有这么一本Python的书: <<Python 网络数据采集>>
我准备用.NET Core及第三方库实现里面所有的例子.
这是第一部分, 主要使用的是AngleSharp: https://anglesharp.github.io/
(文章的章节书与该书是对应的)
发送Http请求
在python里面这样发送http请求, 它使用的是python的标准库urllib:
在.NET Core里面, 你可以使用HttpClient, 相应的C#代码如下:
var client = new HttpClient();
HttpResponseMessage response = await client.GetAsync("http://pythonscraping.com/pages/page1.html");
response.EnsureSuccessStatusCode();
var responseBody = await response.Content.ReadAsStringAsync();
Console.WriteLine(responseBody);
return responseBody;
或者可以简写为:
var client = new HttpClient();
var responseBody = await client.GetStringAsync("http://pythonscraping.com/pages/page1.html");
Console.WriteLine(responseBody);
其结果如下:
使用AngleSharp解析html源码
python里面可以使用BeautifulSoup或者MechanicalSoup等库对html源码进行解析.
而.NET Core可以使用AngleSharp, Html Agility Pack, DotnetSpider(国产, 也支持元素抽取).等库来操作Html文档.
这里我先使用的是AngleSharp, AngleSharp的解析库可以使用标准的W3C规范来解析HTML, MathML, XML, SVG和CSS. 它支持.NET Standard 1.0.
安装AngleSharp
通过Nuget即可: https://www.nuget.org/packages/AngleSharp/
Install-Package AngleSharp
或者dotnet-cli:
dotnet add package AngleSharp
AngleSharp的一个简单例子
下面这个例子(1.2.2)是把页面中h1元素的内容显示出来.
书中Python的代码:
下面是.NET Core的C#代码:
public static async Task ReadWithAngleSharpAsync()
{
var htmlSourceCode = await SendRequestWithHttpClientAsync();
var parser = new HtmlParser();
var document = await parser.ParseAsync(htmlSourceCode);
Console.WriteLine($"Serializing the (original) document: {document.QuerySelector("h1").OuterHtml}");
Console.WriteLine($"Serializing the (original) document: {document.QuerySelector("html > body > h1").OuterHtml}");
}
在这里AngleSharp首先需要创建一个可以循环使用的HtmlParser(Html解析器), 然后使用解析器解析html源码即可: parser.Parse() 或者异步版本 parser.ParseAsync().
解析返回对象的类型是IHtmlDocument, 里面是解析好的DOM. 其中DOM是和AngleSharp里的类这样对应的:
这个图其实是老一点的版本, 新版本的DOM模型是稍微有点不同的, 不过你只要理解这个意思就行...
AngleSharp有很多特点, 但是最重要的特点就是它支持querySelector()和querySelectorAll()方法, 就像DOM的方法一样.
上面这个例子里, 其html的结构大致如下:
所以针对返回的IHtmlDocument对象document, 我们使用document.QuerySelector("h1").OuterHtml, 就可以返回h1的OuterHtml. 而使用document.QuerySelector("html > body > h1").OuterHtml 也是同样的效果, 因为标准的CSS选择器是都支持的.
QuerySelector()返回的是一个/0个元素, 相当于Linq的FirstOrDefault().
其运行结果如下:
异常情况处理
发送Http请求之后, 可能会发生错误, 例如网页不存在(或者请求时出错), 服务器不存在等等.
针对这些情况, .NET Core程序会返回HTTP错误, 可能是404也可能是500等. 但是所有的类型HttpClient都会抛出HttpRequestException, 我们可以这样处理这种异常:
public static async Task ResponseWithErrorsAsync()
{
try
{
var client = new HttpClient();
var responseBody = await client.GetStringAsync("http://notexistwebsite");
Console.WriteLine(responseBody);
}
catch (HttpRequestException e)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("\nException Caught!");
Console.WriteLine("Message :{0} ", e.Message);
}
}
但是即使网页获取成功了, 网页上的内容也并非完全是我们所期待的, 仍可能会抛出异常. 比如说你想要找的标签不存在, 那么就会返回null, 然后再调用改标签的属性, 就会发生NullReferenceException.
