dict.keys() / dict.values() / dict.items()

# Returns a view of all keys in the dictionary.
dict.keys()

# Returns a view of all values in the dictionary.
dict.values()

# Returns a view of all key-value pairs (tuples) in the dictionary.
dict.items()

person = {
    "name": "Alice",
    "age": 20,
    "city": "New York",
    "job": "Engineer"
}

# Iterate over keys using keys().
print(list(person.keys()))  # ['name', 'age', 'city', 'job']
for key in person.keys():
    print(key, end=" ")  # name age city job
# Note: "for key in person:" produces the same result.

# Iterate over values using values().
print(list(person.values()))  # ['Alice', 20, 'New York', 'Engineer']
for value in person.values():
    print(value, end=" ")  # Alice 20 New York Engineer

# Iterate over key-value pairs using items() (the most common pattern).
for key, value in person.items():
    print(f"{key}: {value}")
# name: Alice
# age: 20
# city: New York
# job: Engineer

# Calculate the total and average of scores using values().
scores = {"Math": 85, "English": 92, "History": 78, "Science": 88}
total = sum(scores.values())
average = total / len(scores)
print(f"Total: {total}, Average: {average:.1f}")  # Total: 343, Average: 85.8

# Set operations using keys().
dict1 = {"a": 1, "b": 2, "c": 3}
dict2 = {"b": 20, "c": 30, "d": 40}

# Get the keys that exist in both dictionaries.
common_keys = dict1.keys() & dict2.keys()
print(common_keys)  # {'b', 'c'}

# Get the keys that exist in dict1 but not in dict2.
only_in_dict1 = dict1.keys() - dict2.keys()
print(only_in_dict1)  # {'a'}

# Practical example: transform a dictionary using items().
# Swap keys and values.
original = {"a": 1, "b": 2, "c": 3}
inverted = {v: k for k, v in original.items()}
print(inverted)  # {1: 'a', 2: 'b', 3: 'c'}

# Build a dictionary with only the entries that meet a condition.
filtered = {k: v for k, v in scores.items() if v >= 85}
print(filtered)  # {'Math': 85, 'English': 92, 'Science': 88}

# View objects dynamically reflect changes to the dictionary.
d = {"x": 1}
keys_view = d.keys()
print(list(keys_view))  # ['x']
d["y"] = 2
print(list(keys_view))  # ['x', 'y'] (reflects the updated dictionary)

keys(), values(), and items() each return a view object — a "window" into the dictionary that automatically updates whenever the dictionary changes. To use the result as a list, convert it with list().

Iterating over a dictionary directly with for key in dict: produces the same result as using keys(). Adding or removing entries from a dictionary while iterating over it raises a RuntimeError. If you need to modify the dictionary during iteration, first create a list of the keys (list(dict.keys())) and iterate over that instead.

Since Python 3.7, dictionaries maintain insertion order, so keys() returns keys in the order they were inserted. For safe key lookup in a dictionary, see dict.get() / dict.setdefault().