Advanced Client Usage

Client Session

ClientSession is the heart and the main entry point for all client API operations.

Create the session first, use the instance for performing HTTP requests and initiating WebSocket connections.

The session contains a cookie storage and connection pool, thus cookies and connections are shared between HTTP requests sent by the same session.

Custom Request Headers

If you need to add HTTP headers to a request, pass them in a dict to the headers parameter.

For example, if you want to specify the content-type directly:

url = ''
payload = b'GIF89a\x01\x00\x01\x00\x00\xff\x00,\x00\x00'
headers = {'content-type': 'image/gif'}


You also can set default headers for all session requests:

headers={"Authorization": "Basic bG9naW46cGFzcw=="}
async with aiohttp.ClientSession(headers=headers) as session:
    async with session.get("") as r:
        json_body = await r.json()
        assert json_body['headers']['Authorization'] == \
            'Basic bG9naW46cGFzcw=='

Typical use case is sending JSON body. You can specify content type directly as shown above, but it is more convenient to use special keyword json:

await, json={'example': 'text'})

For text/plain

await, data='Привет, Мир!')


Authorization header will be removed if you get redirected to a different host or protocol.

Custom Cookies

To send your own cookies to the server, you can use the cookies parameter of ClientSession constructor:

url = ''
cookies = {'cookies_are': 'working'}
async with ClientSession(cookies=cookies) as session:
    async with session.get(url) as resp:
        assert await resp.json() == {
           "cookies": {"cookies_are": "working"}}

Note endpoint returns request cookies in JSON-encoded body. To access session cookies see ClientSession.cookie_jar.

ClientSession may be used for sharing cookies between multiple requests:

async with aiohttp.ClientSession() as session:
    await session.get(
    filtered = session.cookie_jar.filter_cookies(
    assert filtered['my_cookie'].value == 'my_value'
    async with session.get('') as r:
        json_body = await r.json()
        assert json_body['cookies']['my_cookie'] == 'my_value'

Response Headers and Cookies

We can view the server’s response ClientResponse.headers using a CIMultiDictProxy:

assert resp.headers == {
    'CONTENT-TYPE': 'application/json',
    'DATE': 'Tue, 15 Jul 2014 16:49:51 GMT',
    'SERVER': 'gunicorn/18.0',
    'CONTENT-LENGTH': '331',
    'CONNECTION': 'keep-alive'}

The dictionary is special, though: it’s made just for HTTP headers. According to RFC 7230, HTTP Header names are case-insensitive. It also supports multiple values for the same key as HTTP protocol does.

So, we can access the headers using any capitalization we want:

assert resp.headers['Content-Type'] == 'application/json'

assert resp.headers.get('content-type') == 'application/json'

All headers are converted from binary data using UTF-8 with surrogateescape option. That works fine on most cases but sometimes unconverted data is needed if a server uses nonstandard encoding. While these headers are malformed from RFC 7230 perspective they may be retrieved by using ClientResponse.raw_headers property:

assert resp.raw_headers == (
    (b'SERVER', b'nginx'),
    (b'DATE', b'Sat, 09 Jan 2016 20:28:40 GMT'),
    (b'CONTENT-TYPE', b'text/html; charset=utf-8'),
    (b'CONTENT-LENGTH', b'12150'),
    (b'CONNECTION', b'keep-alive'))

If a response contains some HTTP Cookies, you can quickly access them:

url = ''
async with session.get(url) as resp:


Response cookies contain only values, that were in Set-Cookie headers of the last request in redirection chain. To gather cookies between all redirection requests please use aiohttp.ClientSession object.

Redirection History

If a request was redirected, it is possible to view previous responses using the history attribute:

resp = await session.get('')
assert resp.status == 200
assert resp.url = URL('')
assert len(resp.history) == 1
assert resp.history[0].status == 301
assert resp.history[0].url = URL(

If no redirects occurred or allow_redirects is set to False, history will be an empty sequence.