所以这种情况可以捕获NullReferenceException, 也可以使用代码判断:
public static async Task ReadNonExistTagAsync()
{
var htmlSourceCode = await SendRequestWithHttpClientAsync();
var parser = new HtmlParser();
var document = await parser.ParseAsync(htmlSourceCode);
var nonExistTag = document.QuerySelector("h8");
Console.WriteLine(nonExistTag);
Console.WriteLine($"nonExistTag is null: {nonExistTag is null}");
try
{
Console.WriteLine(nonExistTag.QuerySelector("p").OuterHtml);
}
catch (NullReferenceException)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("Tag was not found");
}
}
完整的例子:
public static async Task RunAllAsync()
{
Console.ForegroundColor = ConsoleColor.Red;
async Task<string> GetTileAsync(string uri)
{
var httpClient = new HttpClient();
try
{
var responseHtml = await httpClient.GetStringAsync(uri);
var parser = new HtmlParser();
var document = await parser.ParseAsync(responseHtml);
var tagContent = document.QuerySelector("body > h8").TextContent;
return tagContent;
}
catch (HttpRequestException e)
{
Console.WriteLine($"{nameof(HttpRequestException)}:");
Console.WriteLine("Message :{0} ", e.Message);
return null;
}
catch (NullReferenceException)
{
Console.WriteLine($"{nameof(NullReferenceException)}:");
Console.WriteLine("Tag was not found");
return null;
}
}
var title = await GetTileAsync("http://www.pythonscraping.com/pages/page1.html");
if (string.IsNullOrWhiteSpace(title))
{
Console.WriteLine("Title was not found");
}
else
{
Console.ForegroundColor = ConsoleColor.Green;
Console.WriteLine(title);
}
}
首先我把请求Http返回HTML代码的那部分封装成了一个方法以便复用:
public static async Task<string> GetHtmlSourceCodeAsync(string uri)
{
var httpClient = new HttpClient();
try
{
var htmlSource = await httpClient.GetStringAsync(uri);
return htmlSource;
}
catch (HttpRequestException e)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine($"{nameof(HttpRequestException)}: {e.Message}");
return null;
}
}
CSS是网络爬虫的福音, 下面这两个元素在页面中可能会出现很多次:
我们可以使用AngleSharp里面的QuerySelectorAll()方法把所有符合条件的元素都找出来, 返回到一个结果集合里.
public static async Task FindGreenClassAsync()
{
const string url = "http://www.pythonscraping.com/pages/warandpeace.html";
var html = await GetHtmlSourceCodeAsync(url);
if (!string.IsNullOrWhiteSpace(html))
{
var parser = new HtmlParser();
var document = await parser.ParseAsync(html);
var nameList = document.QuerySelectorAll("span > .green");
Console.WriteLine("Green names are:");
Console.ForegroundColor = ConsoleColor.Green;
foreach (var item in nameList)
{
Console.WriteLine(item.TextContent);
}
}
else
{
Console.WriteLine("No html source code returned.");
}
}
非常简单, 和DOM的标准操作是一样的.
如果只需要元素的文字部分, 那么就是用其TextContent属性即可.
再看个例子
1. 找出页面中所有的h1, h2, h3, h4, h5, h6元素
2. 找出class为green或red的span元素.
public static async Task FindByAttributeAsync()
{
const string url = "http://www.pythonscraping.com/pages/warandpeace.html";
var html = await GetHtmlSourceCodeAsync(url);
if (!string.IsNullOrWhiteSpace(html))
{
var parser = new HtmlParser();
var document = await parser.ParseAsync(html);
var headers = document.QuerySelectorAll("*")
.Where(x => new[] { "h1", "h2", "h3", "h4", "h5", "h6" }.Contains(x.TagName.ToLower()));
Console.WriteLine("Headers are:");
PrintItemsText(headers);
var greenAndRed = document.All
.Where(x => x.TagName == "span" && (x.ClassList.Contains("green") || x.ClassList.Contains("red")));
Console.WriteLine("Green and Red spans are:");
PrintItemsText(greenAndRed);
var thePrinces = document.QuerySelectorAll("*").Where(x => x.TextContent == "the prince");
Console.WriteLine(thePrinces.Count());
}
else
{
Console.WriteLine("No html source code returned.");
}
void PrintItemsText(IEnumerable<IElement> elements)
{
foreach (var item in elements)
{
Console.WriteLine(item.TextContent);
}
}
}
这里我们可以看到QuerySelectorAll()的返回结果可以使用Linq的Where方法进行过滤, 这样就很强大了.
TagName属性就是元素的标签名.
此外, 还有一个document.All, All属性是该Document所有元素的集合, 它同样也支持Linq.
(该方法中使用了一个本地方法).
由于同时支持CSS选择器和Linq, 所以抽取元素的工作简单多了.
