21.22. "http.server" — HTTP servers
***********************************

**Source code:** Lib/http/server.py

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

This module defines classes for implementing HTTP servers (Web
servers).

One class, "HTTPServer", is a "socketserver.TCPServer" subclass. It
creates and listens at the HTTP socket, dispatching the requests to a
handler.  Code to create and run the server looks like this:

   def run(server_class=HTTPServer, handler_class=BaseHTTPRequestHandler):
       server_address = ('', 8000)
       httpd = server_class(server_address, handler_class)
       httpd.serve_forever()

class http.server.HTTPServer(server_address, RequestHandlerClass)

   This class builds on the "TCPServer" class by storing the server
   address as instance variables named "server_name" and
   "server_port". The server is accessible by the handler, typically
   through the handler’s "server" instance variable.

The "HTTPServer" must be given a *RequestHandlerClass* on
instantiation, of which this module provides three different variants:

class http.server.BaseHTTPRequestHandler(request, client_address, server)

   This class is used to handle the HTTP requests that arrive at the
   server.  By itself, it cannot respond to any actual HTTP requests;
   it must be subclassed to handle each request method (e.g. GET or
   POST). "BaseHTTPRequestHandler" provides a number of class and
   instance variables, and methods for use by subclasses.

   The handler will parse the request and the headers, then call a
   method specific to the request type. The method name is constructed
   from the request. For example, for the request method "SPAM", the
   "do_SPAM()" method will be called with no arguments. All of the
   relevant information is stored in instance variables of the
   handler.  Subclasses should not need to override or extend the
   "__init__()" method.

   "BaseHTTPRequestHandler" has the following instance variables:

   client_address

      Contains a tuple of the form "(host, port)" referring to the
      client’s address.

   server

      Contains the server instance.

   close_connection

      Boolean that should be set before "handle_one_request()"
      returns, indicating if another request may be expected, or if
      the connection should be shut down.

   requestline

      Contains the string representation of the HTTP request line. The
      terminating CRLF is stripped. This attribute should be set by
      "handle_one_request()". If no valid request line was processed,
      it should be set to the empty string.

   command

      Contains the command (request type). For example, "'GET'".

   path

      Contains the request path.

   request_version

      Contains the version string from the request. For example,
      "'HTTP/1.0'".

   headers

      Holds an instance of the class specified by the "MessageClass"
      class variable. This instance parses and manages the headers in
      the HTTP request. The "parse_headers()" function from
      "http.client" is used to parse the headers and it requires that
      the HTTP request provide a valid **RFC 2822** style header.

   rfile

      Contains an input stream, positioned at the start of the
      optional input data.

   wfile

      Contains the output stream for writing a response back to the
      client. Proper adherence to the HTTP protocol must be used when
      writing to this stream.

   "BaseHTTPRequestHandler" has the following class variables:

   server_version

      Specifies the server software version.  You may want to override
      this. The format is multiple whitespace-separated strings, where
      each string is of the form name[/version]. For example,
      "'BaseHTTP/0.2'".

   sys_version

      Contains the Python system version, in a form usable by the
      "version_string" method and the "server_version" class variable.
      For example, "'Python/1.4'".

   error_message_format

      Specifies a format string for building an error response to the
      client. It uses parenthesized, keyed format specifiers, so the
      format operand must be a dictionary. The *code* key should be an
      integer, specifying the numeric HTTP error code value. *message*
      should be a string containing a (detailed) error message of what
      occurred, and *explain* should be an explanation of the error
      code number. Default *message* and *explain* values can found in
      the "responses" class variable.

   error_content_type

      Specifies the Content-Type HTTP header of error responses sent
      to the client.  The default value is "'text/html'".

   protocol_version

      This specifies the HTTP protocol version used in responses.  If
      set to "'HTTP/1.1'", the server will permit HTTP persistent
      connections; however, your server *must* then include an
      accurate "Content-Length" header (using "send_header()") in all
      of its responses to clients. For backwards compatibility, the
      setting defaults to "'HTTP/1.0'".

   MessageClass

      Specifies an "email.message.Message"-like class to parse HTTP
      headers.  Typically, this is not overridden, and it defaults to
      "http.client.HTTPMessage".

   responses

      This variable contains a mapping of error code integers to two-
      element tuples containing a short and long message. For example,
      "{code: (shortmessage, longmessage)}". The *shortmessage* is
      usually used as the *message* key in an error response, and
      *longmessage* as the *explain* key (see the
      "error_message_format" class variable).

