EDS 217 Cheatsheet

dict Dictionaries

1. Creating Dictionaries

1.1 Empty Dictionary

my_dict = {}

1.2 Dictionary with Initial Values

my_dict = {"name": "Alice", "age": 30}

1.3 Dictionary with Mixed Data Types

mixed_dict = {"number": 42, "text": "hello", "list": [1, 2, 3], "flag": True}

1.4 Nested Dictionaries

nested_dict = {
    "person1": {"name": "Alice", "age": 25},
    "person2": {"name": "Bob", "age": 30}
}

2. Accessing Elements

2.1 Access by Key

print(my_dict["name"])  # Output: Alice

2.2 Safely Access Using .get()

print(my_dict.get("name"))  # Output: Alice
print(my_dict.get("profession", "Unknown"))  # Output: Unknown if not present

3. Modifying Dictionaries

3.1 Add or Update an Element

my_dict["profession"] = "Engineer"  # Adds a new key or updates if exists

3.2 Remove Elements

del my_dict["age"]  # Removes the key 'age'
profession = my_dict.pop("profession", "No profession found")  # Removes and returns
my_dict.clear()  # Clears all elements

4. Dictionary Operations

4.1 Check if Key Exists

"name" in my_dict  # Returns True if 'name' is a key

4.2 Iterate Through Keys, Values, or Items

for key in my_dict.keys():
    print(key)
for value in my_dict.values():
    print(value)
for key, value in my_dict.items():
    print(f"{key}: {value}")

4.3 Dictionary Comprehensions

squared = {x: x**2 for x in range(5)}  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

4.4 Merge Dictionaries

dict1 = {"name": "Alice", "age": 25}
dict2 = {"city": "New York", "age": 30}
merged = {**dict1, **dict2}  # Python 3.5+ method

5. Common Dictionary Methods

5.1 Get Dictionary Length

len(my_dict)  # Returns the number of key-value pairs

5.2 Copy a Dictionary

new_dict = my_dict.copy()  # Creates a shallow copy of the dictionary

5.3 Get All Keys or Values

all_keys = list(my_dict.keys())
all_values = list(my_dict.values())

5.4 Update Dictionary

my_dict.update({"age": 26, "city": "Boston"})  # Updates and adds multiple keys

5.5 Set Default Value for Key

my_dict.setdefault("age", 29)  # Sets 'age' to 29 if key is not present

6. Common Dictionary Pitfalls

6.1 Avoid Modifying a Dictionary While Iterating

# Incorrect
for key in my_dict:
    if key.startswith('a'):
        del my_dict[key]

# Correct (Using a copy of keys)
for key in list(my_dict.keys()):
    if key.startswith('a'):
        del my_dict[key]