Uploading pre-compressed data

To upload data that is already compressed before passing it to aiohttp, call the request function with the used compression algorithm name (usually deflate or gzip) as the value of the Content-Encoding header:

async def my_coroutine(session, headers, my_data):
    data = zlib.compress(my_data)
    headers = {'Content-Encoding': 'deflate'}
    async with'',

Disabling content type validation for JSON responses

The standard explicitly restricts JSON Content-Type HTTP header to application/json or any extended form, e.g. application/vnd.custom-type+json. Unfortunately, some servers send a wrong type, like text/html.

This can be worked around in two ways:

  1. Pass the expected type explicitly (in this case checking will be strict, without the extended form support, so custom/xxx+type won’t be accepted):

    await resp.json(content_type='custom/type').

  2. Disable the check entirely:

    await resp.json(content_type=None).

Client Tracing

The execution flow of a specific request can be followed attaching listeners coroutines to the signals provided by the TraceConfig instance, this instance will be used as a parameter for the ClientSession constructor having as a result a client that triggers the different signals supported by the TraceConfig. By default any instance of ClientSession class comes with the signals ability disabled. The following snippet shows how the start and the end signals of a request flow can be followed:

async def on_request_start(
        session, trace_config_ctx, params):
    print("Starting request")

async def on_request_end(session, trace_config_ctx, params):
    print("Ending request")

trace_config = aiohttp.TraceConfig()
async with aiohttp.ClientSession(
        trace_configs=[trace_config]) as client:

The trace_configs is a list that can contain instances of TraceConfig class that allow run the signals handlers coming from different TraceConfig instances. The following example shows how two different TraceConfig that have a different nature are installed to perform their job in each signal handle:

from mylib.traceconfig import AuditRequest
from mylib.traceconfig import XRay

async with aiohttp.ClientSession(
        trace_configs=[AuditRequest(), XRay()]) as client:

All signals take as a parameters first, the ClientSession instance used by the specific request related to that signals and second, a SimpleNamespace instance called trace_config_ctx. The trace_config_ctx object can be used to share the state through to the different signals that belong to the same request and to the same TraceConfig class, perhaps:

async def on_request_start(
        session, trace_config_ctx, params):
    trace_config_ctx.start = asyncio.get_event_loop().time()

async def on_request_end(session, trace_config_ctx, params):
    elapsed = asyncio.get_event_loop().time() - trace_config_ctx.start
    print("Request took {}".format(elapsed))

The trace_config_ctx param is by default a SimpleNamespace that is initialized at the beginning of the request flow. However, the factory used to create this object can be overwritten using the trace_config_ctx_factory constructor param of the TraceConfig class.

The trace_request_ctx param can given at the beginning of the request execution, accepted by all of the HTTP verbs, and will be passed as a keyword argument for the trace_config_ctx_factory factory. This param is useful to pass data that is only available at request time, perhaps:

async def on_request_start(
        session, trace_config_ctx, params):

            trace_request_ctx={'foo': 'bar'})

See also

Tracing Reference section for more information about the different signals supported.


To tweak or change transport layer of requests you can pass a custom connector to ClientSession and family. For example:

conn = aiohttp.TCPConnector()
session = aiohttp.ClientSession(connector=conn)


By default session object takes the ownership of the connector, among other things closing the connections once the session is closed. If you are keen on share the same connector through different session instances you must give the connector_owner parameter as False for each session instance.

See also

Connectors section for more information about different connector types and configuration options.

Limiting connection pool size

To limit amount of simultaneously opened connections you can pass limit parameter to connector:

conn = aiohttp.TCPConnector(limit=30)

The example limits total amount of parallel connections to 30.

The default is 100.

If you explicitly want not to have limits, pass 0. For example:

conn = aiohttp.TCPConnector(limit=0)

To limit amount of simultaneously opened connection to the same endpoint ((host, port, is_ssl) triple) you can pass limit_per_host parameter to connector:

conn = aiohttp.TCPConnector(limit_per_host=30)

The example limits amount of parallel connections to the same to 30.