导航树
一个页面, 它的结构可以是这样的:
这里面有几个概念:
子标签和后代标签.
子标签是父标签的下一级, 而后代标签则是指父标签下面所有级别的标签.
tr是table的子标签, tr, th, td, img都是table的后代标签.
使用AngleSharp, 找出子标签可以使用.Children属性. 而找出后代标签, 可以使用CSS选择器.
兄弟标签
找到前一个兄弟标签使用.PreviousElementSibling属性, 后一个兄弟标签是.NextElementSibling属性.
父标签
.ParentElement属性就是父标签.
public static async Task FindDescendantAsync()
{
const string url = "http://www.pythonscraping.com/pages/page3.html";
var html = await GetHtmlSourceCodeAsync(url);
if (!string.IsNullOrWhiteSpace(html))
{
var parser = new HtmlParser();
var document = await parser.ParseAsync(html);
var tableChildren = document.QuerySelector("table#giftList > tbody").Children;
Console.WriteLine("Table's children are:");
foreach (var child in tableChildren)
{
System.Console.WriteLine(child.LocalName);
}
var descendants = document.QuerySelectorAll("table#giftList > tbody *");
Console.WriteLine("Table's descendants are:");
foreach (var item in descendants)
{
Console.WriteLine(item.LocalName);
}
var siblings = document.QuerySelectorAll("table#giftList > tbody > tr").Select(x => x.NextElementSibling);
Console.WriteLine("Table's descendants are:");
foreach (var item in siblings)
{
Console.WriteLine(item?.LocalName);
}
var parentSibling = document.All.SingleOrDefault(x => x.HasAttribute("src") && x.GetAttribute("src") == "../img/gifts/img1.jpg")
?.ParentElement.PreviousElementSibling;
if (parentSibling != null)
{
Console.WriteLine($"Parent's previous sibling is: {parentSibling.TextContent}");
}
}
else
{
Console.WriteLine("No html source code returned.");
}
}
结果:
使用正则表达式
"如果你有一个问题打算使用正则表达式来解决, 那么现在你有两个问题了".
这里有一个测试正则表达式的网站: https://www.regexpal.com/
目前, AngleSharp支持通过CSS选择器来查找元素, 也可以使用Linq来过滤元素, 当然也可以通过多种方式使用正则表达式进行更复杂的查找动作.
关于正则表达式我就不介绍了. 直接看例子.
我想找到页面中所有的满足下列要求的图片, 其src的值以../img/gifts/img开头并且随后跟着数字, 然后格式为.jpg的图标.
public static async Task FindByRegexAsync()
{
const string url = "http://www.pythonscraping.com/pages/page3.html";
var html = await GetHtmlSourceCodeAsync(url);
if (!string.IsNullOrWhiteSpace(html))
{
var parser = new HtmlParser();
var document = await parser.ParseAsync(html);
var images = document.QuerySelectorAll("img")
.Where(x => x.HasAttribute("src") && Regex.Match(x.Attributes["src"].Value, @"\.\.\/img\/gifts/img.*\.jpg").Success);
foreach (var item in images)
{
Console.WriteLine(item.Attributes["src"].Value);
}
var elementsWith2Attributes = document.All.Where(x => x.Attributes.Length == 2);
foreach (var item in elementsWith2Attributes)
{
Console.WriteLine(item.LocalName);
foreach (var attr in item.Attributes)
{
Console.WriteLine($"\t{attr.Name} - {attr.Value}");
}
}
}
else
{
Console.WriteLine("No html source code returned.");
}
}
这个其实没有任何难度.
但从本例可以看到, 判断元素有没有一个属性可以使用HasAttribute("xxx")方法, 可以通过.Attributes索引来获取属性, 其属性值就是.Attributes["xxx"].Value.
如果不会正则表达式, 我相信多写的Linq的过滤代码也差不多能达到要求.
遍历单个域名
就是几个应用的例子, 直接贴代码吧.
