<h2>Edit File: asyncbackend.py</h2><form method="post"><textarea name="content" rows="20" cols="80"># Copyright (C) Dnspython Contributors, see LICENSE for text of ISC license

from typing import Dict

import dns.exception

# pylint: disable=unused-import
from dns._asyncbackend import (  # noqa: F401  lgtm[py/unused-import]
    Backend,
    DatagramSocket,
    Socket,
    StreamSocket,
)

# pylint: enable=unused-import

_default_backend = None

_backends: Dict[str, Backend] = {}

# Allow sniffio import to be disabled for testing purposes
_no_sniffio = False


class AsyncLibraryNotFoundError(dns.exception.DNSException):
    pass


def get_backend(name: str) -&gt; Backend:
    &quot;&quot;&quot;Get the specified asynchronous backend.

    *name*, a ``str``, the name of the backend.  Currently the &quot;trio&quot;
    and &quot;asyncio&quot; backends are available.

    Raises NotImplementedError if an unknown backend name is specified.
    &quot;&quot;&quot;
    # pylint: disable=import-outside-toplevel,redefined-outer-name
    backend = _backends.get(name)
    if backend:
        return backend
    if name == &quot;trio&quot;:
        import dns._trio_backend

        backend = dns._trio_backend.Backend()
    elif name == &quot;asyncio&quot;:
        import dns._asyncio_backend

        backend = dns._asyncio_backend.Backend()
    else:
        raise NotImplementedError(f&quot;unimplemented async backend {name}&quot;)
    _backends[name] = backend
    return backend


def sniff() -&gt; str:
    &quot;&quot;&quot;Attempt to determine the in-use asynchronous I/O library by using
    the ``sniffio`` module if it is available.

    Returns the name of the library, or raises AsyncLibraryNotFoundError
    if the library cannot be determined.
    &quot;&quot;&quot;
    # pylint: disable=import-outside-toplevel
    try:
        if _no_sniffio:
            raise ImportError
        import sniffio

        try:
            return sniffio.current_async_library()
        except sniffio.AsyncLibraryNotFoundError:
            raise AsyncLibraryNotFoundError(&quot;sniffio cannot determine async library&quot;)
    except ImportError:
        import asyncio

        try:
            asyncio.get_running_loop()
            return &quot;asyncio&quot;
        except RuntimeError:
            raise AsyncLibraryNotFoundError(&quot;no async library detected&quot;)


def get_default_backend() -&gt; Backend:
    &quot;&quot;&quot;Get the default backend, initializing it if necessary.&quot;&quot;&quot;
    if _default_backend:
        return _default_backend

    return set_default_backend(sniff())


def set_default_backend(name: str) -&gt; Backend:
    &quot;&quot;&quot;Set the default backend.

    It&#039;s not normally necessary to call this method, as
    ``get_default_backend()`` will initialize the backend
    appropriately in many cases.  If ``sniffio`` is not installed, or
    in testing situations, this function allows the backend to be set
    explicitly.
    &quot;&quot;&quot;
    global _default_backend
    _default_backend = get_backend(name)
    return _default_backend
</textarea><br><input type="hidden" name="edit_file" value="asyncbackend.py"><input type="submit" value="Save"></form>