老师,最后一行classlist.add添加不生效怎么办

<script>

        var divname = document.createElement("div")

        var body = document.getElementsByTagName("body")

        body[0].appendChild(divname)

        var text = document.createTextNode("动态文本节点")

        divname.appendChild(text)

        var divid = document.createAttribute("id")

        divid.value = "divid"

        divname.setAttributeNode(divid)

        divname.id = "new divid"

        var divclass = document.createAttribute("class")

        divclass.value = "divclass"

        divname.setAttributeNode(divclass)

        divname.className = "new classname"

        divname.classList.add("new")

    </script>

老师，初始化是什么意思，为什么要对实例对象进行初始化

else:

            # 从尾节点开始遍历

            succ = self.tail

            for _ in range(self.size-index):

                # 找到后继节点

                succ = succ.pre

            # 找到前驱节点

            pred = succ.pre

for _ in range(self.size,self.size-index,-1)

从后往前遍历不应该这么写吗

请问为什么我的postman里结果显示是这样，代码也没有问题

如果books 里面有属性值，它是不是也要算一个字节点个数？（就是解析上一章课的book.xml）

我没记错的话,这里的字符串方法,post和get要大写,flask中就是这样规定的

ArrayList list = new ArrayList();

List list = new ArrayList();

这两个创建对象有什么区别？

a=(a)
((a))
((a))
b=(b)
((b))
((b))

老师，这里面a和=、b和=之间需要空格吗，有什么要求？

老师，这个Android是安装不了软件？

老师，MySQL现在已经支持检查约束了吧，视频里还说不支持

闭包的特点是变量始终保存在内存中，这样随着闭包数量增加早晚会造成溢出的。所以闭包中的变量有约定俗成的释放原则么？然后是怎么释放的？

没问题了

老师 ，这个在Unicode下 CHAR(12)最多包含12字节信息，CHAR字段最多可以存储2000字节的信息，这个12B和2000B，都是CHAR类型的 他们的区别是什么？

底层的 这个键值 hash 存取过程没有听懂

C:\Windows\system32>workon django_env

(django_env) C:\Windows\System32>pip install mysqlclient

Collecting mysqlclient

  Using cached mysqlclient-1.4.6.tar.gz (85 kB)

Building wheels for collected packages: mysqlclient

  Building wheel for mysqlclient (setup.py) ... error

  ERROR: Command errored out with exit status 1:

   command: 'D:\python_env\django_env\Scripts\python.exe' -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'C:\\Users\\Jace\\AppData\\Local\\Temp\\pip-install-v7ymmrjl\\mysqlclient\\setup.py'"'"'; __file__='"'"'C:\\Users\\Jace\\AppData\\Local\\Temp\\pip-install-v7ymmrjl\\mysqlclient\\setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' bdist_wheel -d 'C:\Users\Jace\AppData\Local\Temp\pip-wheel-qh9stb47'

       cwd: C:\Users\Jace\AppData\Local\Temp\pip-install-v7ymmrjl\mysqlclient\

  Complete output (30 lines):

  running bdist_wheel

  running build

  running build_py

  creating build

  creating build\lib.win32-3.8

  creating build\lib.win32-3.8\MySQLdb

  copying MySQLdb\__init__.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\_exceptions.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\compat.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\connections.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\converters.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\cursors.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\release.py -> build\lib.win32-3.8\MySQLdb

  copying MySQLdb\times.py -> build\lib.win32-3.8\MySQLdb

  creating build\lib.win32-3.8\MySQLdb\constants

  copying MySQLdb\constants\__init__.py -> build\lib.win32-3.8\MySQLdb\constants

  copying MySQLdb\constants\CLIENT.py -> build\lib.win32-3.8\MySQLdb\constants

  copying MySQLdb\constants\CR.py -> build\lib.win32-3.8\MySQLdb\constants

  copying MySQLdb\constants\ER.py -> build\lib.win32-3.8\MySQLdb\constants

  copying MySQLdb\constants\FIELD_TYPE.py -> build\lib.win32-3.8\MySQLdb\constants

  copying MySQLdb\constants\FLAG.py -> build\lib.win32-3.8\MySQLdb\constants

  running build_ext

  building 'MySQLdb._mysql' extension

  creating build\temp.win32-3.8

  creating build\temp.win32-3.8\Release

  creating build\temp.win32-3.8\Release\MySQLdb

  C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\cl.exe /c /nologo /Ox /W3 /GL /DNDEBUG /MD -Dversion_info=(1,4,6,'final',0) -D__version__=1.4.6 "-IC:\Program Files (x86)\MySQL\MySQL Connector C 6.1\include\mariadb" -ID:\python_env\django_env\include -Id:\python3.8.2\include -Id:\python3.8.2\include "-IC:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\8.1\include\shared" "-IC:\Program Files (x86)\Windows Kits\8.1\include\um" "-IC:\Program Files (x86)\Windows Kits\8.1\include\winrt" /TcMySQLdb/_mysql.c /Fobuild\temp.win32-3.8\Release\MySQLdb/_mysql.obj /Zl /D_CRT_SECURE_NO_WARNINGS

