使用Python批量下载Plus上的Podcast

Plus是一个介绍数学之美与实际应用的网络杂志，其中包含了数学知识、轶闻趣事、历史典故等许多精彩的内容。该杂志恰好有一个Podcast栏目，提供了不少采访与讲座的mp3音频。于是， 我使用Python脚本将所有的Podcast文件都下载了下来，用于上下班路途上不适宜看书的时候听。

该脚本引入了四个模块：

• re用于正则表达式匹配，将Podcast标题转为音频文件名。这是由于Linux系统下的文件名应避免包含字符/><|:&。所以，如果Podcast标题包含了这些字符，就需要将它们替换掉。

• shutil用于将从网络上下载的音频数据流转存为本地文件。

• requests用于向网站服务器发出GET请求，以获取HTML页面。

• html.parser用于解析得到的HTML页面内容，从中匹配并提取出感兴趣的内容，包括Podcast子页面的链接，mp3文件的url等。使用时，需要由该模块中的HTMLParser类派生出子类，重写其中的成员函数handle_starttag与handle_data，即可以实现对HTML标签及其属性与包含的内容进行提取与处理。

具体代码如下：

import re
import shutil
import requests
from html.parser import HTMLParser

# Remove characters unsuitable to be used in a file name.
def StringToFileName(s):
    s = s.replace('#', 'No.')
    s = re.sub('[?]', '', s)
    s = re.sub('[&+$*!@%^|<>/]', '_', s)
    s = re.sub(':s*', '-', s)

    return s

# Class for parse podcast main page and localize the links to subpages.
class PlusPodcastPageParser(HTMLParser):
    def __init__(self):
        HTMLParser.__init__(self)

        self.root_url = 'https://plus.maths.org'
        self.subpage_url_list = list()
        self.subpage_title_list = list()

        self.is_subpage_div_found = False
        self.is_subpage_span_found = False
        self.is_subpage_link_found = False

    def handle_starttag(self, tag, attrs):
        if (not self.is_subpage_link_found and not self.is_subpage_div_found and tag == 'div'):
            if (len(attrs) > 0):
                for attr in attrs:
                    if (attr[0] == 'class' and attr[1] == 'views-field views-field-title'):
                        self.is_subpage_div_found = True
                        break
        elif (not self.is_subpage_link_found and self.is_subpage_div_found and tag == 'span'):
            if (len(attrs) > 0):
                for attr in attrs:
                    if (attr[0] == 'class' and attr[1] == 'field-content'):
                        self.is_subpage_span_found = True
                        break
        elif (not self.is_subpage_link_found and self.is_subpage_span_found and tag == 'a'):
            if (len(attrs) > 0):
                for attr in attrs:
                    if (attr[0] == 'href'):
                        self.is_subpage_link_found = True
                        self.subpage_url_list.append(self.root_url + attr[1])
                        break

    def handle_data(self, data):
        if (self.is_subpage_link_found):
            podcast_file_name = StringToFileName(data)
            self.subpage_title_list.append(podcast_file_name)

            # Reset pattern searching flags and prepare for the next search.
            self.is_subpage_div_found = False
            self.is_subpage_span_found = False
            self.is_subpage_link_found = False

# Class for parse podcast subpage which contains the mp3 file.
class PlusPodcastSubpageParser(HTMLParser):
    def __init__(self):
        HTMLParser.__init__(self)

        self.root_url = 'https://plus.maths.org'
        self.podcast_url = ''

        self.is_subpage_div_found = False
        self.is_subpage_span_found = False
        self.is_podcast_link_found = False

    def handle_starttag(self, tag, attrs):
        if (not self.is_podcast_link_found and not self.is_subpage_div_found and tag == 'div'):
            if (len(attrs) > 0):
                for attr in attrs:
                    if (attr[0] == 'class' and attr[1] == 'field-item even'):
                        self.is_subpage_div_found = True
                        break
        elif (not self.is_podcast_link_found and self.is_subpage_div_found and tag == 'span'):
            if (len(attrs) > 0):
                for attr in attrs:
                    if (attr[0] == 'class' and attr[1] == 'file'):
                        self.is_subpage_span_found = True
                        break
        elif (not self.is_podcast_link_found and self.is_subpage_span_found and tag == 'a'):
            if (len(attrs) > 0):
                for attr in attrs:
                    if (attr[0] == 'href'):
                        self.is_podcast_link_found = True
                        self.podcast_url = attr[1]
                        break

number_of_podcast_pages = 16
mp3_file_counter = 1

for page_idx in range(0, number_of_podcast_pages):
    if (page_idx > 0):
        page_url = 'https://plus.maths.org/content/podcast?page=' + str(page_idx)
    else:
        page_url = 'https://plus.maths.org/content/podcast'
        
    response = requests.get(page_url)
    if (response.status_code == 200):
        current_page = response.text
        del response
        page_parser = PlusPodcastPageParser()
        page_parser.feed(current_page)
        
        if (len(page_parser.subpage_url_list) == len(page_parser.subpage_title_list)):
            # Iterative over each found subpage url.
            for subpage_idx in range(len(page_parser.subpage_url_list)):
                response = requests.get(page_parser.subpage_url_list[subpage_idx])
                if (response.status_code == 200):
                    current_subpage = response.text
                    del response
                    subpage_parser = PlusPodcastSubpageParser()
                    subpage_parser.feed(current_subpage)

                    if (len(subpage_parser.podcast_url) > 0):
                        print ('*** Downloading ' + subpage_parser.podcast_url + ' ...')
                        response = requests.get(subpage_parser.podcast_url, stream = True)
                        with open(str(mp3_file_counter) + '-' + page_parser.subpage_title_list[subpage_idx] + '.mp3', 'wb') as mp3_file:
                            shutil.copyfileobj(response.raw, mp3_file)
                            del response

                        mp3_file_counter = mp3_file_counter + 1
                else:
                    print ('Cannot get the podcast subpage: ' + page_parser.subpage)
        else:
            print ('The numbers of subpage urls and titles should be the same!')
    else:
        print ('Cannot get the podcast page: ' + page_url)

运行该程序，稍等片刻，就可以得到所有的Podcast资源了。