Python Dictionaries

1. What is a Dictionary?

A dictionary stores key-value pairs. You look up values using their keys — similar to how a list uses indices, except you define your own keys.

x = {2: "hello", "one": 5}

• Keys and values can be any data type (int, string, tuple, etc.)
• Keys and values don't need to be consistent types
• Dictionaries are unordered — don't rely on insertion order

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2. Creating a Dictionary

With initial values

x = {2: "hello", "one": 5}

Empty dictionary

x = {}
x = dict()

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3. Accessing Values

Use square brackets with the key.

x = {2: "hello", "one": 5}
x[2]      # → "hello"
x["one"]  # → 5

⚠️ Accessing a key that doesn't exist raises a KeyError.

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4. Adding & Updating Items

x["key"] = "value"   # adds new key-value pair
x[2] = "world"       # updates existing key (overrides old value)

• If the key doesn't exist → it gets added
• If the key already exists → its value gets overwritten

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5. Deleting Items

Use the del keyword.

del x[2]   # removes the key 2 and its value entirely

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6. Checking if a Key Exists

The in operator checks for keys only (not values).

1 in x    # → True if key 1 exists
1 not in x

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7. Useful Methods

.values() — get all values

x.values()          # → dict_values([...])
list(x.values())    # convert to a regular list

.keys() — get all keys

x.keys()            # → dict_keys([...])
list(x.keys())      # convert to a regular list

.items() — get all key-value pairs as tuples

x.items()           # → dict_items([(key, value), ...])
list(x.items())     # → [(key, value), ...]

.get(key, default) — safe lookup with fallback ⭐

x.get(4, 0)   # returns value at key 4, or 0 if key doesn't exist

Avoids a KeyError when the key might not be present.

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8. Length

len(x)   # number of key-value pairs

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9. Looping Through a Dictionary

By key (then access value manually)

for key in x:
    value = x[key]
    print(key, value)

By key and value together (preferred)

for key, value in x.items():
    print(key, value)

Since dictionaries are unordered, you cannot loop by index using range.

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10. Common Pattern — Frequency Counting

Use .get() to count occurrences without crashing on missing keys.

Count characters in a string

string = "hello world"
characters = {}

for char in string:
    characters[char] = characters.get(char, 0) + 1

print(characters)
# → {'h': 1, 'e': 1, 'l': 3, 'o': 2, ' ': 1, 'w': 1, 'r': 1, 'd': 1}

Count user inputs

counts = {}
while True:
    num = input("Enter a number (q to quit): ")
    if num == "q":
        break
    num = int(num)
    counts[num] = counts.get(num, 0) + 1

print(counts)

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11. Dictionary vs List — Lookup Speed

Operation	List	Dictionary
Check if item exists	Slow — scans every element	Instant
Access by key/index	Fast (by index)	Instant (by key)
Insert item	Fast	Instant

Rule of thumb: If you need to frequently check whether something exists, use a dictionary over a list.

A list must scan every item one by one to find a match. A dictionary looks it up instantly regardless of size.

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Cheat Sheet

# Create
d = {"a": 1, "b": 2}
d = {}

# Access
d["a"]              # → 1 (KeyError if missing)
d.get("a", 0)       # → 1, or 0 if missing

# Add / Update
d["c"] = 3          # add or overwrite

# Delete
del d["a"]

# Check key
"b" in d            # → True

# Length
len(d)              # → number of pairs

# Loop
for key in d: ...
for key, val in d.items(): ...

# All keys / values / pairs
d.keys()
d.values()
d.items()

# Frequency count pattern
d[key] = d.get(key, 0) + 1