python tutorial lxml detailed explanation

lxml is an XML and HTML processing library for Python that provides efficient, flexible, and easy-to-use APIs for parsing, manipulating, and generating XML and HTML documents. lxml is based on libxml2 and libxslt C libraries, so it has excellent performance when processing large XML and HTML documents.

introduce

The following is some detailed explanation about lxml:

1. Parsing XML and HTML documents: lxml provides two main parsers, namely C-based parser and Python-based parser. The C-based parser uses the libxml2 library and is faster, while the Python-based parser is easier to install and use. Using the lxml parser, XML and HTML documents can be parsed into a tree-structured Element object, so that the content of the document can be accessed, modified and manipulated by operating the Element object.
2. Element object: The Element object is the main object in lxml, which represents an element or tag in an XML or HTML document. The Element object has rich properties and methods, which can be used to obtain the element's tag name, attributes, text content, child elements, parent elements and other information, and can also be used to add, delete and modify the attributes and content of the element.
3. XPath and CSS selectors: lxml supports using XPath and CSS selectors to locate and search for elements in XML and HTML documents. XPath is a language used to locate elements in XML and HTML documents. It can specify the positional relationship of elements through path expressions. CSS selectors are a syntax similar to CSS, and elements can be positioned through selectors. Using XPath and CSS selectors, you can flexibly locate and manipulate elements in your document.
4. Tree traversal and search: lxml provides a series of methods for tree traversal and search between Element objects. For example, you can use the getchildren() method of the Element object to obtain child elements, use the iter() method of the Element object to iterate through the child elements, and use the find() and findall() methods of the Element object to search for child elements that match the conditions. etc. These methods make it very convenient to locate and manipulate elements in XML and HTML documents.
5. Element operations: lxml allows rich operations on Element objects. For example, you can use the attrib attribute of the Element object to access and modify the attributes of the element, use the text attribute of the Element object to access and modify the text content of the element, and use the append( of the Element object ) and insert() methods to add child elements, use the Element object's remove() method to delete elements, and so on. These methods make modifications to XML and HTML documents simple and intuitive.
6. Document generation: lxml can also be used to generate XML and HTML documents. You can use the Element() function of the Element object to create new elements, you can use the attrib property of the Element object to add and modify the attributes of the element, you can use the text property of the Element object to set the text content of the element, and so on. lxml is a powerful and efficient Python library when parsing and processing XML and HTML documents through combination and nesting. It is based on the libxml2 and libxslt C libraries and provides a rich set of APIs for parsing, manipulating and generating XML and HTML documents. This article will introduce in detail all aspects of the lxml library, including parsers, Element objects, XPath and CSS selectors, tree traversal and search, element operations, document generation, etc., in order to have an in-depth understanding of the usage and functions of lxml.

1. Parser

lxml provides two main parsers, namely C-based parser and Python-based parser.

The C-based parser uses the libxml2 library and is the default parser for lxml. It has high performance and is especially suitable for processing large XML and HTML documents. To use the C-based parser, the lxml library needs to be installed, and the C libraries libxml2 and libxslt are installed.

The Python-based parser uses lxml's own implementation of a pure Python parser and does not rely on external C libraries. It is easier to install and use, and is suitable for use in environments where the C library is not installed. To use the Python-based parser, just specify it explicitly when importing the lxml library, for example:

from lxml import etree

# 使用基于Python的解析器
parser = etree.XMLParser(parser='python')

When using the parser, you can control the parsing process by setting different parameters, such as whether to validate the document, whether to preserve comments and whitespace characters, whether to enable namespaces, etc. You can refer to the lxml official documentation for more details about the parser.

2. Element object

The Element object is the main object in lxml, which represents an element or tag in an XML or HTML document. The Element object has a rich set of properties and methods that can be used to access, modify, and manipulate the content of the document.

