Directory changers skip retrieval of missing files with a user warnin…

…g. (#636)

Add check if the file that is being retrieved on a directory changer
exists and skip the copy if it does not exist. This is useful in instances
where optional files are requested when executing a physics code and
not all files are created because the user settings differ than the
default behavior.

armi/utils/directoryChangers.py

@@ -50,7 +50,8 @@ class DirectoryChanger:
     filesToMove : list of str, optional
         Filenames to bring from the CWD into the destination
     filesToRetrieve : list of str, optional
-        Filenames to bring back from the destination to the cwd
+        Filenames to bring back from the destination to the cwd. Note that if any of these
+        files do not exist then the file will be skipped and a warning will be provided.
     dumpOnException : bool, optional
         Flag to tell system to retrieve the entire directory if an exception
         is raised within a the context manager.
@@ -185,6 +186,10 @@ def _transferFiles(initialPath, destinationPath, fileList):
 
             for fromName, destName in copies:
                 fromPath = os.path.join(initialPath, fromName)
+                if not os.path.exists(fromPath):
+                    runLog.warning(f"{fromPath} does not exist and will not be copied.")
+                    continue
+
                 toPath = os.path.join(destinationPath, destName)
                 runLog.extra("Copying {} to {}".format(fromPath, toPath))
                 shutil.copy(fromPath, toPath)

armi/utils/tests/test_directoryChangers.py

@@ -142,3 +142,19 @@ def f(name):
         self.assertTrue(os.path.exists(f("file2.txt")))
         os.remove(f("file1.txt"))
         os.remove(f("file2.txt"))
+
+    def test_file_retrieval_missing_file(self):
+        """Tests that the directory changer still returns a subset of files even if all do not exist."""
+
+        def f(name):
+            """Utility to avoid test clashes during cleanups"""
+            return self._testMethodName + name
+
+        with directoryChangers.TemporaryDirectoryChanger(
+            filesToRetrieve=[f("file1.txt"), f("file2.txt")]
+        ):
+            Path(f("file1.txt")).touch()
+
+        self.assertTrue(os.path.exists(f("file1.txt")))
+        self.assertFalse(os.path.exists(f("file2.txt")))
+        os.remove(f("file1.txt"))