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Python Dictionaries

Accessing and modifying elements

1. Accessing values by key:

To retrieve the value associated with a specific key in a dictionary, you use square brackets [] after the dictionary name, followed by the key you want to access:

my_dict = {"name": "Charlie", "age": 40, "city": "Los Angeles"}

# Accessing the value for key "name"
name = my_dict["name"]
print(name)  # Output: Charlie

If you try to access a key that doesn't exist in the dictionary, Python will raise a KeyError. To avoid this, you can use the get() method (explained later).


2.Modifying existing values:

To change the value associated with an existing key, use the same syntax as accessing values, but assign a new value to the key:

my_dict["age"] = 42  # Update the age value

print(my_dict)  # Output: {'name': 'Charlie', 'age': 42, 'city': 'Los Angeles'}


3.Adding new key-value pairs:

To add a new entry to a dictionary, use the same square bracket syntax, but provide a new key that doesn't exist yet and assign its corresponding value:

my_dict["occupation"] = "Software Engineer"

print(my_dict)  # Output: {'name': 'Charlie', 'age': 42, 'city': 'Los Angeles', 'occupation': 'Software Engineer'}


4. Additional considerations:

  • Remember that dictionaries are mutable, meaning you can modify their contents after creation.
  • Keys must be unique and immutable (cannot be changed) to maintain the integrity of the key-value association.
  • It's generally considered good practice to avoid modifying dictionary elements while iterating through them using a loop, as it can lead to unexpected behavior.

5. Alternative for Safe access: get() Method:

The get(key, default=None) method is a safer way to access values in a dictionary. It takes two arguments:

  • key: The key you want to retrieve the value for.
  • default (optional): A default value to return if the key doesn't exist.

value = my_dict.get("occupation", "Not specified")  # Returns "Software Engineer"

# If the key doesn't exist, returns "Not specified"
nonexistent_key = my_dict.get("country", "N/A")

By using get(), you can avoid potential KeyError exceptions and provide a meaningful default value if the key is missing.

It's time to take a quiz!

Test your knowledge and see what you've just learned.

How do you access the value associated with a specific key in a dictionary?

Using parentheses ()
Using square brackets []
Using curly braces {}
Using the get() method
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How do you modify the value of an existing key in a dictionary?

By using the append() method
By directly calling the key
By using the square brackets and assigning a new value
By using the update() method
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How do you add a new key-value pair to a dictionary?

Using the add() method
Using square brackets with an existing key
Using square brackets with a new key
Using the dict.update() method
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What does the get() method do when the specified key does not exist?

Returns None
Returns an empty string
Raises a KeyError
Returns a default value
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What is true about dictionaries in Python?

Dictionaries are mutable
Keys must be mutable
Values must be unique
Dictionaries maintain order
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Creating Dictionaries

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Common dictionary methods