20.5. "urllib" — Open arbitrary resources by URL
************************************************

注解: The "urllib" module has been split into parts and renamed in
  Python 3 to "urllib.request", "urllib.parse", and "urllib.error".
  The *2to3* tool will automatically adapt imports when converting
  your sources to Python 3. Also note that the
  "urllib.request.urlopen()" function in Python 3 is equivalent to
  "urllib2.urlopen()" and that "urllib.urlopen()" has been removed.

This module provides a high-level interface for fetching data across
the World Wide Web.  In particular, the "urlopen()" function is
similar to the built-in function "open()", but accepts Universal
Resource Locators (URLs) instead of filenames.  Some restrictions
apply — it can only open URLs for reading, and no seek operations are
available.

参见: The Requests package is recommended for a higher-level HTTP
  client interface.

在 2.7.9 版更改: For HTTPS URIs, "urllib" performs all the neccessary
certificate and hostname checks by default.

警告: For Python versions earlier than 2.7.9, urllib does not
  attempt to validate the server certificates of HTTPS URIs. Use at
  your own risk!


20.5.1. High-level interface
============================

urllib.urlopen(url[, data[, proxies[, context]]])

   Open a network object denoted by a URL for reading.  If the URL
   does not have a scheme identifier, or if it has "file:" as its
   scheme identifier, this opens a local file (without *universal
   newlines*); otherwise it opens a socket to a server somewhere on
   the network.  If the connection cannot be made the "IOError"
   exception is raised.  If all went well, a file-like object is
   returned.  This supports the following methods: "read()",
   "readline()", "readlines()", "fileno()", "close()", "info()",
   "getcode()" and "geturl()".  It also has proper support for the
   *iterator* protocol. One caveat: the "read()" method, if the size
   argument is omitted or negative, may not read until the end of the
   data stream; there is no good way to determine that the entire
   stream from a socket has been read in the general case.

   Except for the "info()", "getcode()" and "geturl()" methods, these
   methods have the same interface as for file objects — see section
   File Objects in this manual.  (It is not a built-in file object,
   however, so it can’t be used at those few places where a true
   built-in file object is required.)

   The "info()" method returns an instance of the class
   "mimetools.Message" containing meta-information associated with the
   URL.  When the method is HTTP, these headers are those returned by
   the server at the head of the retrieved HTML page (including
   Content-Length and Content-Type).  When the method is FTP, a
   Content-Length header will be present if (as is now usual) the
   server passed back a file length in response to the FTP retrieval
   request. A Content-Type header will be present if the MIME type can
   be guessed.  When the method is local-file, returned headers will
   include a Date representing the file’s last-modified time, a
   Content-Length giving file size, and a Content-Type containing a
   guess at the file’s type. See also the description of the
   "mimetools" module.

   The "geturl()" method returns the real URL of the page.  In some
   cases, the HTTP server redirects a client to another URL.  The
   "urlopen()" function handles this transparently, but in some cases
   the caller needs to know which URL the client was redirected to.
   The "geturl()" method can be used to get at this redirected URL.

   The "getcode()" method returns the HTTP status code that was sent
   with the response, or "None" if the URL is no HTTP URL.

   If the *url* uses the "http:" scheme identifier, the optional
   *data* argument may be given to specify a "POST" request (normally
   the request type is "GET").  The *data* argument must be in
   standard *application/x-www-form-urlencoded* format; see the
   "urlencode()" function below.

   The "urlopen()" function works transparently with proxies which do
   not require authentication.  In a Unix or Windows environment, set
   the "http_proxy", or "ftp_proxy" environment variables to a URL
   that identifies the proxy server before starting the Python
   interpreter.  For example (the "'%'" is the command prompt):

      % http_proxy="http://www.someproxy.com:3128"
      % export http_proxy
      % python
      ...

   The "no_proxy" environment variable can be used to specify hosts
   which shouldn’t be reached via proxy; if set, it should be a comma-
   separated list of hostname suffixes, optionally with ":port"
   appended, for example "cern.ch,ncsa.uiuc.edu,some.host:8080".

   In a Windows environment, if no proxy environment variables are
   set, proxy settings are obtained from the registry’s Internet
   Settings section.

   In a Mac OS X  environment, "urlopen()" will retrieve proxy
   information from the OS X System Configuration Framework, which can
   be managed with Network System Preferences panel.

   Alternatively, the optional *proxies* argument may be used to
   explicitly specify proxies.  It must be a dictionary mapping scheme
   names to proxy URLs, where an empty dictionary causes no proxies to
   be used, and "None" (the default value) causes environmental proxy
   settings to be used as discussed above.  For example:

      # Use http://www.someproxy.com:3128 for HTTP proxying
      proxies = {'http': 'http://www.someproxy.com:3128'}
      filehandle = urllib.urlopen(some_url, proxies=proxies)
      # Don't use any proxies
      filehandle = urllib.urlopen(some_url, proxies={})
      # Use proxies from environment - both versions are equivalent
      filehandle = urllib.urlopen(some_url, proxies=None)
      filehandle = urllib.urlopen(some_url)

   Proxies which require authentication for use are not currently
   supported; this is considered an implementation limitation.

