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The request/response API used by httpcore is kept deliberately simple and explicit.

The Request and Response models are pretty slim wrappers around this core API:

# Pseudo-code expressing the essentials of the request/response model.
    status_code: int,
    headers: List[Tuple(bytes, bytes)],
    stream: Iterable[bytes]
) = handle_request(
    method: bytes,
    url: URL,
    headers: List[Tuple(bytes, bytes)],
    stream: Iterable[bytes]

This is everything that's needed in order to represent an HTTP exchange.

Well... almost.

There is a maxim in Computer Science that "All non-trivial abstractions, to some degree, are leaky". When an expression is leaky, it's important that it ought to at least leak only in well-defined places.

In order to handle cases that don't otherwise fit inside this core abstraction, httpcore requests and responses have 'extensions'. These are a dictionary of optional additional information.

Let's expand on our request/response abstraction...

# Pseudo-code expressing the essentials of the request/response model,
# plus extensions allowing for additional API that does not fit into
# this abstraction.
    status_code: int,
    headers: List[Tuple(bytes, bytes)],
    stream: Iterable[bytes],
    extensions: dict
) = handle_request(
    method: bytes,
    url: URL,
    headers: List[Tuple(bytes, bytes)],
    stream: Iterable[bytes],
    extensions: dict

Several extensions are supported both on the request:

r = httpcore.request(
    extensions={"timeout": {"connect": 5.0}}

And on the response:

r = httpcore.request("GET", "")

# When using HTTP/1.1 on the client side, the server HTTP response
# could feasibly be one of b"HTTP/0.9", b"HTTP/1.0", or b"HTTP/1.1".

Request Extensions


A dictionary of str: Optional[float] timeout values.

May include values for 'connect', 'read', 'write', or 'pool'.

For example:

# Timeout if a connection takes more than 5 seconds to established, or if
# we are blocked waiting on the connection pool for more than 10 seconds.
r = httpcore.request(
    extensions={"timeout": {"connect": 5.0, "pool": 10.0}}


The trace extension allows a callback handler to be installed to monitor the internal flow of events within httpcore. The simplest way to explain this is with an example:

import httpcore

def log(event_name, info):
    print(event_name, info)

r = httpcore.request("GET", "", extensions={"trace": log})
# connection.connect_tcp.started {'host': '', 'port': 443, 'local_address': None, 'timeout': None}
# connection.connect_tcp.complete {'return_value': <httpcore.backends.sync.SyncStream object at 0x1093f94d0>}
# connection.start_tls.started {'ssl_context': <ssl.SSLContext object at 0x1093ee750>, 'server_hostname': b'', 'timeout': None}
# connection.start_tls.complete {'return_value': <httpcore.backends.sync.SyncStream object at 0x1093f9450>}
# http11.send_request_headers.started {'request': <Request [b'GET']>}
# http11.send_request_headers.complete {'return_value': None}
# http11.send_request_body.started {'request': <Request [b'GET']>}
# http11.send_request_body.complete {'return_value': None}
# http11.receive_response_headers.started {'request': <Request [b'GET']>}
# http11.receive_response_headers.complete {'return_value': (b'HTTP/1.1', 200, b'OK', [(b'Age', b'553715'), (b'Cache-Control', b'max-age=604800'), (b'Content-Type', b'text/html; charset=UTF-8'), (b'Date', b'Thu, 21 Oct 2021 17:08:42 GMT'), (b'Etag', b'"3147526947+ident"'), (b'Expires', b'Thu, 28 Oct 2021 17:08:42 GMT'), (b'Last-Modified', b'Thu, 17 Oct 2019 07:18:26 GMT'), (b'Server', b'ECS (nyb/1DCD)'), (b'Vary', b'Accept-Encoding'), (b'X-Cache', b'HIT'), (b'Content-Length', b'1256')])}
# http11.receive_response_body.started {'request': <Request [b'GET']>}
# http11.receive_response_body.complete {'return_value': None}
# http11.response_closed.started {}
# http11.response_closed.complete {'return_value': None}

The event_name and info arguments here will be one of the following:

  • {event_type}.{event_name}.started, <dictionary of keyword arguments>
  • {event_type}.{event_name}.complete, {"return_value": <...>}
  • {event_type}.{event_name}.failed, {"exception": <...>}

Note that when using the async variant of httpcore the handler function passed to "trace" must be an async def ... function.

The following event types are currently exposed...

