'All parameters except ``url`` are optional.
:arg string url: URL to fetch
:arg string method: HTTP method, e.g. "GET" or "POST"
:arg headers: Additional HTTP headers to pass on the request
:type headers: `~tornado.httputil.HTTPHeaders` or `dict`
:arg body: HTTP request body as a string (byte or unicode; if unicode
the utf-8 encoding will be used)
:arg body_producer: Callable used for lazy/asynchronous request bodies.
It is called with one argument, a ``write`` function, and should
return a `.Future`.  It should call the write function with new
data as it becomes available.  The write function returns a
`.Future` which can be used for flow control.
Only one of ``body`` and ``body_producer`` may
be specified.  ``body_producer`` is not supported on
``curl_httpclient``.  When using ``body_producer`` it is recommended
to pass a ``Content-Length`` in the headers as otherwise chunked
encoding will be used, and many servers do not support chunked
encoding on requests.  New in Tornado 4.0
:arg string auth_username: Username for HTTP authentication
:arg string auth_password: Password for HTTP authentication
:arg string auth_mode: Authentication mode; default is "basic".
Allowed values are implementation-defined; ``curl_httpclient``
supports "basic" and "digest"; ``simple_httpclient`` only supports
"basic"
:arg float connect_timeout: Timeout for initial connection in seconds
:arg float request_timeout: Timeout for entire request in seconds
:arg if_modified_since: Timestamp for ``If-Modified-Since`` header
:type if_modified_since: `datetime` or `float`
:arg bool follow_redirects: Should redirects be followed automatically
or return the 3xx response?
:arg int max_redirects: Limit for ``follow_redirects``
:arg string user_agent: String to send as ``User-Agent`` header
:arg bool decompress_response: Request a compressed response from
the server and decompress it after downloading.  Default is True.
New in Tornado 4.0.
:arg bool use_gzip: Deprecated alias for ``decompress_response``
since Tornado 4.0.
:arg string network_interface: Network interface to use for request.
``curl_httpclient`` only; see note below.
:arg callable streaming_callback: If set, ``streaming_callback`` will
be run with each chunk of data as it is received, and
``HTTPResponse.body`` and ``HTTPResponse.buffer`` will be empty in
the final response.
:arg callable header_callback: If set, ``header_callback`` will
be run with each header line as it is received (including the
first line, e.g. ``HTTP/1.0 200 OK\r\n``, and a final line
containing only ``\r\n``.  All lines include the trailing newline
characters).  ``HTTPResponse.headers`` will be empty in the final
response.  This is most useful in conjunction with
``streaming_callback``, because it\'s the only way to get access to
header data while the request is in progress.
:arg callable prepare_curl_callback: If set, will be called with
a ``pycurl.Curl`` object to allow the application to make additional
``setopt`` calls.
:arg string proxy_host: HTTP proxy hostname.  To use proxies,
``proxy_host`` and ``proxy_port`` must be set; ``proxy_username`` and
``proxy_pass`` are optional.  Proxies are currently only supported
with ``curl_httpclient``.
:arg int proxy_port: HTTP proxy port
:arg string proxy_username: HTTP proxy username
:arg string proxy_password: HTTP proxy password
:arg bool allow_nonstandard_methods: Allow unknown values for ``method``
argument?
:arg bool validate_cert: For HTTPS requests, validate the server\'s
certificate?
:arg string ca_certs: filename of CA certificates in PEM format,
or None to use defaults.  See note below when used with
``curl_httpclient``.
:arg bool allow_ipv6: Use IPv6 when available?  Default is false in
``simple_httpclient`` and true in ``curl_httpclient``
:arg string client_key: Filename for client SSL key, if any.  See
note below when used with ``curl_httpclient``.
:arg string client_cert: Filename for client SSL certificate, if any.
See note below when used with ``curl_httpclient``.
:arg bool expect_100_continue: If true, send the
``Expect: 100-continue`` header and wait for a continue response
before sending the request body.  Only supported with
simple_httpclient.
.. note::
When using ``curl_httpclient`` certain options may be
inherited by subsequent fetches because ``pycurl`` does
not allow them to be cleanly reset.  This applies to the
``ca_certs``, ``client_key``, ``client_cert``, and
``network_interface`` arguments.  If you use these
options, you should pass them on every request (you don\'t
have to always use the same values, but it\'s not possible
to mix requests that specify these options with ones that
use the defaults).
.. versionadded:: 3.1
The ``auth_mode`` argument.
.. versionadded:: 4.0
The ``body_producer`` and ``expect_100_continue`` arguments.'

def __init__(self, url, method='GET', headers=None, body=None, auth_username=None, auth_password=None, auth_mode=None, connect_timeout=None, request_timeout=None, if_modified_since=None, follow_redirects=None, max_redirects=None, user_agent=None, use_gzip=None, network_interface=None, streaming_callback=None, header_callback=None, prepare_curl_callback=None, proxy_host=None, proxy_port=None, proxy_username=None, proxy_password=None, allow_nonstandard_methods=None, validate_cert=None, ca_certs=None, allow_ipv6=None, client_key=None, client_cert=None, body_producer=None, expect_100_continue=False, decompress_response=None):

    self.headers = headers
    if if_modified_since:
        self.headers['If-Modified-Since'] = httputil.format_timestamp(if_modified_since)
    self.proxy_host = proxy_host
    self.proxy_port = proxy_port
    self.proxy_username = proxy_username
    self.proxy_password = proxy_password
    self.url = url
    self.method = method
    self.body = body
    self.body_producer = body_producer
    self.auth_username = auth_username
    self.auth_password = auth_password
    self.auth_mode = auth_mode
    self.connect_timeout = connect_timeout
    self.request_timeout = request_timeout
    self.follow_redirects = follow_redirects
    self.max_redirects = max_redirects
    self.user_agent = user_agent
    if (decompress_response is not None):
        self.decompress_response = decompress_response
    else:
        self.decompress_response = use_gzip
    self.network_interface = network_interface
    self.streaming_callback = streaming_callback
    self.header_callback = header_callback
    self.prepare_curl_callback = prepare_curl_callback
    self.allow_nonstandard_methods = allow_nonstandard_methods
    self.validate_cert = validate_cert
    self.ca_certs = ca_certs
    self.allow_ipv6 = allow_ipv6
    self.client_key = client_key
    self.client_cert = client_cert
    self.expect_100_continue = expect_100_continue
    self.start_time = time.time()