Create Element object

You can use the Element() function to create a new Element object, passing in the element's tag name as a parameter. For example, the following code creates an Element object named "book":

from lxml import etree

# 创建一个名为"book"的Element对象
book = etree.Element("book")

You can set the properties and text content of an element by passing in other parameters in the Element() function, for example:

# 创建一个带有属性和文本内容的Element对象
book = etree.Element("book", title="The Great Gatsby", price="10.99")
book.text = "A classic novel"

Access and modify properties of Element objects

The properties of the Element object can be accessed and modified through theattrib properties. attrib is a dictionary that contains all attributes and corresponding values ​​of the element. For example, you can use the following code to access and modify the "title" attribute and value of the "book" element:

# 访问和修改元素的属性
print(book.属性名) 
# 访问属性值 
book.attrib["title"] = "The Catcher in the Rye" 
# 修改属性值

Access and modify the text content of an Element object

The text content of the Element object can be accessed and modified through thetext property. The text attribute stores the text content of the element and can be directly assigned to modify the text content. For example, you can use the following code to access and modify the text content of the "book" element:

# 访问和修改元素的文本内容
print(book.text)  # 访问文本内容
book.text = "A classic novel about teenage angst"  # 修改文本内容

Add child element

You can use theappend() method to add child elements to the Element object. The append() method needs to pass in an Element object as a parameter, indicating the child element to be added. For example, the following code adds a child element named "author" to the "book" element:

from lxml import etree

# 创建一个名为"book"的Element对象
book = etree.Element("book")

# 创建一个名为"author"的Element对象
author = etree.Element("author")
author.text = "J.D. Salinger"

# 将"author"元素添加为"book"元素的子元素
book.append(author)

Remove child elements and attributes

You can use theremove() method to delete child elements from the Element object. The remove() method needs to pass in an Element object as a parameter, indicating the child element to be deleted. For example, the following code removes the child element named "author" from the "book" element:

book.remove(author)  # 从"book"元素中删除"author"子元素

To delete the properties of an Element object, you can use the del keyword, for example:

del book.attrib["title"]  # 删除"book"元素的"title"属性

Other properties and methods of the Element object

The Element object also has many other properties and methods for obtaining and manipulating information about the element. For example:

• tag: Get the tag name of the element
• attrib: Get the attribute dictionary of the element
• get(): Get the value of the specified attribute
• set(): Set the value of the specified attribute
• keys(): Get all attribute names of the element
• items(): Get all attributes and corresponding values ​​of the element
• find(): Find the first element that meets the condition among the child elements of the element
• findall(): Find all elements that match the condition among the child elements of the element
• iter(): Gets the iterator of the element, used to traverse all child elements of the element
• itertext(): Get the text content of the element and its sub-elements, used to iterate through all text content

You can refer to the lxml official documentation for more detailed information about Element objects.

3. XPath and CSS selectors

lxml supports using XPath and CSS selectors to locate and filter elements in the document. XPath is a language used to locate elements in XML and HTML documents, while CSS selectors are a language used to locate elements in HTML documents. lxml provides the xpath() and cssselect() methods, through which elements can be selected and filtered.

Select elements using XPath

XPath uses path expressions to locate elements in the document. Path expressions consist of a series of nodes and operators and are used to describe the positional relationship of elements in the document. For example, the following XPath path expression selects all elements named "book":

# 使用XPath选择元素
books = root.xpath("//book")  # 选择所有名为"book"的元素

// in the XPath path expression means starting from the root node, book means the tag name of the element, so this path expression can select all An element named "book".

You can use various operators and axes in XPath path expressions to position elements more precisely. For example, the following XPath path expression selects the first of all child elements named "book":

# 使用XPath选择元素的子元素
first_book = root.xpath("book[1]")  # 选择第一个名为"book"的元素

In XPath path expressions, [] represents an operator that can be used to filter elements that meet conditions. [1] here means selecting the first element that satisfies the condition.

XPath also supports various functions. For example, the text() function can be used to obtain the text content of the element, and the @ symbol can be used to obtain the attribute value of the element. . For example, the following XPath path expression selects the "title" attribute of all elements named "book":

# 使用XPath选择元素的属性
titles = root.xpath("//book/@title")  # 选择所有名为"book"的元素的"title"属性

You can refer to the XPath syntax rules and function list for more detailed information about XPath.

