理论与实践

数据输入输出

任务

条件语句

任务

for循环

任务

字符串

任务

while循环

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列表

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二维数组

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字典

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集合

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函数与递归

任务

课程 8. Dictionary

难度：

任务«关键含义»

💻 Python

你在电话公司的客服部门工作。你接到的任务是：创建一个程序，用于反转电话簿。原始电话簿以客户姓名作为键，电话号码作为值。你需要创建一个新电话簿，其中电话号码作为键，客户姓名作为值。如果一个电话号码对应多个客户，则将他们的姓名合并到一个列表中。

输入格式

首先输入整数 N & NDash; 表示电话簿中的记录数量。
然后在 N 行中，每行输入由空格分隔的“电话号码”和“姓名”对。

输出格式

反向电话目录以“电话号码”或“电话号码：[姓名1, 姓名2, ...]”的形式显示。电话号码的提取顺序并不重要。

示例

输入

3
Ivan 1234567890
Peter 9876543210
Ivan 9876543210

输出

1234567890: 伊万
9876543210: 彼得, 伊万

提示

Useful methods for working with dictionaries in Python

Dictionaries are one of the most powerful and flexible data structures in Python. To use them effectively, it is important to know the basic methods. Let's take a closer look at them.

The `keys() method`

Returns all dictionary keys as a special representation object dict_keys.

my_dict = {'key1': 'value1', 'key2': 'value2', 'key3': 'value3'}
keys = my_dict.keys()
print(keys)  # Output: dict_keys(['key1', 'key2', 'key3'])

A useful tip: The dict_keys object is iterable, meaning you can use it in a for loop. However, this is not a list. If you need a list of keys (for example, for index access), simply convert it using the list() function.

key_list = list(my_dict.keys())
print(key_list[0]) # Output: key1

The `values() method`

Returns all dictionary values as a representation object dict_values.

values = my_dict.values()
print(values) # Output: dict_values(['value1', 'value2', 'value3'])

A useful tip: Like dict_keys, dict_values can be iterated, but it is not a list. The conversion to list() also works if you need a list of values. This is convenient, for example, to check for the presence of a value.

The `items() method`

Returns all key-value pairs as a dict_items object, where each pair is represented as a tuple.

items = my_dict.items()
print(items) # Output: dict_items([('key1', 'value1'), ('key2', 'value2'), ('key3', 'value3')])

A useful tip: This method is incredibly convenient for iterating through a dictionary when you need both a key and a value at the same time. This is considered the most "Pythonic" way to bypass the dictionary.

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

`get() method`

Returns the value by key. If there is no key, it returns the default value (or None if it is not specified).

value = my_dict.get("key1")
print(value) # Output: value1

value_nonexistent = my_dict.get("key4", "there is no such key")
print(value_nonexistent) # Output: there is no such key

A useful tip: Use get() instead of direct access through square brackets (my_dict["key4"]) when you are not sure if the key exists in the dictionary. This will avoid the KeyError error and the crash of the program. This is much safer when working with data that comes from external sources.

The `popitem() method`

Deletes and returns the last added key-value pair as a tuple.

last_item = my_dict.popitem()
print(last_item) # Output: ('key3', 'value3')
print(my_dict) # Output: {'key1': 'value1', 'key2': 'value2'}

A useful tip: This method works according to the LIFO (Last-In, First-Out) principle. It is useful when you need to process dictionary elements in the reverse order of their addition. (Guaranteed LIFO order is supported in Python 3.7+).

The `update() method`

Updates the dictionary by adding key-value pairs from another dictionary or from an iterable object with pairs.

# The original my_dict dictionary is already without 'key3'
my_dict.update({"key4": "value4", "key1": "new value1"})
print(my_dict)
# Output: {'key1': 'new value1', 'key2': 'value2', 'key4': 'value4'}

A useful tip: Note that if the key from the updating dictionary already exists in the original one, its value will be overwritten. update() is an effective way to merge two dictionaries.

Nested dictionaries

Dictionaries can contain other dictionaries as values. This allows you to create complex hierarchical data structures very similar to the JSON format.

nested_dict = {
    "key1": {"plugg1": "value1"},
"key2": {"plugg2": "value2"}
}

# Accessing the nested value
print(nested_dict["key1"]["plugg1"]) # Output: value1

An example of a multidimensional structure in dictionaries

students = {
    "Alice": {"age": 25, "grades": [88, 92, 85]},
    "Bob": {"age": 22, "grades": [79, 85, 80]},
    "Charlie": {"age": 23, "grades": [90, 87, 85]}
}

# Getting Alice's age
print(students["Alice"]["age"]) # Output: 25

# Getting the first grade Bob
print(students["Bob"]["grades"][0]) # Output: 79

# Adding a new student
students["David"] = {"age": 24, "grades": [82, 79, 88]}
print(students)

A useful tip: When working with nested dictionaries, the risk of getting a KeyError increases. For secure access to the embedded data, you can use a chain of calls get().

# Safely obtaining the age of a student, which may not be
age = students.get("Eve", {}).get("age", "Unknown")
print(age) # Output: Unknown

Here students.get("Eve", {}) will return an empty dictionary {}, since there is no student "Eve", and already this empty dictionary will be called .get("age", ...), which will prevent the error.