   The *context* parameter may be set to a "ssl.SSLContext" instance
   to configure the SSL settings that are used if "urlopen()" makes a
   HTTPS connection.

   在 2.3 版更改: Added the *proxies* support.

   在 2.6 版更改: Added "getcode()" to returned object and support for
   the "no_proxy" environment variable.

   在 2.7.9 版更改: The *context* parameter was added.  All the
   neccessary certificate and hostname checks are done by default.

   2.6 版后已移除: The "urlopen()" function has been removed in Python
   3 in favor of "urllib2.urlopen()".

urllib.urlretrieve(url[, filename[, reporthook[, data[, context]]]])

   Copy a network object denoted by a URL to a local file, if
   necessary. If the URL points to a local file, or a valid cached
   copy of the object exists, the object is not copied.  Return a
   tuple "(filename, headers)" where *filename* is the local file name
   under which the object can be found, and *headers* is whatever the
   "info()" method of the object returned by "urlopen()" returned (for
   a remote object, possibly cached). Exceptions are the same as for
   "urlopen()".

   The second argument, if present, specifies the file location to
   copy to (if absent, the location will be a tempfile with a
   generated name). The third argument, if present, is a callable that
   will be called once on establishment of the network connection and
   once after each block read thereafter.  The callable will be passed
   three arguments; a count of blocks transferred so far, a block size
   in bytes, and the total size of the file.  The third argument may
   be "-1" on older FTP servers which do not return a file size in
   response to a retrieval request.

   If the *url* uses the "http:" scheme identifier, the optional
   *data* argument may be given to specify a "POST" request (normally
   the request type is "GET").  The *data* argument must in standard
   *application/x-www-form-urlencoded* format; see the "urlencode()"
   function below.

   The *context* parameter may be set to a "ssl.SSLContext" instance
   to configure the SSL settings that are used if "urlretrieve()"
   makes a HTTPS connection.

   在 2.5 版更改: "urlretrieve()" will raise "ContentTooShortError"
   when it detects that the amount of data available  was less than
   the expected amount (which is the size reported by a  *Content-
   Length* header). This can occur, for example, when the  download is
   interrupted.The *Content-Length* is treated as a lower bound: if
   there’s more data  to read, "urlretrieve()" reads more data, but if
   less data is available,  it raises the exception.You can still
   retrieve the downloaded data in this case, it is stored  in the
   "content" attribute of the exception instance.If no *Content-
   Length* header was supplied, "urlretrieve()" can not check the size
   of the data it has downloaded, and just returns it.  In this case
   you just have to assume that the download was successful.

   在 2.7.9 版更改: The *context* parameter was added.  All the
   neccessary certificate and hostname checks are done by default.

urllib._urlopener

   The public functions "urlopen()" and "urlretrieve()" create an
   instance of the "FancyURLopener" class and use it to perform their
   requested actions.  To override this functionality, programmers can
   create a subclass of "URLopener" or "FancyURLopener", then assign
   an instance of that class to the "urllib._urlopener" variable
   before calling the desired function. For example, applications may
   want to specify a different *User-Agent* header than "URLopener"
   defines.  This can be accomplished with the following code:

      import urllib

      class AppURLopener(urllib.FancyURLopener):
          version = "App/1.7"

      urllib._urlopener = AppURLopener()

urllib.urlcleanup()

   Clear the cache that may have been built up by previous calls to
   "urlretrieve()".


20.5.2. Utility functions
=========================

urllib.quote(string[, safe])

   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
   the URL. The optional *safe* parameter specifies additional
   characters that should not be quoted — its default value is "'/'".

   Example: "quote('/~connolly/')" yields "'/%7econnolly/'".

urllib.quote_plus(string[, safe])

   Like "quote()", but also replaces 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 "'/'".

urllib.unquote(string)

   Replace "%xx" escapes by their single-character equivalent.

