"urllib.parse"  用于解析 URL
****************************

**源代码:** Lib/urllib/parse.py

======================================================================

该模块定义了一个标准接口，用于URL字符串按组件(协议、网络位置、路径等)
分解，或将组件组合回URL字符串，并将 "相对URL "转换为给定 "基础URL "的
绝对URL。

The module has been designed to match the Internet RFC on Relative
Uniform Resource Locators. It supports the following URL schemes:
"file", "ftp", "gopher", "hdl", "http", "https", "imap", "mailto",
"mms", "news", "nntp", "prospero", "rsync", "rtsp", "rtspu", "sftp",
"shttp", "sip", "sips", "snews", "svn", "svn+ssh", "telnet", "wais",
"ws", "wss".

The "urllib.parse" module defines functions that fall into two broad
categories: URL parsing and URL quoting. These are covered in detail
in the following sections.


URL 解析
========

URL解析功能可以将一个URL字符串分割成其组件，或者将URL组件组合成一个URL
字符串。

urllib.parse.urlparse(urlstring, scheme='', allow_fragments=True)

   Parse a URL into six components, returning a 6-item *named tuple*.
   This corresponds to the general structure of a URL:
   "scheme://netloc/path;parameters?query#fragment". Each tuple item
   is a string, possibly empty. The components are not broken up in
   smaller parts (for example, the network location is a single
   string), and % escapes are not expanded. The delimiters as shown
   above are not part of the result, except for a leading slash in the
   *path* component, which is retained if present.  For example:

   >>> from urllib.parse import urlparse
   >>> o = urlparse('http://www.cwi.nl:80/%7Eguido/Python.html')
   >>> o   
   ParseResult(scheme='http', netloc='www.cwi.nl:80', path='/%7Eguido/Python.html',
               params='', query='', fragment='')
   >>> o.scheme
   'http'
   >>> o.port
   80
   >>> o.geturl()
   'http://www.cwi.nl:80/%7Eguido/Python.html'

   Following the syntax specifications in **RFC 1808**, urlparse
   recognizes a netloc only if it is properly introduced by '//'.
   Otherwise the input is presumed to be a relative URL and thus to
   start with a path component.

      >>> from urllib.parse import urlparse
      >>> urlparse('//www.cwi.nl:80/%7Eguido/Python.html')
      ParseResult(scheme='', netloc='www.cwi.nl:80', path='/%7Eguido/Python.html',
                  params='', query='', fragment='')
      >>> urlparse('www.cwi.nl/%7Eguido/Python.html')
      ParseResult(scheme='', netloc='', path='www.cwi.nl/%7Eguido/Python.html',
                  params='', query='', fragment='')
      >>> urlparse('help/Python.html')
      ParseResult(scheme='', netloc='', path='help/Python.html', params='',
                  query='', fragment='')

   *scheme* 参数给出了默认的协议，只有在 URL 未指定协议的情况下才会被
   使用。 它应该是与 *urlstring* 相同的类型（文本或字节串），除此之外
   默认值 "''" 也总是被允许，并会在适当情况下自动转换为 "b''"。

   If the *allow_fragments* argument is false, fragment identifiers
   are not recognized.  Instead, they are parsed as part of the path,
   parameters or query component, and "fragment" is set to the empty
   string in the return value.

   返回值是一个 *named tuple*，这意味着它的条目可以通过索引或作为命名
   属性来访问，这些属性是：

   +--------------------+---------+----------------------------+------------------------+
   | 属性               | 索引    | 值                         | 值（如果不存在）       |
   |====================|=========|============================|========================|
   | "scheme"           | 0       | URL方案说明符              | *scheme* parameter     |
   +--------------------+---------+----------------------------+------------------------+
   | "netloc"           | 1       | 网络位置部分               | 空字符串               |
   +--------------------+---------+----------------------------+------------------------+
   | "path"             | 2       | 分层路径                   | 空字符串               |
   +--------------------+---------+----------------------------+------------------------+
   | "params"           | 3       | 最后路径元素的参数         | 空字符串               |
   +--------------------+---------+----------------------------+------------------------+
   | "query"            | 4       | 查询组件                   | 空字符串               |
   +--------------------+---------+----------------------------+------------------------+
   | "fragment"         | 5       | 片段识别                   | 空字符串               |
   +--------------------+---------+----------------------------+------------------------+
   | "username"         |         | 用户名                     | "None"                 |
   +--------------------+---------+----------------------------+------------------------+
   | "password"         |         | 密码                       | "None"                 |
   +--------------------+---------+----------------------------+------------------------+
   | "hostname"         |         | 主机名（小写）             | "None"                 |
   +--------------------+---------+----------------------------+------------------------+
   | "port"             |         | 端口号为整数（如果存在）   | "None"                 |
   +--------------------+---------+----------------------------+------------------------+

