--lf now skips colletion of files without failed tests

Fix #5172

changelog/5172.feature.rst

@@ -0,0 +1,2 @@
+The ``--last-failed`` (``--lf``) option got smarter and will now skip entire files if all tests
+of that test file have passed in previous runs, greatly speeding up collection.

src/_pytest/cacheprovider.py

@@ -158,6 +158,33 @@ class LFPlugin(object):
         self.lastfailed = config.cache.get("cache/lastfailed", {})
         self._previously_failed_count = None
         self._report_status = None
+        self._skipped_files = 0  # count skipped files during collection due to --lf
+
+    def last_failed_paths(self):
+        """Returns a set with all Paths()s of the previously failed nodeids (cached).
+        """
+        result = getattr(self, "_last_failed_paths", None)
+        if result is None:
+            rootpath = Path(self.config.rootdir)
+            result = {rootpath / nodeid.split("::")[0] for nodeid in self.lastfailed}
+            self._last_failed_paths = result
+        return result
+
+    def pytest_ignore_collect(self, path):
+        """
+        Ignore this file path if we are in --lf mode and it is not in the list of
+        previously failed files.
+        """
+        if (
+            self.active
+            and self.config.getoption("lf")
+            and path.isfile()
+            and self.lastfailed
+        ):
+            skip_it = Path(path) not in self.last_failed_paths()
+            if skip_it:
+                self._skipped_files += 1
+            return skip_it
 
     def pytest_report_collectionfinish(self):
         if self.active and self.config.getoption("verbose") >= 0:
@@ -206,9 +233,19 @@ class LFPlugin(object):
                     items[:] = previously_failed + previously_passed
 
                 noun = "failure" if self._previously_failed_count == 1 else "failures"
+                if self._skipped_files > 0:
+                    files_noun = "file" if self._skipped_files == 1 else "files"
+                    skipped_files_msg = " (skipped {files} {files_noun})".format(
+                        files=self._skipped_files, files_noun=files_noun
+                    )
+                else:
+                    skipped_files_msg = ""
                 suffix = " first" if self.config.getoption("failedfirst") else ""
-                self._report_status = "rerun previous {count} {noun}{suffix}".format(
-                    count=self._previously_failed_count, suffix=suffix, noun=noun
+                self._report_status = "rerun previous {count} {noun}{suffix}{skipped_files}".format(
+                    count=self._previously_failed_count,
+                    suffix=suffix,
+                    noun=noun,
+                    skipped_files=skipped_files_msg,
                 )
         else:
             self._report_status = "no previously failed tests, "

testing/test_cacheprovider.py

@@ -445,9 +445,9 @@ class TestLastFailed(object):
         result = testdir.runpytest("--lf")
         result.stdout.fnmatch_lines(
             [
-                "collected 4 items / 2 deselected / 2 selected",
-                "run-last-failure: rerun previous 2 failures",
-                "*2 failed, 2 deselected in*",
+                "collected 2 items",
+                "run-last-failure: rerun previous 2 failures (skipped 1 file)",
+                "*2 failed in*",
             ]
         )
 
@@ -718,7 +718,7 @@ class TestLastFailed(object):
         assert self.get_cached_last_failed(testdir) == ["test_foo.py::test_foo_4"]
 
         result = testdir.runpytest("--last-failed")
-        result.stdout.fnmatch_lines(["*1 failed, 3 deselected*"])
+        result.stdout.fnmatch_lines(["*1 failed, 1 deselected*"])
         assert self.get_cached_last_failed(testdir) == ["test_foo.py::test_foo_4"]
 
         # 3. fix test_foo_4, run only test_foo.py
@@ -779,6 +779,58 @@ class TestLastFailed(object):
         result = testdir.runpytest("--lf", "--cache-clear", "--lfnf", "none")
         result.stdout.fnmatch_lines(["*2 desel*"])
 
+    def test_lastfailed_skip_collection(self, testdir):
+        """
+        Test --lf behavior regarding skipping collection of files that are not marked as
+        failed in the cache (#5172).
+        """
+        testdir.makepyfile(
+            **{
+                "pkg1/test_1.py": """
+                import pytest
+
+                @pytest.mark.parametrize('i', range(3))
+                def test_1(i): pass
+            """,
+                "pkg2/test_2.py": """
+                import pytest
+
+                @pytest.mark.parametrize('i', range(5))
+                def test_1(i):
+                    assert i not in (1, 3)
+            """,
+            }
+        )
+        # first run: collects 8 items (test_1: 3, test_2: 5)
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["collected 8 items", "*2 failed*6 passed*"])
+        # second run: collects only 5 items from test_2, because all tests from test_1 have passed
+        result = testdir.runpytest("--lf")
+        result.stdout.fnmatch_lines(
+            [
+                "collected 5 items / 3 deselected / 2 selected",
+                "run-last-failure: rerun previous 2 failures (skipped 1 file)",
+                "*2 failed*3 deselected*",
+            ]
+        )
+
+        # add another file and check if message is correct when skipping more than 1 file
+        testdir.makepyfile(
+            **{
+                "pkg1/test_3.py": """
+                def test_3(): pass
+            """
+            }
+        )
+        result = testdir.runpytest("--lf")
+        result.stdout.fnmatch_lines(
+            [
+                "collected 5 items / 3 deselected / 2 selected",
+                "run-last-failure: rerun previous 2 failures (skipped 2 files)",
+                "*2 failed*3 deselected*",
+            ]
+        )
+
 
 class TestNewFirst(object):
     def test_newfirst_usecase(self, testdir):