   A "BaseHTTPRequestHandler" instance has the following methods:

   handle()

      Calls "handle_one_request()" once (or, if persistent connections
      are enabled, multiple times) to handle incoming HTTP requests.
      You should never need to override it; instead, implement
      appropriate "do_*()" methods.

   handle_one_request()

      This method will parse and dispatch the request to the
      appropriate "do_*()" method.  You should never need to override
      it.

   handle_expect_100()

      When a HTTP/1.1 compliant server receives an "Expect:
      100-continue" request header it responds back with a "100
      Continue" followed by "200 OK" headers. This method can be
      overridden to raise an error if the server does not want the
      client to continue.  For e.g. server can chose to send "417
      Expectation Failed" as a response header and "return False".

      New in version 3.2.

   send_error(code, message=None, explain=None)

      Sends and logs a complete error reply to the client. The numeric
      *code* specifies the HTTP error code, with *message* as an
      optional, short, human readable description of the error.  The
      *explain* argument can be used to provide more detailed
      information about the error; it will be formatted using the
      "error_message_format" class variable and emitted, after a
      complete set of headers, as the response body.  The "responses"
      class variable holds the default values for *message* and
      *explain* that will be used if no value is provided; for unknown
      codes the default value for both is the string "???".

      Changed in version 3.4: The error response includes a Content-
      Length header. Added the *explain* argument.

   send_response(code, message=None)

      Adds a response header to the headers buffer and logs the
      accepted request. The HTTP response line is written to the
      internal buffer, followed by *Server* and *Date* headers. The
      values for these two headers are picked up from the
      "version_string()" and "date_time_string()" methods,
      respectively. If the server does not intend to send any other
      headers using the "send_header()" method, then "send_response()"
      should be followed by an "end_headers()" call.

      Changed in version 3.3: Headers are stored to an internal buffer
      and "end_headers()" needs to be called explicitly.

   send_header(keyword, value)

      Adds the HTTP header to an internal buffer which will be written
      to the output stream when either "end_headers()" or
      "flush_headers()" is invoked. *keyword* should specify the
      header keyword, with *value* specifying its value. Note that,
      after the send_header calls are done, "end_headers()" MUST BE
      called in order to complete the operation.

      Changed in version 3.2: Headers are stored in an internal
      buffer.

   send_response_only(code, message=None)

      Sends the response header only, used for the purposes when "100
      Continue" response is sent by the server to the client. The
      headers not buffered and sent directly the output stream.If the
      *message* is not specified, the HTTP message corresponding the
      response *code*  is sent.

      New in version 3.2.

   end_headers()

      Adds a blank line (indicating the end of the HTTP headers in the
      response) to the headers buffer and calls "flush_headers()".

      Changed in version 3.2: The buffered headers are written to the
      output stream.

   flush_headers()

      Finally send the headers to the output stream and flush the
      internal headers buffer.

      New in version 3.3.

   log_request(code='-', size='-')

      Logs an accepted (successful) request. *code* should specify the
      numeric HTTP code associated with the response. If a size of the
      response is available, then it should be passed as the *size*
      parameter.

   log_error(...)

      Logs an error when a request cannot be fulfilled. By default, it
      passes the message to "log_message()", so it takes the same
      arguments (*format* and additional values).

   log_message(format, ...)

      Logs an arbitrary message to "sys.stderr". This is typically
      overridden to create custom error logging mechanisms. The
      *format* argument is a standard printf-style format string,
      where the additional arguments to "log_message()" are applied as
      inputs to the formatting. The client ip address and current date
      and time are prefixed to every message logged.

   version_string()

      Returns the server software’s version string. This is a
      combination of the "server_version" and "sys_version" class
      variables.

   date_time_string(timestamp=None)

      Returns the date and time given by *timestamp* (which must be
      None or in the format returned by "time.time()"), formatted for
      a message header. If *timestamp* is omitted, it uses the current
      date and time.

      The result looks like "'Sun, 06 Nov 1994 08:49:37 GMT'".

   log_date_time_string()

      Returns the current date and time, formatted for logging.

   address_string()

      Returns the client address.

      Changed in version 3.3: Previously, a name lookup was performed.
      To avoid name resolution delays, it now always returns the IP
      address.

class http.server.SimpleHTTPRequestHandler(request, client_address, server)

   This class serves files from the current directory and below,
   directly mapping the directory structure to HTTP requests.

