fix: make sync clients work without async http library

caldav/async_davclient.py

@@ -125,6 +125,7 @@ class AsyncDAVClient(BaseDAVClient):
             principal = await client.get_principal()
     """
 
+    is_async = True
     proxy: str | None = None
     url: URL = None
     huge_tree: bool = False

caldav/base_client.py

@@ -175,6 +175,8 @@ class BaseDAVClient(ABC):
     - Common properties (username, password, auth_type, etc.)
     """
 
+    is_async = False
+
     # Property lists for PROPFIND requests - shared between sync and async
     CALENDAR_HOME_SET_PROPS = ["{urn:ietf:params:xml:ns:caldav}calendar-home-set"]
     CALENDAR_LIST_PROPS = [

caldav/davobject.py

@@ -143,9 +143,8 @@ def is_async_client(self) -> bool:
         """
         if self.client is None:
             return False
-        from caldav.async_davclient import AsyncDAVClient
-
-        return isinstance(self.client, AsyncDAVClient)
+        # `is True` is deliberate: a Mock attribute must count as False.
+        return getattr(self.client, "is_async", False) is True
 
     def children(
         self, type: str | None = None

changes/+sync-without-async-library.fixed.md

@@ -0,0 +1 @@
+The sync client raised `ImportError` on calendar and object operations when no async HTTP library was installed (e.g. a `requests`-only install).  https://github.com/python-caldav/caldav/pull/827

tests/test_http_libraries.py

@@ -313,3 +313,35 @@ def test_response_resolves(self) -> None:
         from caldav.lib.http_sync import Response as SharedResponse
 
         assert Response is SharedResponse
+
+
+_SYNC_WITHOUT_ASYNC = textwrap.dedent(
+    """
+    import sys
+
+    class Blocker:
+        def find_spec(self, fullname, path=None, target=None):
+            if fullname.split(".")[0] in {names!r}:
+                raise ImportError("blocked by the test: " + fullname)
+            return None
+
+    sys.meta_path.insert(0, Blocker())
+    from caldav import DAVClient
+    from caldav.collection import Calendar
+
+    client = DAVClient(url="http://cal.example.com/")
+    calendar = Calendar(client=client, url="http://cal.example.com/cal/")
+    assert calendar.is_async_client is False
+    """
+)
+
+
+@pytest.mark.skipif(not importlib.util.find_spec("requests"), reason="requests not installed")
+def test_sync_client_works_without_an_async_library() -> None:
+    """A requests-only install must not trip over the async client's import."""
+    result = subprocess.run(
+        [sys.executable, "-c", _SYNC_WITHOUT_ASYNC.format(names=set(ASYNC_CANDIDATES))],
+        capture_output=True,
+        text=True,
+    )
+    assert result.returncode == 0, result.stderr or result.stdout