   Example: "unquote('/%7Econnolly/')" yields "'/~connolly/'".

urllib.unquote_plus(string)

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

urllib.urlencode(query[, doseq])

   Convert a mapping object or a sequence of two-element tuples to a
   “percent-encoded” string, suitable to pass to "urlopen()" above as
   the optional *data* argument.  This is useful to pass a dictionary
   of form fields to a "POST" request.  The resulting string is a
   series of "key=value" pairs separated by "'&'" characters, where
   both *key* and *value* are quoted using "quote_plus()" above.  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 "urlparse" module provides the functions "parse_qs()"
   and "parse_qsl()" which are used to parse query strings into Python
   data structures.

urllib.pathname2url(path)

   Convert the pathname *path* from the local syntax for a path to the
   form used in the path component of a URL.  This does not produce a
   complete URL.  The return value will already be quoted using the
   "quote()" function.

urllib.url2pathname(path)

   Convert the path component *path* from a percent-encoded URL to the
   local syntax for a path.  This does not accept a complete URL.
   This function uses "unquote()" to decode *path*.

urllib.getproxies()

   This helper function returns a dictionary of scheme to proxy server
   URL mappings. It scans the environment for variables named
   "<scheme>_proxy", in case insensitive way, for all operating
   systems first, and when it cannot find it, looks for proxy
   information from Mac OSX System Configuration for Mac OS X and
   Windows Systems Registry for Windows. If both lowercase and
   uppercase environment variables exist (and disagree), lowercase is
   preferred.

   注解: If the environment variable "REQUEST_METHOD" is set, which
     usually indicates your script is running in a CGI environment,
     the environment variable "HTTP_PROXY" (uppercase "_PROXY") will
     be ignored. This is because that variable can be injected by a
     client using the “Proxy:” HTTP header. If you need to use an HTTP
     proxy in a CGI environment, either use "ProxyHandler" explicitly,
     or make sure the variable name is in lowercase (or at least the
     "_proxy" suffix).

注解: urllib also exposes certain utility functions like splittype,
  splithost and others parsing URL into various components. But it is
  recommended to use "urlparse" for parsing URLs rather than using
  these functions directly. Python 3 does not expose these helper
  functions from "urllib.parse" module.


20.5.3. URL Opener objects
==========================

class urllib.URLopener([proxies[, context[, **x509]]])

   Base class for opening and reading URLs.  Unless you need to
   support opening objects using schemes other than "http:", "ftp:",
   or "file:", you probably want to use "FancyURLopener".

   By default, the "URLopener" class sends a *User-Agent* header of
   "urllib/VVV", where *VVV* is the "urllib" version number.
   Applications can define their own *User-Agent* header by
   subclassing "URLopener" or "FancyURLopener" and setting the class
   attribute "version" to an appropriate string value in the subclass
   definition.

   The optional *proxies* parameter should be a dictionary mapping
   scheme names to proxy URLs, where an empty dictionary turns proxies
   off completely.  Its default value is "None", in which case
   environmental proxy settings will be used if present, as discussed
   in the definition of "urlopen()", above.

   The *context* parameter may be a "ssl.SSLContext" instance.  If
   given, it defines the SSL settings the opener uses to make HTTPS
   connections.

   Additional keyword parameters, collected in *x509*, may be used for
   authentication of the client when using the "https:" scheme.  The
   keywords *key_file* and *cert_file* are supported to provide an
   SSL key and certificate; both are needed to support client
   authentication.

   "URLopener" objects will raise an "IOError" exception if the server
   returns an error code.

   在 2.7.9 版更改: The *context* parameter was added.  All the
   neccessary certificate and hostname checks are done by default.

   open(fullurl[, data])

      Open *fullurl* using the appropriate protocol.  This method sets
      up cache and proxy information, then calls the appropriate open
      method with its input arguments.  If the scheme is not
      recognized, "open_unknown()" is called. The *data* argument has
      the same meaning as the *data* argument of "urlopen()".

   open_unknown(fullurl[, data])

      Overridable interface to open unknown URL types.

   retrieve(url[, filename[, reporthook[, data]]])

      Retrieves the contents of *url* and places it in *filename*.
      The return value is a tuple consisting of a local filename and
      either a "mimetools.Message" object containing the response
      headers (for remote URLs) or "None" (for local URLs).  The
      caller must then open and read the contents of *filename*.  If
      *filename* is not given and the URL refers to a local file, the
      input filename is returned.  If the URL is non-local and
      *filename* is not given, the filename is the output of
      "tempfile.mktemp()" with a suffix that matches the suffix of the
      last path component of the input URL.  If *reporthook* is given,
      it must be a function accepting three numeric parameters.  It
      will be called after each chunk of data is read from the
      network.  *reporthook* is ignored for local URLs.

      If the *url* uses the "http:" scheme identifier, the optional
      *data* argument may be given to specify a "POST" request
      (normally the request type is "GET").  The *data* argument must
      in standard *application/x-www-form-urlencoded* format; see the
      "urlencode()" function below.

   version

      Variable that specifies the user agent of the opener object.  To
      get "urllib" to tell servers that it is a particular user agent,
      set this in a subclass as a class variable or in the constructor
      before calling the base constructor.

class urllib.FancyURLopener(...)

