Dictionary Methods and Operations in Python (Class 12 Computer Science)

Understand Dictionary Methods and Operations

Dictionary Methods and Operations: key facts

Reading and testing

d[k]	value of k; KeyError if k is missing
d.get(k)	value, or None if missing
d.get(k, v)	value, or v if missing
k in d	True if k is a key (values are not checked)
len(d)	number of pairs

To test a value, use v in d.values().

Adding, changing and removing

d[k] = v	adds k, or replaces its value
d.update(e)	copies all pairs of e; common keys take e's values
del d[k]	removes the pair; KeyError if missing
d.pop(k)	removes the pair and returns its value
d.popitem()	removes and returns the last inserted pair as a tuple
d.clear()	empties d

Only pop() and popitem() give back what they removed.

Creating dictionaries

dict(pairs)	from a sequence of (key, value) pairs
dict.fromkeys(seq, v)	every key in seq gets v (None if v is omitted)
d.setdefault(k, v)	returns d[k] if present, else adds k: v and returns v
d.copy()	a new dictionary with the same pairs

copy() is shallow: a list value is shared by both dictionaries.

Views and functions on keys

d.keys()	dict_keys view of the keys
d.values()	dict_values view
d.items()	dict_items view of (key, value) tuples
max(d), min(d), sorted(d)	work on keys
sorted(d.values())	a sorted list of the values

Wrap a view in list() before indexing it.

Frequency-count pattern

1. 1.freq = {}
2. 2.for x in data:
3. 3.freq[x] = freq.get(x, 0) + 1

The standard answer to 'count the occurrences of each word or character'.

How to answer Dictionary Methods and Operations questions

1. 1Write the dictionary as a table of keys and values in insertion order.
2. 2For each statement, decide whether it adds a key (new key), changes a value (existing key) or removes a pair; new keys go to the end of the order.
3. 3Note what each call returns: pop() gives the value, popitem() gives a (key, value) tuple, setdefault() gives the existing or new value, update() and clear() give None.
4. 4Remember that in, max(), min() and sorted() look at keys unless you pass d.values().
5. 5For a missing key, d[k] raises KeyError but d.get(k) gives None; check which one the code uses.
6. 6In programs, traverse with for k in d: or for k, v in d.items(): and build results in a separate list or dictionary.
7. 7Write printed dictionaries exactly: {'a': 1, 'b': 2}, with a colon and a space after each key.

Dictionary Methods and Operations in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Dictionary Methods and Operations

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Keys are locks, so they must not change

  A key is like a locker number: it must stay fixed, so only immutable types (int, str, tuple) can be keys.

  Use it when: Why a list cannot be a key.

• in looks at the labels

  in, max, min and sorted read the labels (keys), never the contents (values), unless you hand them d.values().

  Use it when: MCQs such as 12 in d.

• Square brackets shout, get() whispers

  d[k] shouts KeyError when k is missing; d.get(k) quietly returns None (or your default).

  Use it when: Choosing between d[k] and get().

• popitem() is a stack pop

  popitem() takes the most recently added pair, like popping the top of a stack.

  Use it when: Output questions on popitem().

Dictionary Methods and Operations: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

d = {'Pen': 12, 'Ink': 6}
print('Pen' in d, 12 in d, d.get('Ruler'))

stock = {'pen': 10, 'pad': 4}
stock.update({'pad': 7, 'ink': 3})
x = stock.pop('pen')
y = stock.setdefault('ink', 50)
z = stock.setdefault('tape', 2)
print(stock, x, y, z)

d = dict.fromkeys(['a', 'b', 'c'], 0)
d['b'] += 5
k = d.popitem()
print(d, k, max(d), sorted(d.values()))

Where students lose marks in Dictionary Methods and Operations

The slips examiners see most often, and what to write instead.

• ✗Mistake: Using in to look for a value

  The in operator checks only the keys of a dictionary.

  Wrong working

  d = {'Pen': 12}
  print(12 in d) # True

  Right working

  print(12 in d) # False
  print(12 in d.values()) # True

  ✓Do this: Use d.values() when the question is about values.

  Usually costs: 1 mark in MCQs.

• ✗Mistake: Reading a missing key with square brackets in a counting loop

  The first time a word is met, d[w] does not exist, so d[w] + 1 raises KeyError.

  Wrong working

  for w in words:
  d[w] = d[w] + 1

  Right working

  for w in words:
  d[w] = d.get(w, 0) + 1

  ✓Do this: Use get() with default 0, or test if w in d first.

