Test CLI using click.testing.CliRunner (#530)

* Test ‘build’ behaviour using a ‘click.testing.CliRunner’.

This invokes the command behaviour consistently, without assuming any
particular installed command name.

* Use CliRunner for testing CLI standard output streams.

* Document the improvement in a Towncrier news fragment.

* Test ‘--version’ behaviour using a ‘click.testing.CliRunner’.

This invokes the command behaviour consistently, without assuming any
particular installed command name.

* Test ‘--help’ behaviour using a ‘click.testing.CliRunner’.

This invokes the command behaviour consistently, without assuming any
particular installed command name.

* Remove unused imports.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Semantic linefeeds

* Markup

---------

Co-authored-by: Ben Finney <ben+python@benfinney.id.au>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/towncrier/newsfragments/481.misc

@@ -0,0 +1,3 @@
+``click.testing.CliRunner`` is now consistently used for testing CLI behaviour.
+
+This invokes the command behaviour consistently, without assuming any particular installed command name.

src/towncrier/test/test_check.py

@@ -3,16 +3,16 @@
 
 import os
 import os.path
-import sys
 import warnings
 
 from pathlib import Path
-from subprocess import PIPE, Popen, call
+from subprocess import call
 
 from click.testing import CliRunner
 from twisted.trial.unittest import TestCase
 
 from towncrier import check
+from towncrier.build import _main as towncrier_build
 from towncrier.check import _main as towncrier_check
 
 from .helpers import setup_simple_project, with_isolated_runner, write
@@ -182,15 +182,11 @@ def test_none_stdout_encoding_works(self):
             call(["git", "add", fragment_path])
             call(["git", "commit", "-m", "add a newsfragment"])
 
-            proc = Popen(
-                [sys.executable, "-m", "towncrier.check", "--compare-with", "master"],
-                stdout=PIPE,
-                stderr=PIPE,
-            )
-            stdout, stderr = proc.communicate()
+            runner = CliRunner(mix_stderr=False)
+            result = runner.invoke(towncrier_check, ["--compare-with", "master"])
 
-        self.assertEqual(0, proc.returncode)
-        self.assertEqual(b"", stderr)
+        self.assertEqual(0, result.exit_code)
+        self.assertEqual(0, len(result.stderr))
 
     def test_first_release(self):
         """
@@ -207,7 +203,7 @@ def test_first_release(self):
             # Before any release, the NEWS file might no exist.
             self.assertNotIn("NEWS.rst", os.listdir("."))
 
-            call(["towncrier", "build", "--yes", "--version", "1.0"])
+            runner.invoke(towncrier_build, ["--yes", "--version", "1.0"])
             commit("Prepare a release")
             # When missing,
             # the news file is automatically created with a new release.
@@ -235,7 +231,7 @@ def test_release_branch(self):
 
             # Do a first release without any checks.
             # And merge the release branch back into the main branch.
-            call(["towncrier", "build", "--yes", "--version", "1.0"])
+            runner.invoke(towncrier_build, ["--yes", "--version", "1.0"])
             commit("First release")
             # The news file is now created.
             self.assertIn("NEWS.rst", os.listdir("."))
@@ -260,7 +256,7 @@ def test_release_branch(self):
 
             # We now have the new release branch.
             call(["git", "checkout", "-b", "next-release"])
-            call(["towncrier", "build", "--yes", "--version", "2.0"])
+            runner.invoke(towncrier_build, ["--yes", "--version", "2.0"])
             commit("Second release")
 
             # Act

src/towncrier/test/test_project.py

@@ -4,12 +4,13 @@
 import os
 import sys
 
-from subprocess import check_output
 from unittest import skipIf
 
+from click.testing import CliRunner
 from twisted.trial.unittest import TestCase
 
 from .._project import get_project_name, get_version
+from .._shell import cli as towncrier_cli
 from .helpers import write
 
 
@@ -19,6 +20,9 @@
     metadata_version = None
 
 
+towncrier_cli.name = "towncrier"
+
+
 class VersionFetchingTests(TestCase):
     def test_str(self):
         """
@@ -174,6 +178,7 @@ def test_dash_m(self):
         """
         `python -m towncrier` invokes the main entrypoint.
         """
+        runner = CliRunner()
         temp = self.mktemp()
         new_dir = os.path.join(temp, "dashm")
         os.makedirs(new_dir)
@@ -183,15 +188,16 @@ def test_dash_m(self):
             with open("pyproject.toml", "w") as f:
                 f.write("[tool.towncrier]\n" 'directory = "news"\n')
             os.makedirs("news")
-            out = check_output([sys.executable, "-m", "towncrier", "--help"])
-            self.assertIn(b"[OPTIONS] COMMAND [ARGS]...", out)
-            self.assertRegex(out, rb".*--help\s+Show this message and exit.")
+            result = runner.invoke(towncrier_cli, ["--help"])
+            self.assertIn("[OPTIONS] COMMAND [ARGS]...", result.stdout)
+            self.assertRegex(result.stdout, r".*--help\s+Show this message and exit.")
         finally:
             os.chdir(orig_dir)
 
     def test_version(self):
         """
         `--version` command line option is available to show the current production version.
         """
-        out = check_output(["towncrier", "--version"])
-        self.assertTrue(out.startswith(b"towncrier, version 2"))
+        runner = CliRunner()
+        result = runner.invoke(towncrier_cli, ["--version"])
+        self.assertTrue(result.output.startswith("towncrier, version 2"))