Dictionaries - Cataloging Discoveries

Welcome back! Darwin here with another important lesson.

In the previous lesson you learned about lists - ordered collections like expedition trails. But sometimes you need something more - a way to catalog your discoveries with additional information. In the jungle every species has its data card: habitat, size, whether it's dangerous. In Python we use dictionaries for this.

What Are Dictionaries?

A dictionary is a collection of key: value pairs. Imagine a catalog of discoveries where each species (key) has its information card (value).

1# Species catalog - key is the name, value is the details
2species_info = {
3    "Python regius": "Royal python, 1-2m, harmless",
4    "Panthera leo": "Lion, 2-3m, very dangerous",
5    "Loxodonta africana": "African elephant, 3-4m, calm"
6}
7
8# More complex catalog
9detailed_catalog = {
10    "Python regius": {
11        "common_name": "Royal Python",
12        "length_m": 1.5,
13        "habitat": "jungle",
14        "dangerous": False
15    },
16    "Panthera leo": {
17        "common_name": "Lion",
18        "length_m": 2.5,
19        "habitat": "savanna",
20        "dangerous": True
21    }
22}

Key characteristics of dictionaries:

• Key-value pairs - each key leads to a value
• Keep insertion order (since Python 3.7), but have no indices - you get a value by its key
• Keys must be unique and immutable (string, int, tuple)
• Values can be anything - even other dictionaries!
• Mutable - they can be modified

Creating Dictionaries

1# Empty dictionary - clean catalog
2empty_catalog = {}
3empty_dict = dict()
4
5# Dictionary with data
6explorer_data = {
7    "name": "Darwin",
8    "age": 35,
9    "profession": "Researcher",
10    "discoveries": 47
11}
12
13# Different key types (but strings are most common)
14mixed_keys = {
15    "name": "Python",
16    42: "Answer",
17    (10, 20): "Coordinates"
18}
19
20# Nested dictionaries
21expedition = {
22    "location": "Amazonia",
23    "team": {
24        "leader": "Darwin",
25        "count": 5
26    },
27    "equipment": ["Binoculars", "Map", "GPS"]
28}
29
30# dict() constructor
31from_pairs = dict([("A", 1), ("B", 2), ("C", 3)])
32with_kwargs = dict(name="Darwin", age=35, job="Explorer")

Accessing Values

1species = {
2    "Python regius": "Royal Python",
3    "Panthera leo": "Lion",
4    "Loxodonta africana": "Elephant"
5}
6
7# Access by key (square brackets)
8python_name = species["Python regius"]  # "Royal Python"
9lion_name = species["Panthera leo"]     # "Lion"
10
11# KeyError if key doesn't exist!
12# wrong = species["Tyrannosaurus"]  # KeyError!
13
14# Safe access - get()
15python_name = species.get("Python regius")  # "Royal Python"
16trex = species.get("Tyrannosaurus")         # None
17trex = species.get("Tyrannosaurus", "Not found")  # "Not found"
18
19print(f"Python: {python_name}")

get() vs []:

• dict[key] - raises KeyError if key is missing
• dict.get(key) - returns None if key is missing
• dict.get(key, default) - returns default if key is missing

Adding and Modifying

1catalog = {
2    "Python regius": {"length": 1.5, "habitat": "jungle"}
3}
4
5# Adding a new entry
6catalog["Panthera leo"] = {"length": 2.5, "habitat": "savanna"}
7
8# Modifying an existing one
9catalog["Python regius"]["length"] = 1.8
10
11# Adding via update() - dictionary or key-value pairs
12catalog.update({"Loxodonta africana": {"length": 4.0, "habitat": "savanna"}})
13catalog.update(discovered_by="Darwin", year=2024)
14
15print(catalog)

Removing Elements

1species = {
2    "Python": "Python",
3    "Leo": "Lion",
4    "Elephas": "Elephant",
5    "Giraffa": "Giraffe"
6}
7
8# del - remove by key
9del species["Giraffa"]
10
11# pop() - remove and return value
12leo_value = species.pop("Leo")  # "Lion"
13print(leo_value)
14
15# pop() with default (if key doesn't exist)
16dino = species.pop("T-Rex", "Not found")  # "Not found"
17
18# popitem() - remove and return the LAST pair (Python 3.7+)
19last_item = species.popitem()  # ("Elephas", "Elephant")
20
21# clear() - clear the entire dictionary
22species.clear()
23print(species)  # {}

Checking Key Existence

1catalog = {
2    "Python regius": "Python",
3    "Panthera leo": "Lion"
4}
5
6# in - check if key exists
7if "Python regius" in catalog:
8    print("Python is in the catalog!")
9
10if "T-Rex" not in catalog:
11    print("T-Rex is not in the catalog")
12
13# Note: 'in' checks KEYS, not values!
14print("Python" in catalog)  # False (that's a value, not a key!)