   A lot of the work, such as parsing the request, is done by the base
   class "BaseHTTPRequestHandler".  This class implements the
   "do_GET()" and "do_HEAD()" functions.

   The following are defined as class-level attributes of
   "SimpleHTTPRequestHandler":

   server_version

      This will be ""SimpleHTTP/" + __version__", where "__version__"
      is defined at the module level.

   extensions_map

      A dictionary mapping suffixes into MIME types. The default is
      signified by an empty string, and is considered to be
      "application/octet-stream". The mapping is used case-
      insensitively, and so should contain only lower-cased keys.

   The "SimpleHTTPRequestHandler" class defines the following methods:

   do_HEAD()

      This method serves the "'HEAD'" request type: it sends the
      headers it would send for the equivalent "GET" request. See the
      "do_GET()" method for a more complete explanation of the
      possible headers.

   do_GET()

      The request is mapped to a local file by interpreting the
      request as a path relative to the current working directory.

      If the request was mapped to a directory, the directory is
      checked for a file named "index.html" or "index.htm" (in that
      order). If found, the file’s contents are returned; otherwise a
      directory listing is generated by calling the "list_directory()"
      method. This method uses "os.listdir()" to scan the directory,
      and returns a "404" error response if the "listdir()" fails.

      If the request was mapped to a file, it is opened and the
      contents are returned.  Any "OSError" exception in opening the
      requested file is mapped to a "404", "'File not found'" error.
      Otherwise, the content type is guessed by calling the
      "guess_type()" method, which in turn uses the *extensions_map*
      variable.

      A "'Content-type:'" header with the guessed content type is
      output, followed by a "'Content-Length:'" header with the file’s
      size and a "'Last-Modified:'" header with the file’s
      modification time.

      Then follows a blank line signifying the end of the headers, and
      then the contents of the file are output. If the file’s MIME
      type starts with "text/" the file is opened in text mode;
      otherwise binary mode is used.

      For example usage, see the implementation of the "test()"
      function invocation in the "http.server" module.

The "SimpleHTTPRequestHandler" class can be used in the following
manner in order to create a very basic webserver serving files
relative to the current directory:

   import http.server
   import socketserver

   PORT = 8000

   Handler = http.server.SimpleHTTPRequestHandler

   httpd = socketserver.TCPServer(("", PORT), Handler)

   print("serving at port", PORT)
   httpd.serve_forever()

"http.server" can also be invoked directly using the "-m" switch of
the interpreter with a "port number" argument.  Similar to the
previous example, this serves files relative to the current directory:

   python -m http.server 8000

By default, server binds itself to all interfaces.  The option
"-b/--bind" specifies a specific address to which it should bind.  For
example, the following command causes the server to bind to localhost
only:

   python -m http.server 8000 --bind 127.0.0.1

New in version 3.4: "--bind" argument was introduced.

class http.server.CGIHTTPRequestHandler(request, client_address, server)

   This class is used to serve either files or output of CGI scripts
   from the current directory and below. Note that mapping HTTP
   hierarchic structure to local directory structure is exactly as in
   "SimpleHTTPRequestHandler".

   Note: CGI scripts run by the "CGIHTTPRequestHandler" class cannot
     execute redirects (HTTP code 302), because code 200 (script
     output follows) is sent prior to execution of the CGI script.
     This pre-empts the status code.

   The class will however, run the CGI script, instead of serving it
   as a file, if it guesses it to be a CGI script.  Only directory-
   based CGI are used — the other common server configuration is to
   treat special extensions as denoting CGI scripts.

   The "do_GET()" and "do_HEAD()" functions are modified to run CGI
   scripts and serve the output, instead of serving files, if the
   request leads to somewhere below the "cgi_directories" path.

   The "CGIHTTPRequestHandler" defines the following data member:

   cgi_directories

      This defaults to "['/cgi-bin', '/htbin']" and describes
      directories to treat as containing CGI scripts.

   The "CGIHTTPRequestHandler" defines the following method:

   do_POST()

      This method serves the "'POST'" request type, only allowed for
      CGI scripts.  Error 501, “Can only POST to CGI scripts”, is
      output when trying to POST to a non-CGI url.

   Note that CGI scripts will be run with UID of user nobody, for
   security reasons.  Problems with the CGI script will be translated
   to error 403.

"CGIHTTPRequestHandler" can be enabled in the command line by passing
the "--cgi" option:

   python -m http.server --cgi 8000