The default is 0 (no limit on per host bases).

Tuning the DNS cache

By default TCPConnector comes with the DNS cache table enabled, and resolutions will be cached by default for 10 seconds. This behavior can be changed either to change of the TTL for a resolution, as can be seen in the following example:

conn = aiohttp.TCPConnector(ttl_dns_cache=300)

or disabling the use of the DNS cache table, meaning that all requests will end up making a DNS resolution, as the following example shows:

conn = aiohttp.TCPConnector(use_dns_cache=False)

Resolving using custom nameservers

In order to specify the nameservers to when resolving the hostnames, aiodns is required:

from aiohttp.resolver import AsyncResolver

resolver = AsyncResolver(nameservers=["", ""])
conn = aiohttp.TCPConnector(resolver=resolver)

Unix domain sockets

If your HTTP server uses UNIX domain sockets you can use UnixConnector:

conn = aiohttp.UnixConnector(path='/path/to/socket')
session = aiohttp.ClientSession(connector=conn)

Named pipes in Windows

If your HTTP server uses Named pipes you can use NamedPipeConnector:

conn = aiohttp.NamedPipeConnector(path=r'\\.\pipe\<name-of-pipe>')
session = aiohttp.ClientSession(connector=conn)

It will only work with the ProactorEventLoop

SSL control for TCP sockets

By default aiohttp uses strict checks for HTTPS protocol. Certification checks can be relaxed by setting ssl to False:

r = await session.get('', ssl=False)

If you need to setup custom ssl parameters (use own certification files for example) you can create a ssl.SSLContext instance and pass it into the ClientSession.request() methods or set it for the entire session with ClientSession(connector=TCPConnector(ssl=ssl_context)).

There are explicit errors when ssl verification fails


    await session.get('')
except aiohttp.ClientConnectorSSLError as e:
    assert isinstance(e, ssl.SSLError)


    await session.get('')
except aiohttp.ClientConnectorCertificateError as e:
    assert isinstance(e, ssl.CertificateError)

If you need to skip both ssl related errors


    await session.get('')
except aiohttp.ClientSSLError as e:
    assert isinstance(e, ssl.SSLError)

    await session.get('')
except aiohttp.ClientSSLError as e:
    assert isinstance(e, ssl.CertificateError)

Example: Use certifi

By default, Python uses the system CA certificates. In rare cases, these may not be installed or Python is unable to find them, resulting in a error like ssl.SSLCertVerificationError: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: unable to get local issuer certificate

One way to work around this problem is to use the certifi package:

ssl_context = ssl.create_default_context(cafile=certifi.where())
async with ClientSession(connector=TCPConnector(ssl=ssl_context)) as sess:

Example: Use self-signed certificate

If you need to verify self-signed certificates, you need to add a call to ssl.SSLContext.load_cert_chain() with the key pair:

ssl_context = ssl.create_default_context()
async with sess.get("", ssl=ssl_context) as resp:

Example: Verify certificate fingerprint

You may also verify certificates via SHA256 fingerprint:

# Attempt to connect to
# with a pin to a bogus certificate:
bad_fp = b'0'*64
exc = None
    r = await session.get('',
except aiohttp.FingerprintMismatch as e:
    exc = e
assert exc is not None
assert exc.expected == bad_fp

# cert's actual fingerprint
assert == b'...'

Note that this is the fingerprint of the DER-encoded certificate. If you have the certificate in PEM format, you can convert it to DER with e.g:

openssl x509 -in crt.pem -inform PEM -outform DER > crt.der


Tip: to convert from a hexadecimal digest to a binary byte-string, you can use binascii.unhexlify().

ssl parameter could be passed to TCPConnector as default, the value from ClientSession.get() and others override default.