   "FancyURLopener" subclasses "URLopener" providing default handling
   for the following HTTP response codes: 301, 302, 303, 307 and 401.
   For the 30x response codes listed above, the *Location* header is
   used to fetch the actual URL.  For 401 response codes
   (authentication required), basic HTTP authentication is performed.
   For the 30x response codes, recursion is bounded by the value of
   the *maxtries* attribute, which defaults to 10.

   For all other response codes, the method "http_error_default()" is
   called which you can override in subclasses to handle the error
   appropriately.

   注解: According to the letter of **RFC 2616**, 301 and 302
     responses to POST requests must not be automatically redirected
     without confirmation by the user.  In reality, browsers do allow
     automatic redirection of these responses, changing the POST to a
     GET, and "urllib" reproduces this behaviour.

   The parameters to the constructor are the same as those for
   "URLopener".

   注解: When performing basic authentication, a "FancyURLopener"
     instance calls its "prompt_user_passwd()" method.  The default
     implementation asks the users for the required information on the
     controlling terminal.  A subclass may override this method to
     support more appropriate behavior if needed.

   The "FancyURLopener" class offers one additional method that should
   be overloaded to provide the appropriate behavior:

   prompt_user_passwd(host, realm)

      Return information needed to authenticate the user at the given
      host in the specified security realm.  The return value should
      be a tuple, "(user, password)", which can be used for basic
      authentication.

      The implementation prompts for this information on the terminal;
      an application should override this method to use an appropriate
      interaction model in the local environment.

exception urllib.ContentTooShortError(msg[, content])

   This exception is raised when the "urlretrieve()" function detects
   that the amount of the downloaded data is less than the  expected
   amount (given by the *Content-Length* header). The "content"
   attribute stores the downloaded (and supposedly truncated) data.

   2.5 新版功能.


20.5.4. "urllib" Restrictions
=============================

* Currently, only the following protocols are supported: HTTP,
  (versions 0.9 and 1.0),  FTP, and local files.

* The caching feature of "urlretrieve()" has been disabled until I
  find the time to hack proper processing of Expiration time headers.

* There should be a function to query whether a particular URL is in
  the cache.

* For backward compatibility, if a URL appears to point to a local
  file but the file can’t be opened, the URL is re-interpreted using
  the FTP protocol.  This can sometimes cause confusing error
  messages.

* The "urlopen()" and "urlretrieve()" functions can cause
  arbitrarily long delays while waiting for a network connection to be
  set up. This means that it is difficult to build an interactive Web
  client using these functions without using threads.

* The data returned by "urlopen()" or "urlretrieve()" is the raw
  data returned by the server.  This may be binary data (such as an
  image), plain text or (for example) HTML.  The HTTP protocol
  provides type information in the reply header, which can be
  inspected by looking at the *Content-Type* header.  If the returned
  data is HTML, you can use the module "htmllib" to parse it.

* The code handling the FTP protocol cannot differentiate between a
  file and a directory.  This can lead to unexpected behavior when
  attempting to read a URL that points to a file that is not
  accessible.  If the URL ends in a "/", it is assumed to refer to a
  directory and will be handled accordingly.  But if an attempt to
  read a file leads to a 550 error (meaning the URL cannot be found or
  is not accessible, often for permission reasons), then the path is
  treated as a directory in order to handle the case when a directory
  is specified by a URL but the trailing "/" has been left off.  This
  can cause misleading results when you try to fetch a file whose read
  permissions make it inaccessible; the FTP code will try to read it,
  fail with a 550 error, and then perform a directory listing for the
  unreadable file. If fine-grained control is needed, consider using
  the "ftplib" module, subclassing "FancyURLopener", or changing
  *_urlopener* to meet your needs.

* This module does not support the use of proxies which require
  authentication. This may be implemented in the future.

* Although the "urllib" module contains (undocumented) routines to
  parse and unparse URL strings, the recommended interface for URL
  manipulation is in module "urlparse".


20.5.5. Examples
================

Here is an example session that uses the "GET" method to retrieve a
URL containing parameters:

   >>> import urllib
   >>> params = urllib.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
   >>> f = urllib.urlopen("http://www.musi-cal.com/cgi-bin/query?%s" % params)
   >>> print f.read()

The following example uses the "POST" method instead:

   >>> import urllib
   >>> params = urllib.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
   >>> f = urllib.urlopen("http://www.musi-cal.com/cgi-bin/query", params)
   >>> print f.read()

The following example uses an explicitly specified HTTP proxy,
overriding environment settings:

   >>> import urllib
   >>> proxies = {'http': 'http://proxy.example.com:8080/'}
   >>> opener = urllib.FancyURLopener(proxies)
   >>> f = opener.open("http://www.python.org")
   >>> f.read()

The following example uses no proxies at all, overriding environment
settings:

   >>> import urllib
   >>> opener = urllib.FancyURLopener({})
   >>> f = opener.open("http://www.python.org/")
   >>> f.read()
