Fix/improve handling of chdir with no-args and testpaths

Fixes #4332.

Author: blueyed

src/_pytest/config/__init__.py

@@ -603,7 +603,6 @@ def __init__(self, pluginmanager):
         self._warn = self.pluginmanager._warn
         self.pluginmanager.register(self, "pytestconfig")
         self._configured = False
-        self.cwd = os.getcwd()
 
         def do_setns(dic):
             import pytest
@@ -848,10 +847,13 @@ def parse(self, args, addopts=True):
                 args, self.option, namespace=self.option
             )
             if not args:
-                if self.cwd == self.rootdir:
-                    args = self.getini("testpaths")
+                if self.invocation_dir == self.rootdir:
+                    args = [
+                        str(self.invocation_dir.join(x)) if not os.path.isabs(x) else x
+                        for x in self.getini("testpaths")
+                    ]
                 if not args:
-                    args = [self.cwd]
+                    args = [self.invocation_dir]
             self.args = args
         except PrintHelp:
             pass

testing/test_collection.py

@@ -1040,6 +1040,19 @@ def test_1():
         """
         % (str(subdir),)
     )
-    result = testdir.runpytest()
+    with testdir.tmpdir.as_cwd():
+        result = testdir.runpytest()
     result.stdout.fnmatch_lines(["*1 passed in*"])
     assert result.ret == 0
+
+    # Handles relative testpaths.
+    testdir.makeini(
+        """
+        [pytest]
+        testpaths = .
+    """
+    )
+    with testdir.tmpdir.as_cwd():
+        result = testdir.runpytest("--collect-only")
+    result.stdout.fnmatch_lines(["collected 1 item"])
+    assert result.ret == 0