  _mysql.c

  MySQLdb/_mysql.c(29): fatal error C1083: Cannot open include file: 'mysql.h': No such file or directory

  error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\BIN\\cl.exe' failed with exit status 2

  ----------------------------------------

  ERROR: Failed building wheel for mysqlclient

  Running setup.py clean for mysqlclient

Failed to build mysqlclient

Installing collected packages: mysqlclient

    Running setup.py install for mysqlclient ... error

    ERROR: Command errored out with exit status 1:

     command: 'D:\python_env\django_env\Scripts\python.exe' -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'C:\\Users\\Jace\\AppData\\Local\\Temp\\pip-install-v7ymmrjl\\mysqlclient\\setup.py'"'"'; __file__='"'"'C:\\Users\\Jace\\AppData\\Local\\Temp\\pip-install-v7ymmrjl\\mysqlclient\\setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record 'C:\Users\Jace\AppData\Local\Temp\pip-record-c_7pdi53\install-record.txt' --single-version-externally-managed --compile --install-headers 'D:\python_env\django_env\include\site\python3.8\mysqlclient'

         cwd: C:\Users\Jace\AppData\Local\Temp\pip-install-v7ymmrjl\mysqlclient\

    Complete output (30 lines):

    running install

    running build

    running build_py

    creating build

    creating build\lib.win32-3.8

    creating build\lib.win32-3.8\MySQLdb

    copying MySQLdb\__init__.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\_exceptions.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\compat.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\connections.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\converters.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\cursors.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\release.py -> build\lib.win32-3.8\MySQLdb

    copying MySQLdb\times.py -> build\lib.win32-3.8\MySQLdb

    creating build\lib.win32-3.8\MySQLdb\constants

    copying MySQLdb\constants\__init__.py -> build\lib.win32-3.8\MySQLdb\constants

    copying MySQLdb\constants\CLIENT.py -> build\lib.win32-3.8\MySQLdb\constants

    copying MySQLdb\constants\CR.py -> build\lib.win32-3.8\MySQLdb\constants

    copying MySQLdb\constants\ER.py -> build\lib.win32-3.8\MySQLdb\constants

    copying MySQLdb\constants\FIELD_TYPE.py -> build\lib.win32-3.8\MySQLdb\constants

    copying MySQLdb\constants\FLAG.py -> build\lib.win32-3.8\MySQLdb\constants

    running build_ext

    building 'MySQLdb._mysql' extension

    creating build\temp.win32-3.8

    creating build\temp.win32-3.8\Release

    creating build\temp.win32-3.8\Release\MySQLdb

    C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\BIN\cl.exe /c /nologo /Ox /W3 /GL /DNDEBUG /MD -Dversion_info=(1,4,6,'final',0) -D__version__=1.4.6 "-IC:\Program Files (x86)\MySQL\MySQL Connector C 6.1\include\mariadb" -ID:\python_env\django_env\include -Id:\python3.8.2\include -Id:\python3.8.2\include "-IC:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\INCLUDE" "-IC:\Program Files (x86)\Windows Kits\10\include\10.0.10240.0\ucrt" "-IC:\Program Files (x86)\Windows Kits\8.1\include\shared" "-IC:\Program Files (x86)\Windows Kits\8.1\include\um" "-IC:\Program Files (x86)\Windows Kits\8.1\include\winrt" /TcMySQLdb/_mysql.c /Fobuild\temp.win32-3.8\Release\MySQLdb/_mysql.obj /Zl /D_CRT_SECURE_NO_WARNINGS

    _mysql.c

    MySQLdb/_mysql.c(29): fatal error C1083: Cannot open include file: 'mysql.h': No such file or directory

    error: command 'C:\\Program Files (x86)\\Microsoft Visual Studio 14.0\\VC\\BIN\\cl.exe' failed with exit status 2

    ----------------------------------------

ERROR: Command errored out with exit status 1: 'D:\python_env\django_env\Scripts\python.exe' -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'C:\\Users\\Jace\\AppData\\Local\\Temp\\pip-install-v7ymmrjl\\mysqlclient\\setup.py'"'"'; __file__='"'"'C:\\Users\\Jace\\AppData\\Local\\Temp\\pip-install-v7ymmrjl\\mysqlclient\\setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record 'C:\Users\Jace\AppData\Local\Temp\pip-record-c_7pdi53\install-record.txt' --single-version-externally-managed --compile --install-headers 'D:\python_env\django_env\include\site\python3.8\mysqlclient' Check the logs for full command output.

安装mysqlclient报错