# Advanced Usage

## Customizing pool behavior

The `PoolManager` class automatically handles creating `ConnectionPool` instances for each host as needed. By default, it will keep a maximum of 10 `ConnectionPool` instances. If you’re making requests to many different hosts it might improve performance to increase this number:

    >>> import urllib3
    >>> http = urllib3.PoolManager(num_pools=50)

However, keep in mind that this does increase memory and socket consumption.

Similarly, the `ConnectionPool` class keeps a pool of individual `HTTPConnection` instances. These connections are used during an individual request and returned to the pool when the request is complete. By default only one connection will be saved for re-use. If you are making many requests to the same host simultaneously it might improve performance to increase this number:

    >>> import urllib3
    >>> http = urllib3.PoolManager(maxsize=10)
    # Alternatively
    >>> http = urllib3.HTTPConnectionPool('google.com', maxsize=10)

The behavior of the pooling for `ConnectionPool` is different from `PoolManager`. By default, if a new request is made and there is no free connection in the pool then a new connection will be created. However, this connection will not be saved if more than `maxsize` connections exist. This means that `maxsize` does not determine the maximum number of connections that can be open to a particular host, just the maximum number of connections to keep in the pool. However, if you specify `block=True` then there can be at most `maxsize` connections open to a particular host:

    >>> http = urllib3.PoolManager(maxsize=10, block=True)
    # Alternatively
    >>> http = urllib3.HTTPConnectionPool('google.com', maxsize=10, block=True)

Any new requests will block until a connection is available from the pool. This is a great way to prevent flooding a host with too many connections in multi-threaded applications.

## Streaming and IO

When dealing with large responses it’s often better to stream the response content:

    >>> import urllib3
    >>> http = urllib3.PoolManager()
    >>> r = http.request(
    ...     'GET',
    ...     'http://httpbin.org/bytes/1024',
    ...     preload_content=False)
    >>> for chunk in r.stream(32):
    ...     print(chunk)
    b'...'
    b'...'
    ...
    >>> r.release_conn()

Setting `preload_content` to `False` means that urllib3 will stream the response content. `stream()` lets you iterate over chunks of the response content.

Note

When using `preload_content=False`, you should call `release_conn()` to release the http connection back to the connection pool so that it can be re-used.

However, you can also treat the `HTTPResponse` instance as a file-like object. This allows you to do buffering:

    >>> r = http.request(
    ...     'GET',
    ...     'http://httpbin.org/bytes/1024',
    ...     preload_content=False)
    >>> r.read(4)
    b'\x88\x1f\x8b\xe5'

Calls to `read()` will block until more response data is available.

    >>> import io
    >>> reader = io.BufferedReader(r, 8)
    >>> reader.read(4)
    >>> r.release_conn()

You can use this file-like object to do things like decode the content using `codecs`:

    >>> import codecs
    >>> reader = codecs.getreader('utf-8')
    >>> r = http.request(
    ...     'GET',
    ...     'http://httpbin.org/ip',
    ...     preload_content=False)
    >>> json.load(reader(r))
    {'origin': '127.0.0.1'}
    >>> r.release_conn()

## Proxies

You can use `ProxyManager` to tunnel requests through an HTTP proxy:

    >>> import urllib3
    >>> proxy = urllib3.ProxyManager('http://localhost:3128/')
    >>> proxy.request('GET', 'http://google.com/')

The usage of `ProxyManager` is the same as `PoolManager`.

You can use `SOCKSProxyManager` to connect to SOCKS4 or SOCKS5 proxies. In order to use SOCKS proxies you will need to install PySocks or install urllib3 with the `socks` extra:

    pip install urllib3[socks]

Once PySocks is installed, you can use `SOCKSProxyManager`:

    >>> from urllib3.contrib.socks import SOCKSProxyManager
    >>> proxy = SOCKSProxyManager('socks5://localhost:8889/')
    >>> proxy.request('GET', 'http://google.com/')

## Custom SSL certificates

Instead of using certifi you can provide your own certificate authority bundle. This is useful for cases where you’ve generated your own certificates or when you’re using a private certificate authority. Just provide the full path to the certificate bundle when creating a `PoolManager`:

    >>> import urllib3
    >>> http = urllib3.PoolManager(
    ...     cert_reqs='CERT_REQUIRED',
    ...     ca_certs='/path/to/your/certificate_bundle')

When you specify your own certificate bundle only requests that can be verified with that bundle will succeed. It’s recommended to use a separate `PoolManager` to make requests to URLs that do not need the custom certificate.

