Show testpaths option in the header if it has been used for collection

Fix #4875

changelog/4875.feature.rst

@@ -0,0 +1,3 @@
+The `testpaths <https://docs.pytest.org/en/latest/reference.html#confval-testpaths>`__ configuration option is now displayed next
+to the ``rootdir`` and ``inifile`` lines in the pytest header if the option is in effect, i.e., directories or file names were
+not explicitly passed in the command line.

src/_pytest/terminal.py

@@ -586,8 +586,13 @@ class TerminalReporter(object):
         inifile = ""
         if config.inifile:
             inifile = " " + config.rootdir.bestrelpath(config.inifile)
-        lines = ["rootdir: %s, inifile:%s" % (config.rootdir, inifile)]
 
+        line = "rootdir: %s, inifile:%s" % (config.rootdir, inifile)
+        testpaths = config.getini("testpaths")
+        if testpaths and config.args == testpaths:
+            rel_paths = [config.rootdir.bestrelpath(x) for x in testpaths]
+            line += ", testpaths: {}".format(", ".join(rel_paths))
+        lines = [line]
         plugininfo = config.pluginmanager.list_plugin_distinfo()
         if plugininfo:
 

testing/test_terminal.py

@@ -567,6 +567,26 @@ class TestTerminalFunctional(object):
         if request.config.pluginmanager.list_plugin_distinfo():
             result.stdout.fnmatch_lines(["plugins: *"])
 
+    def test_header(self, testdir, request):
+        testdir.tmpdir.join("tests").ensure_dir()
+        testdir.tmpdir.join("gui").ensure_dir()
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(["rootdir: *test_header0, inifile:"])
+
+        testdir.makeini(
+            """
+            [pytest]
+            testpaths = tests gui
+        """
+        )
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines(
+            ["rootdir: *test_header0, inifile: tox.ini, testpaths: tests, gui"]
+        )
+
+        result = testdir.runpytest("tests")
+        result.stdout.fnmatch_lines(["rootdir: *test_header0, inifile: tox.ini"])
+
     def test_showlocals(self, testdir):
         p1 = testdir.makepyfile(
             """