Iterating Through a Dictionary

1species = {
2    "Python regius": "Royal Python",
3    "Panthera leo": "Lion",
4    "Loxodonta africana": "African Elephant"
5}
6
7# Iterating through keys (default)
8for species_name in species:
9    print(species_name)
10
11# Explicitly through keys - keys()
12for key in species.keys():
13    print(key)
14
15# Iterating through values - values()
16for common_name in species.values():
17    print(common_name)
18
19# Iterating through key-value pairs - items() (most common!)
20for species_name, common_name in species.items():
21    print(f"{species_name}: {common_name}")
22
23# Enumerate for numbering
24for i, (key, value) in enumerate(species.items(), start=1):
25    print(f"{i}. {key} -> {value}")

Dictionary Methods

keys(), values(), items()

1catalog = {
2    "Python": {"habitat": "jungle", "size": 1.5},
3    "Leo": {"habitat": "savanna", "size": 2.5}
4}
5
6# All keys
7all_keys = catalog.keys()  # dict_keys(['Python', 'Leo'])
8keys_list = list(catalog.keys())  # List: ['Python', 'Leo']
9
10# All values
11all_values = catalog.values()  # dict_values([{...}, {...}])
12
13# All pairs
14all_items = catalog.items()  # dict_items([('Python', {...}), ('Leo', {...})])
15
16# Conversion to lists
17keys = list(catalog.keys())
18values = list(catalog.values())
19items = list(catalog.items())

get() - safe access

1species = {"Python": "Python", "Leo": "Lion"}
2
3# get(key) - None if missing
4result = species.get("Python")  # "Python"
5result = species.get("T-Rex")   # None
6
7# get(key, default) - custom default
8result = species.get("T-Rex", "Not found!")  # "Not found!"

setdefault() - return value OR set a default

1catalog = {"Python": {"count": 5}}
2
3# If key exists - return value
4python_data = catalog.setdefault("Python", {"count": 0})
5print(python_data)  # {'count': 5}
6
7# If key does NOT exist - add with default
8leo_data = catalog.setdefault("Leo", {"count": 0})
9print(leo_data)  # {'count': 0}
10print(catalog)   # {'Python': {...}, 'Leo': {'count': 0}}

update() - update/merge dictionaries

1catalog1 = {"Python": "Python", "Leo": "Lion"}
2catalog2 = {"Elephas": "Elephant", "Giraffa": "Giraffe"}
3new_data = {"Python": "Royal Python"}  # Will overwrite
4
5# Update with dictionary
6catalog1.update(catalog2)
7print(catalog1)  # {'Python': 'Python', 'Leo': 'Lion', 'Elephas': 'Elephant', 'Giraffa': 'Giraffe'}
8
9# Update with key=value pairs
10catalog1.update(discovered_by="Darwin", year=2024)

copy() - copying a dictionary

1# WARNING - this does NOT copy!
2original = {"A": 1, "B": 2}
3reference = original  # Same reference!
4reference["A"] = 999
5print(original)  # {'A': 999, 'B': 2} - changed!
6
7# Proper copying
8original = {"A": 1, "B": 2}
9copy1 = original.copy()  # Shallow copy
10copy2 = dict(original)   # Alternative
11
12copy1["A"] = 999
13print(original)  # {'A': 1, 'B': 2} - unchanged!
14
15# Deep copy (for nested dictionaries)
16import copy
17nested = {"team": {"leader": "Darwin", "size": 5}}
18deep = copy.deepcopy(nested)
19deep["team"]["leader"] = "Alex"
20print(nested)  # {'team': {'leader': 'Darwin', 'size': 5}} - unchanged!