## Client certificates

You can also specify a client certificate. This is useful when both the server and the client need to verify each other’s identity. Typically these certificates are issued from the same authority. To use a client certificate, provide the full path when creating a `PoolManager`:

    >>> http = urllib3.PoolManager(
    ...     cert_file='/path/to/your/client_cert.pem',
    ...     cert_reqs='CERT_REQUIRED',
    ...     ca_certs='/path/to/your/certificate_bundle')

If you have an encrypted client certificate private key you can use the `key_password` parameter to specify a password to decrypt the key.

    >>> http = urllib3.PoolManager(
    ...     cert_file='/path/to/your/client_cert.pem',
    ...     cert_reqs='CERT_REQUIRED',
    ...     key_file='/path/to/your/client.key',
    ...     key_password='keyfile_password')

If your key isn’t encrypted the `key_password` parameter isn’t required.

## Certificate validation and Mac OS X

Apple-provided Python and OpenSSL libraries contain a patches that make them automatically check the system keychain’s certificates. This can be surprising if you specify custom certificates and see requests unexpectedly succeed. For example, if you are specifying your own certificate for validation and the server presents a different certificate you would expect the connection to fail. However, if that server presents a certificate that is in the system keychain then the connection will succeed.

This article has more in-depth analysis and explanation.

## SSL Warnings

urllib3 will issue several different warnings based on the level of certificate verification support. These warnings indicate particular situations and can be resolved in different ways.

- `InsecureRequestWarning`
    This happens when a request is made to an HTTPS URL without certificate verification enabled. Follow the certificate verification guide to resolve this warning.
- `InsecurePlatformWarning`
    This happens on Python 2 platforms that have an outdated `ssl` module. These older `ssl` modules can cause some insecure requests to succeed where they should fail and secure requests to fail where they should succeed. Follow the pyOpenSSL guide to resolve this warning.

- `SNIMissingWarning`
    This happens on Python 2 versions older than 2.7.9. These older versions lack SNI support. This can cause servers to present a certificate that the client thinks is invalid. Follow the pyOpenSSL guide to resolve this warning.

Making unverified HTTPS requests is strongly discouraged, however, if you understand the risks and wish to disable these warnings, you can use `disable_warnings()`:

    >>> import urllib3
    >>> urllib3.disable_warnings()

Alternatively you can capture the warnings with the standard `logging` module:

    >>> logging.captureWarnings(True)

Finally, you can suppress the warnings at the interpreter level by setting the `PYTHONWARNINGS` environment variable or by using the -W flag.

## Google App Engine

urllib3 supports Google App Engine with some caveats.

If you’re using the Flexible environment, you do not have to do any configuration- urllib3 will just work. However, if you’re using the Standard environment then you either have to use `urllib3.contrib.appengine`’s `AppEngineManager` or use the Sockets API

To use `AppEngineManager`:

    >>> from urllib3.contrib.appengine import AppEngineManager
    >>> http = AppEngineManager()
    >>> http.request('GET', 'https://google.com/')

To use the Sockets API, add the following to your app.yaml and use `PoolManager` as usual:

    env_variables:
        GAE_USE_SOCKETS_HTTPLIB : 'true'

For more details on the limitations and gotchas, see `urllib3.contrib.appengine`.

## Brotli Encoding

Brotli is a compression algorithm created by Google with better compression than gzip and deflate and is supported by urllib3 if the brotlipy package is installed. You may also request the package be installed via the `urllib3[brotli]` extra:

    python -m pip install urllib3[brotli]

Here’s an example using brotli encoding via the `Accept-Encoding` header:

    >>> from urllib3 import PoolManager
    >>> http = PoolManager()
    >>> http.request('GET', 'https://www.google.com/', headers={'Accept-Encoding': 'br'})