Raise ConfigError for missing newsfragment directory (#298)

* Treat a missing newsfragment directory as if it is empty

#85

* OSError for py2

* add missing sys import

* Add test docstrings

* Raise a ConfigError when the newsfragment directory is missing

* Update src/towncrier/newsfragments/85.bugfix.rst

Co-authored-by: Adi Roiban <adiroiban@gmail.com>

* reduce unneeded test diff

* remove probably copy/paste error comment

* Update src/towncrier/test/test_build.py

Co-authored-by: Adi Roiban <adiroiban@gmail.com>

Co-authored-by: Adi Roiban <adiroiban@gmail.com>

src/towncrier/_builder.py

@@ -4,12 +4,16 @@
 from __future__ import absolute_import, division, print_function
 
 import os
+import sys
 import textwrap
+import traceback
 
 from collections import OrderedDict
 
 from jinja2 import Template
 
+from ._settings import ConfigError
+
 
 def strip_if_integer_string(s):
     try:
@@ -86,7 +90,18 @@ def find_fragments(base_directory, sections, fragment_directory, definitions):
         else:
             section_dir = os.path.join(base_directory, val)
 
-        files = os.listdir(section_dir)
+        if sys.version_info >= (3,):
+            expected_exception = FileNotFoundError
+        else:
+            expected_exception = OSError
+
+        try:
+            files = os.listdir(section_dir)
+        except expected_exception as e:
+            message = "Failed to list the news fragment files.\n{}".format(
+                ''.join(traceback.format_exception_only(type(e), e)),
+            )
+            raise ConfigError(message)
 
         file_content = {}
 

src/towncrier/newsfragments/85.bugfix.rst

@@ -0,0 +1 @@
+A configuration error is triggered when the newsfragment files couldn't be discovered. 

src/towncrier/test/test_build.py

@@ -82,6 +82,35 @@ def test_command(self):
     def test_subcommand(self):
         self._test_command(_main)
 
+    def test_no_newsfragment_directory(self):
+        """
+        A missing newsfragment directory acts as if there are no changes.
+        """
+        runner = CliRunner()
+
+        with runner.isolated_filesystem():
+            setup_simple_project()
+            os.rmdir("foo/newsfragments")
+
+            result = runner.invoke(_main, ["--draft", "--date", "01-01-2001"])
+
+        self.assertEqual(1, result.exit_code, result.output)
+        self.assertIn("Failed to list the news fragment files.\n", result.output)
+
+    def test_no_newsfragments(self):
+        """
+        An empty newsfragment directory acts as if there are no changes.
+        """
+        runner = CliRunner()
+
+        with runner.isolated_filesystem():
+            setup_simple_project()
+
+            result = runner.invoke(_main, ["--draft", "--date", "01-01-2001"])
+
+        self.assertEqual(0, result.exit_code)
+        self.assertIn("No significant changes.\n", result.output)
+
     def test_collision(self):
         runner = CliRunner()