Proxy support

aiohttp supports plain HTTP proxies and HTTP proxies that can be upgraded to HTTPS via the HTTP CONNECT method. aiohttp has a limited support for proxies that must be connected to via https:// — see the info box below for more details. To connect, use the proxy parameter:

async with aiohttp.ClientSession() as session:
    async with session.get("",
                           proxy="") as resp:

It also supports proxy authorization:

async with aiohttp.ClientSession() as session:
    proxy_auth = aiohttp.BasicAuth('user', 'pass')
    async with session.get("",
                           proxy_auth=proxy_auth) as resp:

Authentication credentials can be passed in proxy URL:


Contrary to the requests library, it won’t read environment variables by default. But you can do so by passing trust_env=True into aiohttp.ClientSession constructor.:

async with aiohttp.ClientSession(trust_env=True) as session:
    async with session.get("") as resp:


aiohttp uses urllib.request.getproxies() for reading the proxy configuration (e.g. from the HTTP_PROXY etc. environment variables) and applies them for the HTTP, HTTPS, WS and WSS schemes.

Hosts defined in no_proxy will bypass the proxy.

New in version 3.8: WS_PROXY and WSS_PROXY are supported since aiohttp v3.8.

Proxy credentials are given from ~/.netrc file if present (see aiohttp.ClientSession for more details).


As of now (Python 3.10), support for TLS in TLS is disabled for the transports that asyncio uses. If the further release of Python (say v3.11) toggles one attribute, it’ll just work™.

aiohttp v3.8 and higher is ready for this to happen and has code in place supports TLS-in-TLS, hence sending HTTPS requests over HTTPS proxy tunnels.

⚠️ For as long as your Python runtime doesn’t declare the support for TLS-in-TLS, please don’t file bugs with aiohttp but rather try to help the CPython upstream enable this feature. Meanwhile, if you really need this to work, there’s a patch that may help you make it happen, include it into your app’s code base:


When supplying a custom ssl.SSLContext instance, bear in mind that it will be used not only to establish a TLS session with the HTTPS endpoint you’re hitting but also to establish a TLS tunnel to the HTTPS proxy. To avoid surprises, make sure to set up the trust chain that would recognize TLS certificates used by both the endpoint and the proxy.

Graceful Shutdown

When ClientSession closes at the end of an async with block (or through a direct ClientSession.close() call), the underlying connection remains open due to asyncio internal details. In practice, the underlying connection will close after a short while. However, if the event loop is stopped before the underlying connection is closed, a ResourceWarning: unclosed transport warning is emitted (when warnings are enabled).

To avoid this situation, a small delay must be added before closing the event loop to allow any open underlying connections to close.

For a ClientSession without SSL, a simple zero-sleep (await asyncio.sleep(0)) will suffice:

async def read_website():
    async with aiohttp.ClientSession() as session:
        async with session.get('') as resp:
    # Zero-sleep to allow underlying connections to close
    await asyncio.sleep(0)

For a ClientSession with SSL, the application must wait a short duration before closing:

# Wait 250 ms for the underlying SSL connections to close
await asyncio.sleep(0.250)

Note that the appropriate amount of time to wait will vary from application to application.

All if this will eventually become obsolete when the asyncio internals are changed so that aiohttp itself can wait on the underlying connection to close. Please follow issue #1925 for the progress on this.

Character Set Detection

If you encounter a UnicodeDecodeError when using ClientResponse.text() this may be because the response does not include the charset needed to decode the body.

If you know the correct encoding for a request, you can simply specify the encoding as a parameter (e.g. resp.text("windows-1252")).

Alternatively, ClientSession accepts a fallback_charset_resolver parameter which can be used to introduce charset guessing functionality. When a charset is not found in the Content-Type header, this function will be called to get the charset encoding. For example, this can be used with the chardetng_py library.:

from chardetng_py import detect

def charset_resolver(resp: ClientResponse, body: bytes) -> str:
    tld =".", maxsplit=1)[-1]
    return detect(body, allow_utf8=True, tld=tld.encode())


Or, if chardetng_py doesn’t work for you, then charset-normalizer is another option:

from charset_normalizer import detect

ClientSession(fallback_charset_resolver=lambda r, b: detect(b)["encoding"] or "utf-8")