Avoid importing coverage for test runs that don't need it

src/pytest_cov/plugin.py

@@ -4,11 +4,9 @@
 import warnings
 
 import pytest
-from coverage.misc import CoverageException
 
 from . import compat
 from . import embed
-from . import engine
 
 
 class CoverageError(Exception):
@@ -146,6 +144,10 @@ def __init__(self, options, pluginmanager, start=True):
             self.options.cov_report = {}
         self.options.cov_source = _prepare_cov_source(self.options.cov_source)
 
+        # import engine lazily here to avoid importing
+        # it for unit tests that don't need it
+        from . import engine
+
         if is_dist and start:
             self.start(engine.DistMaster)
         elif start:
@@ -188,6 +190,10 @@ def pytest_sessionstart(self, session):
             self._disabled = True
             return
 
+        # import engine lazily here to avoid importing
+        # it for unit tests that don't need it
+        from . import engine
+
         self.pid = os.getpid()
         if self._is_worker(session):
             nodeid = (
@@ -239,6 +245,11 @@ def pytest_runtestloop(self, session):
             self.cov_controller.finish()
 
         if not self._is_worker(session) and self._should_report():
+
+            # import coverage lazily here to avoid importing
+            # it for unit tests that don't need it
+            from coverage.misc import CoverageException
+
             try:
                 self.cov_total = self.cov_controller.summary(self.cov_report)
             except CoverageException as exc: