improve async coverage

1 files changed, 7 insertions, 1 deletions

diff --git a/dns/asyncbackend.py b/dns/asyncbackend.py
index 26f2397..f028417 100644
--- a/dns/asyncbackend.py
+++ b/dns/asyncbackend.py
@@ -10,6 +10,8 @@ _default_backend = None
 
 _backends = {}
 
+# Allow sniffio import to be disabled for testing purposes
+_no_sniffio = False
 
 class AsyncLibraryNotFoundError(dns.exception.DNSException):
     pass
@@ -40,6 +42,7 @@ def get_backend(name):
     _backends[name] = backend
     return backend
 
+
 def sniff():
     """Attempt to determine the in-use asynchronous I/O library by using
     the ``sniffio`` module if it is available.
@@ -48,6 +51,8 @@ def sniff():
     if the library cannot be determined.
     """
     try:
+        if _no_sniffio:
+            raise ImportError
         import sniffio
         try:
             return sniffio.current_async_library()
@@ -61,11 +66,12 @@ def sniff():
             return 'asyncio'
         except RuntimeError:
             raise AsyncLibraryNotFoundError('no async library detected')
-        except AttributeError:
+        except AttributeError:  # pragma: no cover
             # we have to check current_task on 3.6
             if not asyncio.Task.current_task():
                 raise AsyncLibraryNotFoundError('no async library detected')
 
+
 def get_default_backend():
     """Get the default backend, initializing it if necessary.
     """