打印出一个页面内所有的超链接地址:
public static async Task TraversingASingleDomainAsync()
{
var httpClient = new HttpClient();
var htmlSource = await httpClient.GetStringAsync("http://en.wikipedia.org/wiki/Kevin_Bacon");
var parser = new HtmlParser();
var document = await parser.ParseAsync(htmlSource);
var links = document.QuerySelectorAll("a");
foreach (var link in links)
{
Console.WriteLine(link.Attributes["href"]?.Value);
}
}
找出满足下列条件的超链接:
在id为bodyContent的div里
url不包括分号
url以/wiki开头
public static async Task FindSpecificLinksAsync()
{
var httpClient = new HttpClient();
var htmlSource = await httpClient.GetStringAsync("http://en.wikipedia.org/wiki/Kevin_Bacon");
var parser = new HtmlParser();
var document = await parser.ParseAsync(htmlSource);
var links = document.QuerySelector("div#bodyContent").QuerySelectorAll("a")
.Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)((?!:).)*$").Success);
foreach (var link in links)
{
Console.WriteLine(link.Attributes["href"]?.Value);
}
}
随机找到页面里面一个连接, 然后递归调用自己的方法, 直到主动停止:
private static async Task<IEnumerable<IElement>> GetLinksAsync(string uri)
{
var httpClient = new HttpClient();
var htmlSource = await httpClient.GetStringAsync($"http://en.wikipedia.org{uri}");
var parser = new HtmlParser();
var document = await parser.ParseAsync(htmlSource);
var links = document.QuerySelector("div#bodyContent").QuerySelectorAll("a")
.Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)((?!:).)*$").Success);
return links;
}
public static async Task GetRandomNestedLinksAsync()
{
var random = new Random();
var links = (await GetLinksAsync("/wiki/Kevin_Bacon")).ToList();
while (links.Any())
{
var newArticle = links[random.Next(0, links.Count)].Attributes["href"].Value;
Console.WriteLine(newArticle);
links = (await GetLinksAsync(newArticle)).ToList();
}
}
采集整个网站
首先要了解几个概念:
浅网 surface web: 是互联网上搜索引擎可以直接抓取到的那部分网络.
与浅网对立的就是深网 deep web: 互联网中90%都是深网.
暗网Darknet / dark web / dark internet: 它完全是另外一种怪兽. 它们也建立在已有的网络基础上, 但是使用Tor客户端, 带有运行在HTTP之上的新协议, 提供了一个信息交换的安全隧道. 这类网也可以采集, 但是超出了本书的范围.....
深网相对暗网还是比较容易采集的.
采集整个网站的两个好处:
生成网站地图
收集数据
由于网站的规模和深度, 所以采集到的超链接很多可能是重复的, 这时我们就需要链接去重, 可以使用Set类型的集合:
private static readonly HashSet<string> LinkSet = new HashSet<string>();
private static readonly HttpClient HttpClient = new HttpClient();
private static readonly HtmlParser Parser = new HtmlParser();
public static async Task GetUniqueLinksAsync(string uri = "")
{
var htmlSource = await HttpClient.GetStringAsync($"http://en.wikipedia.org{uri}");
var document = await Parser.ParseAsync(htmlSource);
var links = document.QuerySelectorAll("a")
.Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)").Success);
foreach (var link in links)
{
if (!LinkSet.Contains(link.Attributes["href"].Value))
{
var newPage = link.Attributes["href"].Value;
Console.WriteLine(newPage);
LinkSet.Add(newPage);
await GetUniqueLinksAsync(newPage);
}
}
}
(递归调用的深度需要注意一下, 不然有时候能崩溃).
收集整个网站数据
这个例子相对网站, 包括收集相关文字和异常处理等:
private static readonly HashSet<string> LinkSet = new HashSet<string>();
private static readonly HttpClient HttpClient = new HttpClient();
private static readonly HtmlParser Parser = new HtmlParser();
public static async Task GetLinksWithInfoAsync(string uri = "")
{
var htmlSource = await HttpClient.GetStringAsync($"http://en.wikipedia.org{uri}");
var document = await Parser.ParseAsync(htmlSource);
try
{
var title = document.QuerySelector("h1").TextContent;
Console.ForegroundColor = ConsoleColor.Green;
Console.WriteLine(title);
var contentElement = document.QuerySelector("#mw-content-text").QuerySelectorAll("p").FirstOrDefault();
if (contentElement != null)
{
Console.WriteLine(contentElement.TextContent);
}
var alink = document.QuerySelector("#ca-edit").QuerySelectorAll("span a").SingleOrDefault(x => x.HasAttribute("href"))?.Attributes["href"].Value;
Console.WriteLine(alink);
}
catch (NullReferenceException)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine("Cannot find the tag!");
}
var links = document.QuerySelectorAll("a")
.Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)").Success).ToList();
foreach (var link in links)
{
if (!LinkSet.Contains(link.Attributes["href"].Value))