Establishing the connection

  • "connection.connect_tcp"
  • "connection.connect_unix_socket"
  • "connection.start_tls"

HTTP/1.1 events

  • "http11.send_request_headers"
  • "http11.send_request_body"
  • "http11.receive_response"
  • "http11.receive_response_body"
  • "http11.response_closed"

HTTP/2 events

  • "http2.send_connection_init"
  • "http2.send_request_headers"
  • "http2.send_request_body"
  • "http2.receive_response_headers"
  • "http2.receive_response_body"
  • "http2.response_closed"

The exact set of trace events may be subject to change across different versions of httpcore. If you need to rely on a particular set of events it is recommended that you pin installation of the package to a fixed version.


The server's hostname, which is used to confirm the hostname supplied by the SSL certificate.

For example:

headers = {"Host": ""}
extensions = {"sni_hostname": ""}
response = httpcore.request(

Response Extensions


The HTTP version, as bytes. Eg. b"HTTP/1.1".

When using HTTP/1.1 the response line includes an explicit version, and the value of this key could feasibly be one of b"HTTP/0.9", b"HTTP/1.0", or b"HTTP/1.1".

When using HTTP/2 there is no further response versioning included in the protocol, and the value of this key will always be b"HTTP/2".


The reason-phrase of the HTTP response, as bytes. For example b"OK". Some servers may include a custom reason phrase, although this is not recommended.

HTTP/2 onwards does not include a reason phrase on the wire.

When no key is included, a default based on the status code may be used.


When HTTP/2 is being used the "stream_id" response extension can be accessed to determine the ID of the data stream that the response was sent on.


The "network_stream" extension allows developers to handle HTTP CONNECT and Upgrade requests, by providing an API that steps outside the standard request/response model, and can directly read or write to the network.

The interface provided by the network stream:

  • read(max_bytes, timeout = None) -> bytes
  • write(buffer, timeout = None)
  • close()
  • start_tls(ssl_context, server_hostname = None, timeout = None) -> NetworkStream
  • get_extra_info(info) -> Any

This API can be used as the foundation for working with HTTP proxies, WebSocket upgrades, and other advanced use-cases.

See the network backends documentation for more information on working directly with network streams.

CONNECT requests

A proxy CONNECT request using the network stream:

# Formulate a CONNECT request...
# This will establish a connection to, and then send the following...
# Host:
url = httpcore.URL(b"http", b"", 8080, b"")
with"CONNECT", url) as response:
    network_stream = response.extensions["network_stream"]

    # Upgrade to an SSL stream...
    network_stream = network_stream.start_tls(

    # Manually send an HTTP request over the network stream, and read the response...
    # For a more complete example see the httpcore `TunnelHTTPConnection` implementation.
    network_stream.write(b"GET / HTTP/1.1\r\nHost:\r\n\r\n")
    data =
Upgrade requests

Using the wsproto package to handle a websockets session:

import httpcore
import wsproto
import os
import base64

url = ""
headers = {
    b"Connection": b"Upgrade",
    b"Upgrade": b"WebSocket",
    b"Sec-WebSocket-Key": base64.b64encode(os.urandom(16)),
    b"Sec-WebSocket-Version": b"13"
with"GET", url, headers=headers) as response:
    if response.status != 101:
        raise Exception("Failed to upgrade to websockets", response)

    # Get the raw network stream.
    network_steam = response.extensions["network_stream"]

    # Write a WebSocket text frame to the stream.
    ws_connection = wsproto.Connection(wsproto.ConnectionType.CLIENT)
    message ="hello, world!")
    outgoing_data = ws_connection.send(message)

    # Wait for a response.
    incoming_data =
    for event in
        if isinstance(event,
            print("Got data:",

    # Write a WebSocket close to the stream.
    message =
    outgoing_data = ws_connection.send(message)
Extra network information

The network stream abstraction also allows access to various low-level information that may be exposed by the underlying socket:

response = httpcore.request("GET", "")
network_stream = response.extensions["network_stream"]

client_addr = network_stream.get_extra_info("client_addr")
server_addr = network_stream.get_extra_info("server_addr")
print("Client address", client_addr)
print("Server address", server_addr)

The socket SSL information is also available through this interface, although you need to ensure that the underlying connection is still open, in order to access it...

with"GET", "") as response:
    network_stream = response.extensions["network_stream"]

    ssl_object = network_stream.get_extra_info("ssl_object")
    print("TLS version", ssl_object.version())