  Usually costs: 1 mark in a 3-mark program.

• ✗Mistake: Thinking max(d) gives the largest value

  max, min and sorted on a dictionary work on its keys.

  Wrong working

  max({'a': 9, 'b': 2}) → 9

  Right working

  max({'a': 9, 'b': 2}) → 'b'; max(d.values()) → 9

  ✓Do this: Pass d.values() when values are wanted.

  Usually costs: 1 mark.

• ✗Mistake: Writing the output of keys() as a plain list

  keys(), values() and items() return view objects that print with their type name.

  Wrong working

  print({'a': 1}.keys()) → ['a']

  Right working

  print({'a': 1}.keys()) → dict_keys(['a'])

  ✓Do this: Copy the view's name into the answer, or use list(d.keys()) in code.

  Usually costs: Half to 1 mark.

Practice questions on Dictionary Methods and Operations

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which dictionary method returns the value of a key, or a default value if the key is not present, without raising an error?
   (a) pop() (b) get() (c) keys() (d) items()

   Hint

   It whispers instead of shouting.

   Answer

   (b) get()

2. Q2 · 1 mark · Very short answerBoard level

   Write the output:

   d = {1: 'a', 2: 'b'}
   d[3] = 'c'
   d[1] = 'z'
   print(d, len(d))

   Hint

   One new key and one changed value.

   Answer

   {1: 'z', 2: 'b', 3: 'c'} 3

3. Q3 · 1 mark · MCQBoard level

   What happens when the following code runs?

   d = {'a': 1}
   print(d['b'])

   (a) prints None (b) prints 0 (c) KeyError (d) IndexError

   Hint

   Square brackets on a missing key.

   Answer

   (c) KeyError

4. Q4 · 2 marks · Short answerHarder board level

   Predict the output:

   D = {'x': 3, 'y': 1, 'z': 2}
   T = 0
   for k, v in D.items():
       if v > 1:
           T += v
           D[k] = v * 10
   print(D, T)

   Hint

   Only the values 3 and 2 pass the test.

   Answer

   {'x': 30, 'y': 1, 'z': 20} 5

5. Q5 · 3 marks · Short answerHarder board level

   A dictionary E stores employee names and salaries. Write a function high_salary(E, limit) that returns a list of names whose salary is more than limit. Show the result for E = {'Anil': 52000, 'Bina': 47000, 'Charu': 61000} and limit 50000.

   Hint

   Loop over items() and append the key.

   Answer

   def high_salary(E, limit):
       names = []
       for n, s in E.items():
           if s > limit:
               names.append(n)
       return names
   print(high_salary({'Anil': 52000, 'Bina': 47000, 'Charu': 61000}, 50000))

   Output: ['Anil', 'Charu']

6. Q6 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): A list cannot be used as a key in a Python dictionary.
   Reason (R): Dictionary keys must be of an immutable type.
   (a) Both A and R are true and R is the correct explanation of A. (b) Both A and R are true but R is not the correct explanation of A. (c) A is true but R is false. (d) A is false but R is true.

   Hint

   What error does a list key raise?

   Answer

   (a) Both are true and R explains A: a list is mutable, so it raises TypeError: unhashable type: 'list'.

7. Q7 · 2 marks · Short answerHOTS / competency

   Predict the output and explain why a changed:

   a = {'k': [1, 2], 'n': 5}
   b = a
   c = a.copy()
   b['n'] = 6
   c['k'].append(3)
   c['n'] = 7
   print(a)

   Hint

   b is an alias; c is a new dictionary whose values are shared.

   Answer

   {'k': [1, 2, 3], 'n': 6}. b is the same dictionary as a, so b['n'] = 6 changes a. c is a separate dictionary, so c['n'] = 7 does not, but c['k'] is the same list object as a['k'], so appending 3 shows in a.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What does d.get('x', 0) return if 'x' is not a key?Show answer

   0.

2. 2.Which method removes and returns the last inserted pair?Show answer

   popitem(), as a (key, value) tuple.

3. 3.What does dict.fromkeys(['p', 'q']) give?Show answer

   {'p': None, 'q': None}.

4. 4.Can a tuple be a dictionary key?Show answer

   Yes (if its items are immutable too), because a tuple is immutable.

5. 5.What does setdefault() return when the key already exists?Show answer

   The existing value; the dictionary is not changed.

6. 6.What does sorted({'b': 1, 'a': 2}) return?Show answer

   ['a', 'b'], the sorted keys.

Dictionary Methods and Operations: frequently asked questions