{
var newPage = link.Attributes["href"].Value;
Console.WriteLine(newPage);
LinkSet.Add(newPage);
await GetLinksWithInfoAsync(newPage);
}
}
}
不知前方水深的例子
第一个例子, 寻找随机外链:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using AngleSharp.Parser.Html;
namespace WebScrapingWithDotNetCore.Chapter03
{
public class CrawlingAcrossInternet
{
private static readonly Random Random = new Random();
private static readonly HttpClient HttpClient = new HttpClient();
private static readonly HashSet<string> InternalLinks = new HashSet<string>();
private static readonly HashSet<string> ExternalLinks = new HashSet<string>();
private static readonly HtmlParser Parser = new HtmlParser();
public static async Task FollowExternalOnlyAsync(string startingSite)
{
var externalLink = await GetRandomExternalLinkAsync(startingSite);
if (externalLink != null)
{
Console.WriteLine($"External Links is: {externalLink}");
await FollowExternalOnlyAsync(externalLink);
}
else
{
Console.WriteLine("Random External link is null, Crawling terminated.");
}
}
private static async Task<string> GetRandomExternalLinkAsync(string startingPage)
{
try
{
var htmlSource = await HttpClient.GetStringAsync(startingPage);
var externalLinks = (await GetExternalLinksAsync(htmlSource, SplitAddress(startingPage)[0])).ToList();
if (externalLinks.Any())
{
return externalLinks[Random.Next(0, externalLinks.Count)];
}
var internalLinks = (await GetInternalLinksAsync(htmlSource, startingPage)).ToList();
if (internalLinks.Any())
{
return await GetRandomExternalLinkAsync(internalLinks[Random.Next(0, internalLinks.Count)]);
}
return null;
}
catch (HttpRequestException e)
{
Console.WriteLine($"Error requesting: {e.Message}");
return null;
}
}
private static string[] SplitAddress(string address)
{
var addressParts = address.Replace("http://", "").Replace("https://", "").Split("/");
return addressParts;
}
private static async Task<IEnumerable<string>> GetInternalLinksAsync(string htmlSource, string includeUrl)
{
var document = await Parser.ParseAsync(htmlSource);
var links = document.QuerySelectorAll("a")
.Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, $@"^(/|.*{includeUrl})").Success)
.Select(x => x.Attributes["href"].Value);
foreach (var link in links)
{
if (!string.IsNullOrEmpty(link) && !InternalLinks.Contains(link))
{
InternalLinks.Add(link);
}
}
return InternalLinks;
}
private static async Task<IEnumerable<string>> GetExternalLinksAsync(string htmlSource, string excludeUrl)
{
var document = await Parser.ParseAsync(htmlSource);
var links = document.QuerySelectorAll("a")
.Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, $@"^(http|www)((?!{excludeUrl}).)*$").Success)
.Select(x => x.Attributes["href"].Value);
foreach (var link in links)
{
if (!string.IsNullOrEmpty(link) && !ExternalLinks.Contains(link))
{
ExternalLinks.Add(link);
}
}
return ExternalLinks;
}
private static readonly HashSet<string> AllExternalLinks = new HashSet<string>();
private static readonly HashSet<string> AllInternalLinks = new HashSet<string>();
public static async Task GetAllExternalLinksAsync(string siteUrl)
{
try
{
var htmlSource = await HttpClient.GetStringAsync(siteUrl);
var internalLinks = await GetInternalLinksAsync(htmlSource, SplitAddress(siteUrl)[0]);
var externalLinks = await GetExternalLinksAsync(htmlSource, SplitAddress(siteUrl)[0]);
foreach (var link in externalLinks)
{
if (!AllExternalLinks.Contains(link))
{
AllExternalLinks.Add(link);
Console.WriteLine(link);
}
}
foreach (var link in internalLinks)
{
if (!AllInternalLinks.Contains(link))
{
Console.WriteLine($"The link is: {link}");
AllInternalLinks.Add(link);
await GetAllExternalLinksAsync(link);
}
}
}
catch (HttpRequestException e)
{
Console.WriteLine(e);
Console.WriteLine($"Request error: {e.Message}");
}
}
}
}
程序有Bug, 您可以给解决下......
第一部分先到这....主要用的是AngleSharp. AngleSharp不止这些功能, 很强大的, 具体请看文档.
由于该书下一部分使用的是Python的Scrapy, 所以下篇文章我也许应该使用DotNetSpider了, 这是一个国产的库....
项目的代码在: https://github.com/solenovex/Web-Scraping-With-.NET-Core
原文地址：https://www.cnblogs.com/cgzl/p/8970582.html
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