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 = 'http://example.com/image'
payload = b'GIF89a\x01\x00\x01\x00\x00\xff\x00,\x00\x00'
          b'\x00\x00\x01\x00\x01\x00\x00\x02\x00;'
headers = {'content-type': 'image/gif'}

await session.post(url,
                   data=payload,
                   headers=headers)

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("http://httpbin.org/headers") 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 session.post(url, json={'example': 'text'})

The same for text/plain:

await session.post(url, text='Привет, Мир!')

Custom Cookies

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

url = 'http://httpbin.org/cookies'
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

httpbin.org/cookies 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(
        'http://httpbin.org/cookies/set?my_cookie=my_value')
    filtered = session.cookie_jar.filter_cookies('http://httpbin.org')
    assert filtered['my_cookie'].value == 'my_value'
    async with session.get('http://httpbin.org/cookies') 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:

>>> resp.headers
{'ACCESS-CONTROL-ALLOW-ORIGIN': '*',
 '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:

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

>>> 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:

>>> 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 = 'http://example.com/some/cookie/setting/url'
async with session.get(url) as resp:
    print(resp.cookies['example_cookie_name'])

Note

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('http://example.com/some/redirect/')
>>> resp
<ClientResponse(http://example.com/some/other/url/) [200]>
>>> resp.history
(<ClientResponse(http://example.com/some/redirect/) [301]>,)

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 session.post('http://httpbin.org/post',
                            data=data,
                            headers=headers)
        pass

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()
trace_config.on_request_start.append(on_request_start)
trace_config.on_request_end.append(on_request_end)
async with aiohttp.ClientSession(trace_configs=[trace_config]) as client:
    client.get('http://example.com/some/redirect/')

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:
    client.get('http://example.com/some/redirect/')

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 = session.loop.time()

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

The trace_config_ctx param is by default a SimpleNampespace 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):
    print(trace_config_ctx.trace_request_ctx)


session.get('http://example.com/some/redirect/',
            trace_request_ctx={'foo': 'bar'})

See also

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

Connectors

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)

Note

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=["8.8.8.8", "8.8.4.4"])
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)

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('https://example.com', 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 proper ClientSession method:

sslcontext = ssl.create_default_context(
   cafile='/path/to/ca-bundle.crt')
r = await session.get('https://example.com', ssl=sslcontext)

If you need to verify self-signed certificates, you can do the same thing as the previous example, but add another call to ssl.SSLContext.load_cert_chain() with the key pair:

sslcontext = ssl.create_default_context(
   cafile='/path/to/ca-bundle.crt')
sslcontext.load_cert_chain('/path/to/client/public/device.pem',
                           '/path/to/client/private/device.jey')
r = await session.get('https://example.com', ssl=sslcontext)

There is explicit errors when ssl verification fails

aiohttp.ClientConnectorSSLError:

try:
    await session.get('https://expired.badssl.com/')
except aiohttp.ClientConnectorSSLError as e:
    assert isinstance(e, ssl.SSLError)

aiohttp.ClientConnectorCertificateError:

try:
    await session.get('https://wrong.host.badssl.com/')
except aiohttp.ClientConnectorCertificateError as e:
    assert isinstance(e, ssl.CertificateError)

If you need to skip both ssl related errors

aiohttp.ClientSSLError:

try:
    await session.get('https://expired.badssl.com/')
except aiohttp.ClientSSLError as e:
    assert isinstance(e, ssl.SSLError)

try:
    await session.get('https://wrong.host.badssl.com/')
except aiohttp.ClientSSLError as e:
    assert isinstance(e, ssl.CertificateError)

You may also verify certificates via SHA256 fingerprint:

# Attempt to connect to https://www.python.org
# with a pin to a bogus certificate:
bad_fingerprint = b'0'*64
exc = None
try:
    r = await session.get('https://www.python.org',
                          ssl=aiohttp.Fingerprint(bad_fingerprint))
except aiohttp.FingerprintMismatch as e:
    exc = e
assert exc is not None
assert exc.expected == bad_fingerprint

# www.python.org cert's actual fingerprint
assert exc.got == 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

Note

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 HTTP/HTTPS proxies. You have to use proxy parameter:

async with aiohttp.ClientSession() as session:
    async with session.get("http://python.org",
                           proxy="http://some.proxy.com") as resp:
        print(resp.status)

It also supports proxy authorization:

async with aiohttp.ClientSession() as session:
    proxy_auth = aiohttp.BasicAuth('user', 'pass')
    async with session.get("http://python.org",
                           proxy="http://some.proxy.com",
                           proxy_auth=proxy_auth) as resp:
        print(resp.status)

Authentication credentials can be passed in proxy URL:

session.get("http://python.org",
            proxy="http://user:[email protected]")

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 for extracting proxy configuration from HTTP_PROXY or HTTPS_PROXY environment variables (both are case insensitive):

async with aiohttp.ClientSession(trust_env=True) as session:
    async with session.get("http://python.org") as resp:
        print(resp.status)

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

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, an 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('http://example.org/') as response:
            await response.read()

loop = asyncio.get_event_loop()
loop.run_until_complete(read_website())
# Zero-sleep to allow underlying connections to close
loop.run_until_complete(asyncio.sleep(0))
loop.close()

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

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

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.