Select elements using CSS selectors

CSS selector is a language commonly used to locate elements in HTML documents. lxml also supports the use of CSS selectors to select elements. Use the cssselect() method to select elements through CSS selectors. For example, the following code selects all elements named "book":

# 使用CSS选择器选择元素
books = root.cssselect("book")  # 选择所有名为"book"的元素

In the CSS selector, the tag name represents the tag name of the element, and spaces can be used to represent the hierarchical relationship of the elements. For example, the following code selects the first of all child elements named "book":

# 使用CSS选择器选择元素的子元素
first_book = root.cssselect("book:first-child")  # 选择第一个名为"book"的元素

The CSS selector also supports various pseudo-classes and pseudo-elements for more precise positioning of elements. For example, :first-child means selecting the first child element, :last-child means selecting the last child element, :nth-child(n) means selecting the nth child element ,etc. You can refer to the syntax rules of CSS selectors and the list of pseudo-classes and pseudo-elements for more detailed information about CSS selectors.

Modify elements

lxml provides rich methods to modify elements in HTML documents. You can use these methods to add, delete, and modify the tags, attributes, and text content of the element.

Add element

can be added using the , and methods of the Element class element. append()insert()extend()

• append(element): Adds an element as the last element of the current element's children.
• insert(index, element): Add an element as a child element of the current element at the specified position.
• extend(elements): Add multiple elements as the last elements of the current element's child elements.

For example, the following code will add a child element named "book" under the element named "books":

# 添加元素
new_book = Element("book")
new_book.text = "New Book"
books.append(new_book)

Delete element

You can use the method of the Element class to delete elements. remove()

• remove(element): Remove the specified element from the child elements of the current element.

For example, the following code will delete the element named "book":

# 删除元素
book_to_delete = root.cssselect("book")[0]
root.remove(book_to_delete)

Modify element tags and attributes

You can use the and attributes of the Element class to modify the element's tags and attributes. tagattrib

• tag: The tag name of the element, which can be modified directly.
• attrib: The attribute dictionary of the element. The attributes of the element can be modified by modifying the dictionary.

For example, the following code changes the tag name of the element named "book" to "new_book" and changes the value of its "category" attribute to "fiction":

# 修改元素的标签和属性
book_to_modify = root.cssselect("book")[0]
book_to_modify.tag = "new_book"
book_to_modify.attrib["category"] = "fiction"

Modify the text content of an element

You can use the attribute of the Element class to modify the text content of the element. text

• text: The text content of the element can be modified directly.

For example, the following code changes the text content of an element named "title" to "New Title":

# 修改元素的文本内容
title_element = root.cssselect("title")[0]
title_element.text = "New Title"

Serialize HTML documents

lxml provides the function of serializing HTML documents into strings. You can use the method of the Element class. tostring()

• tostring(element, encoding=None, pretty_print=False, method="xml", xml_declaration=None, with_tail=True, standalone=None): Serializes elements into strings.

For example, the following code serializes an element named "root" to a string:

# 序列化HTML文档
html_string = tostring(root, encoding="utf-8", pretty_print=True).decode("utf-8")
print(html_string)

You can specify the encoding method of the output string by modifying the encoding parameter, and use the pretty_print parameter to control whether to use the indented format for output. a>method parameter to specify the output serialization method (default is "xml", you can also select "html").

Summarize

lxml is a powerful and flexible Python library for processing XML and HTML documents. It provides rich functionality including parsing, traversing, searching, modifying and serializing XML and HTML documents. lxml excels when processing large and complex XML and HTML documents because it is C-based, fast and has a low memory footprint.

When using lxml, you can use theElement class to represent elements in XML and HTML documents, and use the methods it provides to parse, traverse, search, modify and serialize operate. You can use XPath and CSS selectors to locate elements, and use the properties and methods of the Element class to obtain and modify the tags, attributes, and text content of the element.

It should be noted that when processing user-entered XML and HTML data, care should be taken to guard against potential security vulnerabilities, such as XXE attacks and XSS attacks. Input data can be validated and filtered using some of the security options provided by lxml to prevent security risks.

I hope that through the detailed explanation in this article, you will have a deeper understanding of the lxml library and be able to give full play to its functions and advantages in actual projects.