collect: python: fix AssertionError with broken symlinks

Fixes #4782.

changelog/4782.bugfix.rst

@@ -0,0 +1 @@
+Fix ``AssertionError`` with collection of broken symlinks with packages.

src/_pytest/python.py

@@ -632,7 +632,8 @@ def collect(self):
         pkg_prefixes = set()
         for path in this_path.visit(rec=self._recurse, bf=True, sort=True):
             # We will visit our own __init__.py file, in which case we skip it.
-            if path.isfile():
+            is_file = path.isfile()
+            if is_file:
                 if path.basename == "__init__.py" and path.dirpath() == this_path:
                     continue
 
@@ -646,6 +647,9 @@ def collect(self):
             if path.isdir():
                 if path.join("__init__.py").check(file=1):
                     pkg_prefixes.add(path)
+            elif not is_file:
+                # Broken symlinks.
+                continue
             else:
                 for x in self._collectfile(path):
                     yield x

testing/test_collection.py

@@ -1186,3 +1186,48 @@ def test_collect_pkg_init_and_file_in_args(testdir):
             "*2 passed in*",
         ]
     )
+
+
+def test_collect_pkg_with_symlinks(testdir):
+    pkg = testdir.mkpydir("pkg")
+    pkg.ensure("test_file.py").write("def test_file(): pass")
+
+    # Create a broken symlink.
+    pkg.join("test_broken.py").mksymlinkto("test_doesnotexist.py")
+
+    # Symlink that gets collected.
+    pkg.join("test_symlink.py").mksymlinkto("test_file.py")
+
+    result = testdir.runpytest("-v", str(pkg))
+    result.stdout.fnmatch_lines(
+        [
+            "pkg/test_file.py::test_file PASSED*",
+            "pkg/test_symlink.py::test_file PASSED*",
+            "*2 passed in*",
+        ]
+    )
+
+
+def test_collect_sub_with_symlinks(testdir):
+    """Similar to test_collect_pkg_with_symlinks above, without __init__.py.
+
+    Here the _visit_filter only considers files already, and skips the broken
+    symlink.
+    """
+    pkg = testdir.mkdir("sub")
+    pkg.ensure("test_file.py").write("def test_file(): pass")
+
+    # Create a broken symlink.
+    pkg.join("test_broken.py").mksymlinkto("test_doesnotexist.py")
+
+    # Symlink that gets collected.
+    pkg.join("test_symlink.py").mksymlinkto("test_file.py")
+
+    result = testdir.runpytest("-v", str(pkg))
+    result.stdout.fnmatch_lines(
+        [
+            "sub/test_file.py::test_file PASSED*",
+            "sub/test_symlink.py::test_file PASSED*",
+            "*2 passed in*",
+        ]
+    )