## 18.4.5 `wsgiref.handlers` - server/gateway base classes

This module provides base handler classes for implementing WSGI servers and gateways. These base classes handle most of the work of communicating with a WSGI application, as long as they are given a CGI-like environment, along with input, output, and error streams.

class `CGIHandler`( | )

  CGI-based invocation via `sys.stdin`, `sys.stdout`, `sys.stderr` and `os.environ`. This is useful when you have a WSGI application and want to run it as a CGI script. Simply invoke `CGIHandler().run(app)`, where `app` is the WSGI application object you wish to invoke.

  This class is a subclass of `BaseCGIHandler` that sets `wsgi.run_once` to true, `wsgi.multithread` to false, and `wsgi.multiprocess` to true, and always uses `sys` and `os` to obtain the necessary CGI streams and environment.

class `BaseCGIHandler`( | stdin, stdout, stderr, environ [, multithread=True [, multiprocess=False]])

  Similar to `CGIHandler`, but instead of using the `sys` and `os` modules, the CGI environment and I/O streams are specified explicitly. The multithread and multiprocess values are used to set the `wsgi.multithread` and `wsgi.multiprocess` flags for any applications run by the handler instance.

  This class is a subclass of `SimpleHandler` intended for use with software other than HTTP ``origin servers''. If you are writing a gateway protocol implementation (such as CGI, FastCGI, SCGI, etc.) that uses a `Status:` header to send an HTTP status, you probably want to subclass this instead of `SimpleHandler`.

class `SimpleHandler`( | stdin, stdout, stderr, environ [,multithread=True [, multiprocess=False]])

  Similar to `BaseCGIHandler`, but designed for use with HTTP origin servers. If you are writing an HTTP server implementation, you will probably want to subclass this instead of `BaseCGIHandler`

  This class is a subclass of `BaseHandler`. It overrides the `__init__()`, `get_stdin()`, `get_stderr()`, `add_cgi_vars()`, `_write()`, and `_flush()` methods to support explicitly setting the environment and streams via the constructor. The supplied environment and streams are stored in the `stdin`, `stdout`, `stderr`, and `environ` attributes.

class `BaseHandler`( | )

  This is an abstract base class for running WSGI applications. Each instance will handle a single HTTP request, although in principle you could create a subclass that was reusable for multiple requests.

  `BaseHandler` instances have only one method intended for external use:

  `run`( | app)

    Run the specified WSGI application, app.

  All of the other `BaseHandler` methods are invoked by this method in the process of running the application, and thus exist primarily to allow customizing the process.

  The following methods MUST be overridden in a subclass:

  `_write`( | data)

    Buffer the string data for transmission to the client. It's okay if this method actually transmits the data; `BaseHandler` just separates write and flush operations for greater efficiency when the underlying system actually has such a distinction.

  `_flush`( | )

    Force buffered data to be transmitted to the client. It's okay if this method is a no-op (i.e., if `_write()` actually sends the data).

  `get_stdin`( | )

    Return an input stream object suitable for use as the `wsgi.input` of the request currently being processed.

  `get_stderr`( | )

    Return an output stream object suitable for use as the `wsgi.errors` of the request currently being processed.

  `add_cgi_vars`( | )

    Insert CGI variables for the current request into the `environ` attribute.

  Here are some other methods and attributes you may wish to override. This list is only a summary, however, and does not include every method that can be overridden. You should consult the docstrings and source code for additional information before attempting to create a customized `BaseHandler` subclass.

  Attributes and methods for customizing the WSGI environment:

  `wsgi_multithread`
    The value to be used for the `wsgi.multithread` environment variable. It defaults to true in `BaseHandler`, but may have a different default (or be set by the constructor) in the other subclasses.

  `wsgi_multiprocess`
    The value to be used for the `wsgi.multiprocess` environment variable. It defaults to true in `BaseHandler`, but may have a different default (or be set by the constructor) in the other subclasses.