Dictionary Comprehensions

Similar to list comprehensions, you can create dictionaries in a single line!

1# Traditional way
2squares = {}
3for x in range(1, 6):
4    squares[x] = x ** 2
5# {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
6
7# Dict comprehension!
8squares = {x: x ** 2 for x in range(1, 6)}
9# {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}
10
11# With condition
12even_squares = {x: x ** 2 for x in range(1, 11) if x % 2 == 0}
13# {2: 4, 4: 16, 6: 36, 8: 64, 10: 100}
14
15# Swapping keys and values
16species = {"Python": "Python", "Leo": "Lion"}
17reversed_dict = {value: key for key, value in species.items()}
18# {"Python": "Python", "Lion": "Leo"}
19
20# From two lists
21names = ["Python", "Leo", "Elephas"]
22common_names = ["Python", "Lion", "Elephant"]
23catalog = {name: common for name, common in zip(names, common_names)}

Nested Dictionaries

Dictionaries can contain other dictionaries - perfect for complex data!

1# Expedition catalog with details for each species
2species_database = {
3    "Python regius": {
4        "common_name": "Royal Python",
5        "characteristics": {
6            "length_m": 1.5,
7            "weight_kg": 2.5,
8            "habitat": "jungle"
9        },
10        "status": "discovered",
11        "discoverer": "Darwin",
12        "date": "2024-01-15"
13    },
14    "Panthera leo": {
15        "common_name": "Lion",
16        "characteristics": {
17            "length_m": 2.5,
18            "weight_kg": 190,
19            "habitat": "savanna"
20        },
21        "status": "observed",
22        "discoverer": "Team B"
23    }
24}
25
26# Accessing nested values
27python_length = species_database["Python regius"]["characteristics"]["length_m"]
28print(f"Python length: {python_length}m")
29
30# Safe access to deeply nested values
31leo_weight = species_database.get("Panthera leo", {}).get("characteristics", {}).get("weight_kg", "No data")
32print(f"Lion weight: {leo_weight}kg")

Operations on Dictionaries

1# Length (number of key-value pairs)
2species = {"Python": "Python", "Leo": "Lion", "Elephas": "Elephant"}
3count = len(species)  # 3
4
5# Merging dictionaries (Python 3.9+)
6dict1 = {"A": 1, "B": 2}
7dict2 = {"C": 3, "D": 4}
8merged = dict1 | dict2  # {'A': 1, 'B': 2, 'C': 3, 'D': 4}
9
10# Merging with overwriting (Python 3.9+)
11dict1 = {"A": 1, "B": 2}
12dict2 = {"B": 999, "C": 3}
13merged = dict1 | dict2  # {'A': 1, 'B': 999, 'C': 3} - B overwritten!
14
15# |= operator (modifies the first dictionary)
16dict1 |= dict2  # dict1 now contains merged data
17
18# Min/Max of keys
19species = {"Python": 5, "Leo": 12, "Elephas": 3}
20max_key = max(species)  # "Python" (alphabetically)
21max_value = max(species.values())  # 12
22max_by_value = max(species, key=species.get)  # "Leo" (highest value)

Sorting Dictionaries

1species_count = {
2    "Python": 5,
3    "Leo": 12,
4    "Elephas": 3,
5    "Giraffa": 8
6}
7
8# Sort by keys
9sorted_by_key = dict(sorted(species_count.items()))
10# {'Elephas': 3, 'Giraffa': 8, 'Leo': 12, 'Python': 5}
11
12# Sort by values
13sorted_by_value = dict(sorted(species_count.items(), key=lambda x: x[1]))
14# {'Elephas': 3, 'Python': 5, 'Giraffa': 8, 'Leo': 12}
15
16# Sort descending by values
17sorted_desc = dict(sorted(species_count.items(), key=lambda x: x[1], reverse=True))
18# {'Leo': 12, 'Giraffa': 8, 'Python': 5, 'Elephas': 3}