   如果在 URL 中指定了无效的端口，读取 "port" 属性将引发 "ValueError"
   。 有关结果对象的更多信息请参阅 结构化解析结果 一节。

   Unmatched square brackets in the "netloc" attribute will raise a
   "ValueError".

   Characters in the "netloc" attribute that decompose under NFKC
   normalization (as used by the IDNA encoding) into any of "/", "?",
   "#", "@", or ":" will raise a "ValueError". If the URL is
   decomposed before parsing, no error will be raised.

   As is the case with all named tuples, the subclass has a few
   additional methods and attributes that are particularly useful. One
   such method is "_replace()". The "_replace()" method will return a
   new ParseResult object replacing specified fields with new values.

      >>> from urllib.parse import urlparse
      >>> u = urlparse('//www.cwi.nl:80/%7Eguido/Python.html')
      >>> u
      ParseResult(scheme='', netloc='www.cwi.nl:80', path='/%7Eguido/Python.html',
                  params='', query='', fragment='')
      >>> u._replace(scheme='http')
      ParseResult(scheme='http', netloc='www.cwi.nl:80', path='/%7Eguido/Python.html',
                  params='', query='', fragment='')

   在 3.2 版更改: 添加了IPv6 URL解析功能。

   在 3.3 版更改: The fragment is now parsed for all URL schemes
   (unless *allow_fragment* is false), in accordance with **RFC
   3986**.  Previously, a whitelist of schemes that support fragments
   existed.

   在 3.6 版更改: Out-of-range port numbers now raise "ValueError",
   instead of returning "None".

   在 3.8 版更改: Characters that affect netloc parsing under NFKC
   normalization will now raise "ValueError".

urllib.parse.parse_qs(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None, separator='&')

   Parse a query string given as a string argument (data of type
   *application/x-www-form-urlencoded*).  Data are returned as a
   dictionary.  The dictionary keys are the unique query variable
   names and the values are lists of values for each name.

   The optional argument *keep_blank_values* is a flag indicating
   whether blank values in percent-encoded queries should be treated
   as blank strings. A true value indicates that blanks should be
   retained as  blank strings.  The default false value indicates that
   blank values are to be ignored and treated as if they were not
   included.

   The optional argument *strict_parsing* is a flag indicating what to
   do with parsing errors.  If false (the default), errors are
   silently ignored.  If true, errors raise a "ValueError" exception.

   The optional *encoding* and *errors* parameters specify how to
   decode percent-encoded sequences into Unicode characters, as
   accepted by the "bytes.decode()" method.

   The optional argument *max_num_fields* is the maximum number of
   fields to read. If set, then throws a "ValueError" if there are
   more than *max_num_fields* fields read.

   The optional argument *separator* is the symbol to use for
   separating the query arguments. It defaults to "&".

   Use the "urllib.parse.urlencode()" function (with the "doseq"
   parameter set to "True") to convert such dictionaries into query
   strings.

   在 3.2 版更改: Add *encoding* and *errors* parameters.

   在 3.8 版更改: Added *max_num_fields* parameter.

   在 3.8.8 版更改: Added *separator* parameter with the default value
   of "&". Python versions earlier than Python 3.8.8 allowed using
   both ";" and "&" as query parameter separator. This has been
   changed to allow only a single separator key, with "&" as the
   default separator.

urllib.parse.parse_qsl(qs, keep_blank_values=False, strict_parsing=False, encoding='utf-8', errors='replace', max_num_fields=None, separator='&')

   Parse a query string given as a string argument (data of type
   *application/x-www-form-urlencoded*).  Data are returned as a list
   of name, value pairs.

   The optional argument *keep_blank_values* is a flag indicating
   whether blank values in percent-encoded queries should be treated
   as blank strings. A true value indicates that blanks should be
   retained as  blank strings.  The default false value indicates that
   blank values are to be ignored and treated as if they were not
   included.

