目次
1. What is a Python dictionary (dict)
A Python dictionary (dict) is a data type that stores key‑value pairs, enabling highly efficient data management. Unlike lists or tuples, a dictionary does not maintain order, but it allows you to quickly retrieve values using keys. It is one of the most commonly used data types in Python programs.Basic characteristics of dictionaries
- Key‑value pairs: A dictionary consists of unique keys and their corresponding values. Keys must be of an immutable type (such as strings or numbers), while values can be of any type.
- No ordering: Python dictionaries have no storage order, so you look up values by specifying a key.
- Value lookup: You can quickly reference a value by specifying its key.
Example of creating a dictionary
The following example demonstrates the basic operations of creating a dictionary and retrieving values using keys.Creating a dictionary
my_dict = {
"apple": "apple",
"banana": "banana",
"cherry": "cherry"
}
Retrieving a value
print(my_dict["apple"]) # Output: apple2. Basic method for adding elements to a Python dictionary
In Python, you add elements to an existing dictionary using a simple assignment statement. This approach is intuitive and easy to understand even for those handling dictionaries for the first time.Basic way to add elements
The syntax for adding a new element to a dictionary is as follows.Adding a new element
my_dict["orange"] = "orange"
print(my_dict)
Output: {'apple': 'apple', 'banana': 'banana', 'cherry': 'cherry', 'orange': 'orange'}Overwriting when using the same key
If you add a new value to a dictionary using an existing key, that value is overwritten.my_dict["banana"] = "banana juice"
print(my_dict)
Output: {'apple': 'apple', 'banana': 'banana juice', 'cherry': 'cherry', 'orange': 'orange'}"banana" is changed, and the original value is overwritten.
3. How to Add Elements to a Dictionary While Avoiding Overwrites
If you want to avoid overwriting, use thesetdefault() method. This method adds a new element only when the specified key does not already exist in the dictionary, so you don’t have to worry about accidentally overwriting a value.setdefault() Example
The following code demonstrates how to use setdefault() to add a new element without overwriting existing ones.my_dict.setdefault("banana", "banana smoothie")
my_dict.setdefault("grape", "grape")
print(my_dict)
Output: {'apple': 'apple', 'banana': 'banana juice', 'cherry': 'cherry', 'orange': 'orange', 'grape': 'grape'}"banana" already exists, so its value is not overwritten, and only the new key "grape" is added.</final
4. How to Merge Multiple Dictionaries and Add Elements
When merging multiple dictionaries into one, you can use theupdate() method. This allows you to add elements from another dictionary all at once, making data management more efficient.update() Method Example
Below is an example of merging two dictionaries using the update() method.dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}
dict1.update(dict2)
print(dict1)
Output: {'a': 1, 'b': 3, 'c': 4}"b" exists in both dictionaries, the value in dict1 was overwritten by the value from dict2. The new key "c" was added to dict1.How to Avoid Overwrites
If you want to merge dictionaries without overwriting, you can combinesetdefault() with a for loop.for key, value in dict2.items():
dict1.setdefault(key, value)
print(dict1)
Output: {'a': 1, 'b': 2, 'c': 4}
5. Advanced Technique: Adding Elements Conditionally
In real-world development, there are cases where you add elements to a dictionary based on conditions. For example, when you only add elements that meet specific criteria to a dictionary, you can dynamically add them usingif statements or for loops.Example of Adding Elements Conditionally
The following code shows an example of adding only fruits whose price meets a certain threshold to the dictionary.items = [("apple", 100), ("banana", 150), ("cherry", 200)]
fruit_dict = {}
for fruit, price in items:
if price > 120:
fruit_dict[fruit] = price
print(fruit_dict)
Output: {'banana': 150, 'cherry': 200}
6. Error Handling When Working with Dictionaries
A common error that occurs when manipulating dictionaries isKeyError. This error occurs when the specified key does not exist in the dictionary. To avoid the error, it’s common to check for the key’s existence or use the get() method.How to Avoid KeyError
The following example shows how to avoid the error using the get() method.value = my_dict.get("pear", "Key not found")
print(value)
Output: Key not found
