
C. open(path, "r") creates the file when it is missing.
D. write() puts a line break at the end of every call.

Section D — Debugging

3 questions, 4 points each. For each bug: say what it prints now, name the bad line, and write the fix.

Q13

The log keeps losing its history

4 pts

python

def append_log(path, message):    with open(path, "w") as log_file:        log_file.write(message + "\n")append_log("log.txt", "opened the door")append_log("log.txt", "took the keycard")with open("log.txt", "r") as log_file:    print(log_file.read())

Q14

Adding up numbers from a file

4 pts

python

with open("scores.txt", "w") as score_file:    score_file.write("ana 30\n")    score_file.write("bo 12\n")total = 0with open("scores.txt", "r") as score_file:    for line in score_file:        parts = line.split()        total = total + parts[1]print(total)

Q15

Searching for a value

4 pts

This should print True, because one room really is red.

python

def has_color(rooms, color):    if color in rooms:        return True    return Falserooms = {"hall": "red", "library": "blue"}print(has_color(rooms, "red"))

Section E — Short Answer And Coding

2 questions, 10 points total. Use clear steps and simple Python.

Q16

Describe the round trip

4 pts

In 4-6 sentences, describe both directions. Name the Python pieces you would use, and say where str() and int() are needed.

python

bag = {"potion": 2, "keycard": 1}# The save file has to end up looking exactly like this:##   item potion 2#   item keycard 1## (a) How do you turn the bag into those lines?# (b) How do you turn those lines back into a bag?

Q17

Write save_scores and load_scores

6 pts

Complete both functions. Saving then loading must give back exactly the same dict, and a missing file must not crash.

python

def save_scores(path, scores):    # Input:    #   path   - a file name, for example "scores.txt"    #   scores - a dict, for example {"ana": 30, "bo": 12}    # Output:    #   Return nothing. Write one line per pair, like:  ana 30    #   Running it a second time must REPLACE the old file, not grow it.    passdef load_scores(path):    # Input:    #   path - a file name    # Output:    #   Return a dict rebuilt from the file.    #   Return an empty dict {} when the file does not exist.    passsave_scores("scores.txt", {"ana": 30, "bo": 12})print(load_scores("scores.txt"))# expected: {'ana': 30, 'bo': 12}print(load_scores("missing.txt"))# expected: {}