   The optional argument *strict_parsing* is a flag indicating what to
   do with parsing errors.  If false (the default), errors are
   silently ignored.  If true, errors raise a "ValueError" exception.

   The optional *encoding* and *errors* parameters specify how to
   decode percent-encoded sequences into Unicode characters, as
   accepted by the "bytes.decode()" method.

   The optional argument *max_num_fields* is the maximum number of
   fields to read. If set, then throws a "ValueError" if there are
   more than *max_num_fields* fields read.

   The optional argument *separator* is the symbol to use for
   separating the query arguments. It defaults to "&".

   Use the "urllib.parse.urlencode()" function to convert such lists
   of pairs into query strings.

   在 3.2 版更改: Add *encoding* and *errors* parameters.

   在 3.8 版更改: Added *max_num_fields* parameter.

   在 3.8.8 版更改: Added *separator* parameter with the default value
   of "&". Python versions earlier than Python 3.8.8 allowed using
   both ";" and "&" as query parameter separator. This has been
   changed to allow only a single separator key, with "&" as the
   default separator.

urllib.parse.urlunparse(parts)

   Construct a URL from a tuple as returned by "urlparse()". The
   *parts* argument can be any six-item iterable. This may result in a
   slightly different, but equivalent URL, if the URL that was parsed
   originally had unnecessary delimiters (for example, a "?" with an
   empty query; the RFC states that these are equivalent).

urllib.parse.urlsplit(urlstring, scheme='', allow_fragments=True)

   This is similar to "urlparse()", but does not split the params from
   the URL. This should generally be used instead of "urlparse()" if
   the more recent URL syntax allowing parameters to be applied to
   each segment of the *path* portion of the URL (see **RFC 2396**) is
   wanted.  A separate function is needed to separate the path
   segments and parameters.  This function returns a 5-item *named
   tuple*:

      (addressing scheme, network location, path, query, fragment identifier).

   The return value is a *named tuple*, its items can be accessed by
   index or as named attributes:

   +--------------------+---------+---------------------------+------------------------+
   | 属性               | 索引    | 值                        | 值（如果不存在）       |
   |====================|=========|===========================|========================|
   | "scheme"           | 0       | URL方案说明符             | *scheme* parameter     |
   +--------------------+---------+---------------------------+------------------------+
   | "netloc"           | 1       | 网络位置部分              | 空字符串               |
   +--------------------+---------+---------------------------+------------------------+
   | "path"             | 2       | 分层路径                  | 空字符串               |
   +--------------------+---------+---------------------------+------------------------+
   | "query"            | 3       | 查询组件                  | 空字符串               |
   +--------------------+---------+---------------------------+------------------------+
   | "fragment"         | 4       | 片段识别                  | 空字符串               |
   +--------------------+---------+---------------------------+------------------------+
   | "username"         |         | 用户名                    | "None"                 |
   +--------------------+---------+---------------------------+------------------------+
   | "password"         |         | 密码                      | "None"                 |
   +--------------------+---------+---------------------------+------------------------+
   | "hostname"         |         | 主机名（小写）            | "None"                 |
   +--------------------+---------+---------------------------+------------------------+
   | "port"             |         | 端口号为整数（如果存在）  | "None"                 |
   +--------------------+---------+---------------------------+------------------------+

   如果在 URL 中指定了无效的端口，读取 "port" 属性将引发 "ValueError"
   。 有关结果对象的更多信息请参阅 结构化解析结果 一节。

   Unmatched square brackets in the "netloc" attribute will raise a
   "ValueError".

   Characters in the "netloc" attribute that decompose under NFKC
   normalization (as used by the IDNA encoding) into any of "/", "?",
   "#", "@", or ":" will raise a "ValueError". If the URL is
   decomposed before parsing, no error will be raised.

   在 3.6 版更改: Out-of-range port numbers now raise "ValueError",
   instead of returning "None".

