Creating a Dictionary in Python

In Python, a dictionary is a mutable container model used to store objects of any type. Its elements are key-value pairs. Dictionaries are defined using curly braces {}. Each element consists of a key and a value, separated by a colon (:). Different key-value pairs are separated by a comma (,).

For example, we can use a dictionary to represent a student’s information:

student = {'name': '张三', 'age': 18, 'gender': '男', 'score': {'math': 90, 'chinese': 95, 'english': 85}}

The above code defines a dictionary named student, which contains four key-value pairs:

• The key is name, the value is ‘张三’
• The key is age, the value is 18
• The key is gender, the value is ‘男’
• The key is score, whose value is a nested dictionary representing the student’s grades in each subject.

Creating an Empty Dictionary

If we want to create an empty dictionary, we can use the following method:

empty_dict = {}

Or we can use the dict() function to create one:

empty_dict = dict()

Adding Elements

We can access elements in a dictionary using a key. If the key does not exist, a KeyError exception will be raised.

Now, let’s see how to add new elements to a dictionary. We can achieve this as follows:

# Define an empty dictionary
person = {}

# Add an element to the dictionary
person[‘name’] = ‘张三’
person[‘age’] = 18
person[‘gender’] = ‘男’

print(person) # Output: {‘name’: ‘张三’, ‘age’: 18, ‘gender’: ‘男’}

The above code defines an empty dictionary, person, and adds three key-value pairs to it.

Modifying Elements

If we need to modify an element in a dictionary, we can do so by specifying its key name. For example, the following code changes the age value in the person dictionary to 20:

person['age'] = 20
print(person) # Output: {'name': '张三', 'age': 20, 'gender': '男'}

Deleting Elements

We can use the del or pop methods to delete elements from a dictionary.

Use the del method to delete a key-value pair based on the key name:

# Define a dictionary
person = {'name': '张三', 'age': 18, 'gender': '男'}

# Delete the key-value pair with the gender key
del person['gender']

print(person) # Output: {'name': '张三', 'age': 18}

Use the pop method to delete a key-value pair and return the value corresponding to the key:

person = {'name': '张三', 'age': 18, 'gender': '男'}
gender_value = person.pop('gender')
print(person) # Output: {'name': 'Zhang San', 'age': 18}
print(gender_value) # Output: Male

Accessing Elements

We can access elements in a dictionary using a specified key. If the key does not exist, a KeyError exception will be thrown.

# Define a dictionary
person = {'name': 'Zhang San', 'age': 18, 'gender': 'Male'}

# Accessing Elements in a Dictionary
print(person['name']) # Output: Zhang San
print(person['age']) # Output: 18
print(person['gender']) # Output: Male

You can also use the get method to access elements in a dictionary. Unlike using brackets, if the key does not exist, an exception is not thrown. Instead, None or the specified default value is returned.

person = {'name': '张三', 'age': 18, 'gender': '男'}

# Accessing an element in a dictionary
print(person.get('name')) # Output: Zhang San
print(person.get('score')) # Output: None

The get method can also specify a default value. If the key does not exist, the specified default value is returned.

```python
person = {'name': '张三', 'age': 18, 'gender': '男'}

# Accessing an element in a dictionary
print(person.get('name', 'Default value')) # Output: 张三
print(person.get('score', 'Default value')) # Output: Default value

Traversing a dictionary

We can iterate over all elements in a dictionary using a loop.

person = {'name': '张三', 'age': 18, 'gender': '男'}

# Iterate over the keys in the dictionary
for key in person:
print(key)

# Iterate over the values in the dictionary
for value in person.values():
print(value)

# Iterate over the key-value pairs in the dictionary
for key, value in person.items():
print(key, value)

Running the above code prints the keys, values, and key-value pairs in the dictionary, in order.

Dictionary Methods

Python provides many useful dictionary methods, as shown below:

clear Method

Clears all elements in a dictionary.

person = {'name': '张三', 'age': 18, 'gender': '男'}
person.clear()

copy Method

Copies a dictionary.

person = {'name': '张三', 'age': 18, 'gender': '男'}
person_copy = person.copy()

fromkeys Method

Creates a new dictionary where the values corresponding to the specified keys are default values.

keys = ['name', 'age', 'gender']
person = dict.fromkeys(keys, 'default value')

The above code creates a dictionary named person with three keys: name, age, and gender, each with its own default value.

setdefault Method

Gets the value corresponding to a specified key. If the key does not exist, sets its key-value pair and returns the default value.

person = {'name': '张三', 'age': 18, 'gender': '男'}
name_value = person.setdefault('name', '无名氏')
score_value = person.setdefault('score', {'math': 90, 'chinese': 95, 'english': 85})

print(name_value) # Output: 张三
print(score_value) # Output: {'math': 90, 'chinese': 95, 'english': 85}

The above code uses the setdefault method to retrieve the values corresponding to the existing key ‘name’ and the non-existent key ‘score’, and then sets the key-value pair.

update method

Used to update elements in a dictionary.

person = {'name': '张三', 'age': 18, 'gender': '男'}
person_update = {'age': 20, 'score': {'math': 90, 'chinese': 95, 'english': 85}}
person.update(person_update)

print(person) # Output: {'name': '张三', 'age': 20, 'gender': '男', 'score': {'math': 90, 'chinese': 95, 'english': 85}}

The above code uses the update method to update the element in the dictionary person, changing the value corresponding to the key ‘age’ from 18 to 20, and added a key-value pair, where the key is ‘score’ and the value is a dictionary.

Conclusion

Dictionaries in Python are very practical data structures. They can be used to store objects of any type and provide a rich set of operations. Dictionaries are frequently used in daily development. Mastering dictionary usage is crucial for improving development efficiency and code quality.