Practical Example - Cataloging System

1# System for cataloging discovered species
2
3species_catalog = {}
4
5def add_species(name, common_name, habitat, dangerous, notes=""):
6    """Add a species to the catalog"""
7    species_catalog[name] = {
8        "common_name": common_name,
9        "habitat": habitat,
10        "dangerous": dangerous,
11        "notes": notes,
12        "discovery_date": "2024-11-24"  # Example date
13    }
14    print(f"Added: {name}")
15
16def show_catalog():
17    """Display the entire catalog"""
18    if not species_catalog:
19        print("Catalog is empty!")
20        return
21
22    print(f"\n=== SPECIES CATALOG ({len(species_catalog)} entries) ===")
23    for name, data in species_catalog.items():
24        danger = "DANGEROUS" if data["dangerous"] else "Safe"
25        print(f"\n{name} ({data['common_name']})")
26        print(f"  Habitat: {data['habitat']}")
27        print(f"  Status: {danger}")
28        if data["notes"]:
29            print(f"  Notes: {data['notes']}")
30
31def search_species(name):
32    """Search for a species by name"""
33    if name in species_catalog:
34        data = species_catalog[name]
35        print(f"\nFound: {name}")
36        print(f"Common name: {data['common_name']}")
37        print(f"Habitat: {data['habitat']}")
38        print(f"Dangerous: {'Yes' if data['dangerous'] else 'No'}")
39    else:
40        print(f"\nNot found: {name}")
41
42def filter_by_habitat(habitat):
43    """Filter species by habitat"""
44    filtered = {name: data for name, data in species_catalog.items()
45                if data["habitat"] == habitat}
46
47    print(f"\nSpecies in habitat '{habitat}': {len(filtered)}")
48    for name in filtered:
49        print(f"  - {name}")
50
51# Using the system
52add_species("Python regius", "Royal Python", "jungle", False, "Calm species")
53add_species("Panthera leo", "Lion", "savanna", True, "Observe from a distance!")
54add_species("Gorilla gorilla", "Gorilla", "jungle", False)
55
56show_catalog()
57search_species("Python regius")
58filter_by_habitat("jungle")

List vs Dictionary - When to Use Which?

Use a list when:

• Order is important
• You need access by numeric index
• You can have duplicates
• Simple collection of values

Use a dictionary when:

• You need fast access by key
• Each element has its own unique label
• Data has a key-value structure
• You want to easily check if an element exists

Practical Exercise

The editor holds the expedition register safari_registry. Complete it:

1. Add the key "vehicle" with the value "jeep"
2. Change "budget_usd" to 20000
3. Use update() to add "species_found": 12 and "photos_taken": 340
4. Remove the key "days_remaining" with del

1# Adding and updating dictionary entries
2
3safari_registry = {
4    "expedition_name": "Serengeti 2024",
5    "leader": "Darwin",
6    "team_size": 5
7}
8
9print("Initial registry:")
10print(safari_registry)
11
12# Adding a new key
13safari_registry["location"] = "Tanzania"
14safari_registry["budget_usd"] = 15000
15
16# Updating an existing key
17safari_registry["team_size"] = 7
18
19print("\nAfter changes:")
20print(safari_registry)
21
22# update() - updates multiple keys at once
23safari_registry.update({
24    "status": "active",
25    "days_remaining": 14,
26    "leader": "Jane"  # Overwrites existing
27})
28
29print("\nAfter update():")
30print(safari_registry)
31
32# TODO: Add key "vehicle" with value "jeep"
33
34
35# TODO: Change "budget_usd" to 20000
36
37
38# TODO: Use update() to add "species_found": 12 and "photos_taken": 340
39
40
41# TODO: Remove the key "days_remaining" using del
42# del safari_registry["?"]
43
44print("\nFinal registry:")
45for key, value in safari_registry.items():
46    print(f"  {key}: {value}")
47

Summary

In this lesson you learned:

• What dictionaries are (key-value pairs)
• Creating and accessing dictionaries
• Adding, modifying, removing elements
• Methods: keys(), values(), items(), get(), update(), setdefault()
• Dictionary comprehensions
• Nested dictionaries
• Iterating through dictionaries
• Sorting and operations on dictionaries

Checkpoint

Before moving on:

• You understand the difference between a list and a dictionary
• You can use get() instead of []
• You know items(), keys(), values()
• You can iterate through a dictionary
• You can create dict comprehensions
• You understand nested dictionaries

In the next lesson Darwin will show you tuples and sets - permanent records and a register of unique species!