   在 3.8 版更改: Characters that affect netloc parsing under NFKC
   normalization will now raise "ValueError".

urllib.parse.urlunsplit(parts)

   Combine the elements of a tuple as returned by "urlsplit()" into a
   complete URL as a string. The *parts* argument can be any five-item
   iterable. This may result in a slightly different, but equivalent
   URL, if the URL that was parsed originally had unnecessary
   delimiters (for example, a ? with an empty query; the RFC states
   that these are equivalent).

urllib.parse.urljoin(base, url, allow_fragments=True)

   Construct a full ("absolute") URL by combining a "base URL"
   (*base*) with another URL (*url*).  Informally, this uses
   components of the base URL, in particular the addressing scheme,
   the network location and (part of) the path, to provide missing
   components in the relative URL.  For example:

   >>> from urllib.parse import urljoin
   >>> urljoin('http://www.cwi.nl/%7Eguido/Python.html', 'FAQ.html')
   'http://www.cwi.nl/%7Eguido/FAQ.html'

   The *allow_fragments* argument has the same meaning and default as
   for "urlparse()".

   注解:

     If *url* is an absolute URL (that is, starting with "//" or
     "scheme://"), the *url*'s host name and/or scheme will be present
     in the result.  For example:

      >>> urljoin('http://www.cwi.nl/%7Eguido/Python.html',
      ...         '//www.python.org/%7Eguido')
      'http://www.python.org/%7Eguido'

   If you do not want that behavior, preprocess the *url* with
   "urlsplit()" and "urlunsplit()", removing possible *scheme* and
   *netloc* parts.

   在 3.5 版更改: Behaviour updated to match the semantics defined in
   **RFC 3986**.

urllib.parse.urldefrag(url)

   If *url* contains a fragment identifier, return a modified version
   of *url* with no fragment identifier, and the fragment identifier
   as a separate string.  If there is no fragment identifier in *url*,
   return *url* unmodified and an empty string.

   The return value is a *named tuple*, its items can be accessed by
   index or as named attributes:

   +--------------------+---------+---------------------------+------------------------+
   | 属性               | 索引    | 值                        | 值（如果不存在）       |
   |====================|=========|===========================|========================|
   | "url"              | 0       | URL with no fragment      | 空字符串               |
   +--------------------+---------+---------------------------+------------------------+
   | "fragment"         | 1       | 片段识别                  | 空字符串               |
   +--------------------+---------+---------------------------+------------------------+

   See section 结构化解析结果 for more information on the result
   object.

   在 3.2 版更改: Result is a structured object rather than a simple
   2-tuple.

urllib.parse.unwrap(url)

   Extract the url from a wrapped URL (that is, a string formatted as
   "<URL:scheme://host/path>", "<scheme://host/path>",
   "URL:scheme://host/path" or "scheme://host/path"). If *url* is not
   a wrapped URL, it is returned without changes.


解析ASCII编码字节
=================

The URL parsing functions were originally designed to operate on
character strings only. In practice, it is useful to be able to
manipulate properly quoted and encoded URLs as sequences of ASCII
bytes. Accordingly, the URL parsing functions in this module all
operate on "bytes" and "bytearray" objects in addition to "str"
objects.

If "str" data is passed in, the result will also contain only "str"
data. If "bytes" or "bytearray" data is passed in, the result will
contain only "bytes" data.

Attempting to mix "str" data with "bytes" or "bytearray" in a single
function call will result in a "TypeError" being raised, while
attempting to pass in non-ASCII byte values will trigger
"UnicodeDecodeError".

To support easier conversion of result objects between "str" and
"bytes", all return values from URL parsing functions provide either
an "encode()" method (when the result contains "str" data) or a
"decode()" method (when the result contains "bytes" data). The
signatures of these methods match those of the corresponding "str" and
"bytes" methods (except that the default encoding is "'ascii'" rather
than "'utf-8'"). Each produces a value of a corresponding type that
contains either "bytes" data (for "encode()" methods) or "str" data
(for "decode()" methods).

Applications that need to operate on potentially improperly quoted
URLs that may contain non-ASCII data will need to do their own
decoding from bytes to characters before invoking the URL parsing
methods.

The behaviour described in this section applies only to the URL
parsing functions. The URL quoting functions use their own rules when
producing or consuming byte sequences as detailed in the documentation
of the individual URL quoting functions.

在 3.2 版更改: URL parsing functions now accept ASCII encoded byte
sequences


结构化解析结果
==============

The result objects from the "urlparse()", "urlsplit()"  and
"urldefrag()" functions are subclasses of the "tuple" type. These
subclasses add the attributes listed in the documentation for those
functions, the encoding and decoding support described in the previous
section, as well as an additional method:

urllib.parse.SplitResult.geturl()

   Return the re-combined version of the original URL as a string.
   This may differ from the original URL in that the scheme may be
   normalized to lower case and empty components may be dropped.
   Specifically, empty parameters, queries, and fragment identifiers
   will be removed.

