Python中字典的基本知识初步介绍
字典是可变的，并且可以存储任意数量的Python对象，包括其他容器类型另一个容器类型。字典包括键对(称为项目)及其相应的值。
Python字典也被称为关联数组或哈希表。字典的一般语法如下：
dict = {'Alice': '2341', 'Beth': '9102', 'Cecil': '3258'}可以用下面的方式创建字典：
dict1 = { 'abc': 456 };
dict2 = { 'abc': 123, 98.6: 37 };
每个按键都来自它的值用冒号(:)，该项目以逗号分隔，整个事情是包含在大括号分隔。没有任何项目一个空的字典是写只有两个大括号，就像这样：{}
键在一个字典中是唯一的，而值可能不是。字典的值可以是任何类型的，但键必须是不可变的数据类型，例如字符串，数字，或元组。
访问字典的值：
要访问字典元素，您可以使用熟悉的方括号一起的关键，获得它的值。下面是一个简单的例子：
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Name']: ", dict['Name'];
print "dict['Age']: ", dict['Age'];
当执行上面的代码中，产生以下结果：
dict['Name']: Zara dict['Age']: 7
如果要访问一个不存在的键，这会得到一个错误，如下所示：
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
print "dict['Alice']: ", dict['Alice'];
当执行上面的代码，产生以下结果：
dict['Zara']: Traceback (most recent call last): File "test.py", line 4, in <module> print "dict['Alice']: ", dict['Alice']; KeyError: 'Alice'
更新字典：
可以通过添加一个新条目或项目（即一个键 - 值对），修改现有条目或删除。作为简单的例子，如下图所示在现有条目更新字词：
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
dict['Age'] = 8; # update existing entry
dict['School'] = "DPS School"; # Add new entry
print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];
当执行上面的代码，产生以下结果：
dict['Age']: 8 dict['School']: DPS School
删除字典元素：
可以删除单个字典元素或清除字典中的全部内容。也可以删除整个字典在一个单一的操作。
要删除整个字典，只要用del语句。下面是一个简单的例子：
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Class': 'First'};
del dict['Name']; # remove entry with key 'Name'
dict.clear(); # remove all entries in dict
del dict ; # delete entire dictionary
print "dict['Age']: ", dict['Age'];
print "dict['School']: ", dict['School'];
这将产生以下结果。注意引发异常，这是因为经过del dict删除，字典已经不存在了：
dict['Age']: Traceback (most recent call last): File "test.py", line 8, in <module> print "dict['Age']: ", dict['Age']; TypeError: 'type' object is unsubscriptable
注：del()方法会在后续的章节中讨论。
字典的键的属性：
字典值没有限制。它们可以是任意Python对象，无论是标准的对象或用户定义的对象。但是作为键，是不可以这样的。
要记住字典中的键的两个要点：
（一）不准一个键对应多个条目。这意味着不能有重复的键。当有重复的键，在分配过程中以最后分配的为准。下面是一个简单的例子：
#!/usr/bin/python
dict = {'Name': 'Zara', 'Age': 7, 'Name': 'Manni'};
print "dict['Name']: ", dict['Name'];
当执行上面的代码，产生以下结果：
dict['Name']: Manni
（二）键的值字必须是不可变的。这意味着可以使用字符串，数字或元组作为字典的键，但像['key']是不允许的。下面是一个简单的例子：
#!/usr/bin/python
dict = {['Name']: 'Zara', 'Age': 7};
print "dict['Name']: ", dict['Name'];
当执行上面的代码，产生以下结果：
Traceback (most recent call last):
File "test.py", line 3, in <module>
dict = {['Name']: 'Zara', 'Age': 7};
TypeError: list objects are unhashable
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
Is there any mobile app that can convert XML into PDF?
Apr 02, 2025 pm 08:54 PM
An application that converts XML directly to PDF cannot be found because they are two fundamentally different formats. XML is used to store data, while PDF is used to display documents. To complete the transformation, you can use programming languages and libraries such as Python and ReportLab to parse XML data and generate PDF documents.
Is the conversion speed fast when converting XML to PDF on mobile phone?
Apr 02, 2025 pm 10:09 PM
The speed of mobile XML to PDF depends on the following factors: the complexity of XML structure. Mobile hardware configuration conversion method (library, algorithm) code quality optimization methods (select efficient libraries, optimize algorithms, cache data, and utilize multi-threading). Overall, there is no absolute answer and it needs to be optimized according to the specific situation.
How to convert XML files to PDF on your phone?
Apr 02, 2025 pm 10:12 PM
It is impossible to complete XML to PDF conversion directly on your phone with a single application. It is necessary to use cloud services, which can be achieved through two steps: 1. Convert XML to PDF in the cloud, 2. Access or download the converted PDF file on the mobile phone.
How to control the size of XML converted to images?
Apr 02, 2025 pm 07:24 PM
To generate images through XML, you need to use graph libraries (such as Pillow and JFreeChart) as bridges to generate images based on metadata (size, color) in XML. The key to controlling the size of the image is to adjust the values of the <width> and <height> tags in XML. However, in practical applications, the complexity of XML structure, the fineness of graph drawing, the speed of image generation and memory consumption, and the selection of image formats all have an impact on the generated image size. Therefore, it is necessary to have a deep understanding of XML structure, proficient in the graphics library, and consider factors such as optimization algorithms and image format selection.
How to open xml format
Apr 02, 2025 pm 09:00 PM
Use most text editors to open XML files; if you need a more intuitive tree display, you can use an XML editor, such as Oxygen XML Editor or XMLSpy; if you process XML data in a program, you need to use a programming language (such as Python) and XML libraries (such as xml.etree.ElementTree) to parse.
Recommended XML formatting tool
Apr 02, 2025 pm 09:03 PM
XML formatting tools can type code according to rules to improve readability and understanding. When selecting a tool, pay attention to customization capabilities, handling of special circumstances, performance and ease of use. Commonly used tool types include online tools, IDE plug-ins, and command-line tools.
Is there a mobile app that can convert XML into PDF?
Apr 02, 2025 pm 09:45 PM
There is no APP that can convert all XML files into PDFs because the XML structure is flexible and diverse. The core of XML to PDF is to convert the data structure into a page layout, which requires parsing XML and generating PDF. Common methods include parsing XML using Python libraries such as ElementTree and generating PDFs using ReportLab library. For complex XML, it may be necessary to use XSLT transformation structures. When optimizing performance, consider using multithreaded or multiprocesses and select the appropriate library.
What is the function of C language sum?
Apr 03, 2025 pm 02:21 PM
There is no built-in sum function in C language, so it needs to be written by yourself. Sum can be achieved by traversing the array and accumulating elements: Loop version: Sum is calculated using for loop and array length. Pointer version: Use pointers to point to array elements, and efficient summing is achieved through self-increment pointers. Dynamically allocate array version: Dynamically allocate arrays and manage memory yourself, ensuring that allocated memory is freed to prevent memory leaks.