  `wsgi_run_once`
    The value to be used for the `wsgi.run_once` environment variable. It defaults to false in `BaseHandler`, but `CGIHandler` sets it to true by default.

  `os_environ`
    The default environment variables to be included in every request's WSGI environment. By default, this is a copy of `os.environ` at the time that `wsgiref.handlers` was imported, but subclasses can either create their own at the class or instance level. Note that the dictionary should be considered read-only, since the default value is shared between multiple classes and instances.

  `server_software`
    If the `origin_server` attribute is set, this attribute's value is used to set the default `SERVER_SOFTWARE` WSGI environment variable, and also to set a default `Server:` header in HTTP responses. It is ignored for handlers (such as `BaseCGIHandler` and `CGIHandler`) that are not HTTP origin servers.

  `get_scheme`( | )

    Return the URL scheme being used for the current request. The default implementation uses the `guess_scheme()` function from `wsgiref.util` to guess whether the scheme should be ``http'' or ``https'', based on the current request's `environ` variables.

  `setup_environ`( | )

    Set the `environ` attribute to a fully-populated WSGI environment. The default implementation uses all of the above methods and attributes, plus the `get_stdin()`, `get_stderr()`, and `add_cgi_vars()` methods and the `wsgi_file_wrapper` attribute. It also inserts a `SERVER_SOFTWARE` key if not present, as long as the `origin_server` attribute is a true value and the `server_software` attribute is set.

  Methods and attributes for customizing exception handling:

  `log_exception`( | exc_info)

    Log the exc_info tuple in the server log. exc_info is a `(type, value, traceback)` tuple. The default implementation simply writes the traceback to the request's `wsgi.errors` stream and flushes it. Subclasses can override this method to change the format or retarget the output, mail the traceback to an administrator, or whatever other action may be deemed suitable.

  `traceback_limit`
    The maximum number of frames to include in tracebacks output by the default `log_exception()` method. If `None`, all frames are included.

  `error_output`( | environ, start_response)

    This method is a WSGI application to generate an error page for the user. It is only invoked if an error occurs before headers are sent to the client.

    This method can access the current error information using `sys.exc_info()`, and should pass that information to start_response when calling it (as described in the ``Error Handling'' section of PEP 333).

    The default implementation just uses the `error_status`, `error_headers`, and `error_body` attributes to generate an output page. Subclasses can override this to produce more dynamic error output.

    Note, however, that it's not recommended from a security perspective to spit out diagnostics to any old user; ideally, you should have to do something special to enable diagnostic output, which is why the default implementation doesn't include any.

  `error_status`
    The HTTP status used for error responses. This should be a status string as defined in PEP 333; it defaults to a 500 code and message.

  `error_headers`
    The HTTP headers used for error responses. This should be a list of WSGI response headers (`(name, value)` tuples), as described in PEP 333. The default list just sets the content type to `text/plain`.

  `error_body`
    The error response body. This should be an HTTP response body string. It defaults to the plain text, ``A server error occurred. Please contact the administrator.''

  Methods and attributes for PEP 333's ``Optional Platform-Specific File Handling'' feature:

  `wsgi_file_wrapper`
    A `wsgi.file_wrapper` factory, or `None`. The default value of this attribute is the `FileWrapper` class from `wsgiref.util`.

  `sendfile`( | )

    Override to implement platform-specific file transmission. This method is called only if the application's return value is an instance of the class specified by the `wsgi_file_wrapper` attribute. It should return a true value if it was able to successfully transmit the file, so that the default transmission code will not be executed. The default implementation of this method just returns a false value.

  Miscellaneous methods and attributes:

  `origin_server`
    This attribute should be set to a true value if the handler's `_write()` and `_flush()` are being used to communicate directly to the client, rather than via a CGI-like gateway protocol that wants the HTTP status in a special `Status:` header.

    This attribute's default value is true in `BaseHandler`, but false in `BaseCGIHandler` and `CGIHandler`.

  `http_version`
    If `origin_server` is true, this string attribute is used to set the HTTP version of the response set to the client. It defaults to `"1.0"`.