   For "urldefrag()" results, only empty fragment identifiers will be
   removed. For "urlsplit()" and "urlparse()" results, all noted
   changes will be made to the URL returned by this method.

   The result of this method remains unchanged if passed back through
   the original parsing function:

   >>> from urllib.parse import urlsplit
   >>> url = 'HTTP://www.Python.org/doc/#'
   >>> r1 = urlsplit(url)
   >>> r1.geturl()
   'http://www.Python.org/doc/'
   >>> r2 = urlsplit(r1.geturl())
   >>> r2.geturl()
   'http://www.Python.org/doc/'

The following classes provide the implementations of the structured
parse results when operating on "str" objects:

class urllib.parse.DefragResult(url, fragment)

   Concrete class for "urldefrag()" results containing "str" data. The
   "encode()" method returns a "DefragResultBytes" instance.

   3.2 新版功能.

class urllib.parse.ParseResult(scheme, netloc, path, params, query, fragment)

   Concrete class for "urlparse()" results containing "str" data. The
   "encode()" method returns a "ParseResultBytes" instance.

class urllib.parse.SplitResult(scheme, netloc, path, query, fragment)

   Concrete class for "urlsplit()" results containing "str" data. The
   "encode()" method returns a "SplitResultBytes" instance.

The following classes provide the implementations of the parse results
when operating on "bytes" or "bytearray" objects:

class urllib.parse.DefragResultBytes(url, fragment)

   Concrete class for "urldefrag()" results containing "bytes" data.
   The "decode()" method returns a "DefragResult" instance.

   3.2 新版功能.

class urllib.parse.ParseResultBytes(scheme, netloc, path, params, query, fragment)

   Concrete class for "urlparse()" results containing "bytes" data.
   The "decode()" method returns a "ParseResult" instance.

   3.2 新版功能.

class urllib.parse.SplitResultBytes(scheme, netloc, path, query, fragment)

   Concrete class for "urlsplit()" results containing "bytes" data.
   The "decode()" method returns a "SplitResult" instance.

   3.2 新版功能.


URL Quoting
===========

The URL quoting functions focus on taking program data and making it
safe for use as URL components by quoting special characters and
appropriately encoding non-ASCII text. They also support reversing
these operations to recreate the original data from the contents of a
URL component if that task isn't already covered by the URL parsing
functions above.

urllib.parse.quote(string, safe='/', encoding=None, errors=None)

   Replace special characters in *string* using the "%xx" escape.
   Letters, digits, and the characters "'_.-~'" are never quoted. By
   default, this function is intended for quoting the path section of
   URL. The optional *safe* parameter specifies additional ASCII
   characters that should not be quoted --- its default value is
   "'/'".

   *string* may be either a "str" or a "bytes".

   在 3.7 版更改: Moved from **RFC 2396** to **RFC 3986** for quoting
   URL strings. "~" is now included in the set of unreserved
   characters.

   The optional *encoding* and *errors* parameters specify how to deal
   with non-ASCII characters, as accepted by the "str.encode()"
   method. *encoding* defaults to "'utf-8'". *errors* defaults to
   "'strict'", meaning unsupported characters raise a
   "UnicodeEncodeError". *encoding* and *errors* must not be supplied
   if *string* is a "bytes", or a "TypeError" is raised.

   Note that "quote(string, safe, encoding, errors)" is equivalent to
   "quote_from_bytes(string.encode(encoding, errors), safe)".

   Example: "quote('/El Niño/')" yields "'/El%20Ni%C3%B1o/'".

urllib.parse.quote_plus(string, safe='', encoding=None, errors=None)

   Like "quote()", but also replace spaces by plus signs, as required
   for quoting HTML form values when building up a query string to go
   into a URL. Plus signs in the original string are escaped unless
   they are included in *safe*.  It also does not have *safe* default
   to "'/'".

   Example: "quote_plus('/El Niño/')" yields "'%2FEl+Ni%C3%B1o%2F'".

urllib.parse.quote_from_bytes(bytes, safe='/')

   Like "quote()", but accepts a "bytes" object rather than a "str",
   and does not perform string-to-bytes encoding.

   Example: "quote_from_bytes(b'a&\xef')" yields "'a%26%EF'".

urllib.parse.unquote(string, encoding='utf-8', errors='replace')

   Replace "%xx" escapes by their single-character equivalent. The
   optional *encoding* and *errors* parameters specify how to decode
   percent-encoded sequences into Unicode characters, as accepted by
   the "bytes.decode()" method.

   *string* must be a "str".

   *encoding* defaults to "'utf-8'". *errors* defaults to "'replace'",
   meaning invalid sequences are replaced by a placeholder character.

   Example: "unquote('/El%20Ni%C3%B1o/')" yields "'/El Niño/'".

urllib.parse.unquote_plus(string, encoding='utf-8', errors='replace')

   Like "unquote()", but also replace plus signs by spaces, as
   required for unquoting HTML form values.

   *string* must be a "str".

   Example: "unquote_plus('/El+Ni%C3%B1o/')" yields "'/El Niño/'".

urllib.parse.unquote_to_bytes(string)

   Replace "%xx" escapes by their single-octet equivalent, and return
   a "bytes" object.

   *string* may be either a "str" or a "bytes".

   If it is a "str", unescaped non-ASCII characters in *string* are
   encoded into UTF-8 bytes.

   Example: "unquote_to_bytes('a%26%EF')" yields "b'a&\xef'".

urllib.parse.urlencode(query, doseq=False, safe='', encoding=None, errors=None, quote_via=quote_plus)

   Convert a mapping object or a sequence of two-element tuples, which
   may contain "str" or "bytes" objects, to a percent-encoded ASCII
   text string.  If the resultant string is to be used as a *data* for
   POST operation with the "urlopen()" function, then it should be
   encoded to bytes, otherwise it would result in a "TypeError".

   The resulting string is a series of "key=value" pairs separated by
   "'&'" characters, where both *key* and *value* are quoted using the
   *quote_via* function.  By default, "quote_plus()" is used to quote
   the values, which means spaces are quoted as a "'+'" character and
   '/' characters are encoded as "%2F", which follows the standard for
   GET requests ("application/x-www-form-urlencoded").  An alternate
   function that can be passed as *quote_via* is "quote()", which will
   encode spaces as "%20" and not encode '/' characters.  For maximum
   control of what is quoted, use "quote" and specify a value for
   *safe*.

   When a sequence of two-element tuples is used as the *query*
   argument, the first element of each tuple is a key and the second
   is a value. The value element in itself can be a sequence and in
   that case, if the optional parameter *doseq* is evaluates to
   "True", individual "key=value" pairs separated by "'&'" are
   generated for each element of the value sequence for the key.  The
   order of parameters in the encoded string will match the order of
   parameter tuples in the sequence.

   The *safe*, *encoding*, and *errors* parameters are passed down to
   *quote_via* (the *encoding* and *errors* parameters are only passed
   when a query element is a "str").

   To reverse this encoding process, "parse_qs()" and "parse_qsl()"
   are provided in this module to parse query strings into Python data
   structures.

   Refer to urllib examples to find out how urlencode method can be
   used for generating query string for a URL or data for POST.

   在 3.2 版更改: 查询参数支持字节和字符串对象。

   3.5 新版功能: *quote_via* 参数.

参见:

  **RFC 3986** - 统一资源标识符
     This is the current standard (STD66). Any changes to urllib.parse
     module should conform to this. Certain deviations could be
     observed, which are mostly for backward compatibility purposes
     and for certain de-facto parsing requirements as commonly
     observed in major browsers.

  **RFC 2732** - Format for Literal IPv6 Addresses in URL's.
     This specifies the parsing requirements of IPv6 URLs.

  **RFC 2396** - 统一资源标识符（URI）：通用语法
     Document describing the generic syntactic requirements for both
     Uniform Resource Names (URNs) and Uniform Resource Locators
     (URLs).

  **RFC 2368** - The mailto URL scheme.
     Parsing requirements for mailto URL schemes.

  **RFC 1808** - Relative Uniform Resource Locators
     This Request For Comments includes the rules for joining an
     absolute and a relative URL, including a fair number of "Abnormal
     Examples" which govern the treatment of border cases.

  **RFC 1738** - Uniform Resource Locators (URL)
     This specifies the formal syntax and semantics of absolute URLs.
