Include detailed tests for forcefield metadata (#435)

* Include detailed tests for forcefield metadata

Previously, the tests in test_forcefield.py were assumed to test that
forcefield metadata like `version`, `name`, etc. were properly loaded
into the `Forcefield` object.

This is only partially true, as there are cases where the files either
go out of scope before that information is gathered, etc. This PR
includes additional tests to ensure that this information is not being
lost prematurely. Big thanks to: @mattwthompson, @ahy3nz for discovering
this issue, creating MWE's for some of these cases, and taking my
investigation further and most likely pinpointing the issue.

* Change ff load logic to make options consistent

When a forcefield file is provided by using the internal name:
`foyer.Forcefield(name='oplsaa)`, the final file processing would happen
in a separate part of the logic and use variables that were only
accessed if the user used `forcefield_files` in the `Forcefield`
constructor method.

Now the same final processing occurs for all input types to load the
forcefields. All tests currently pass with this fix in place.

* Remove named temp files after final parsing

foyer/forcefield.py

@@ -522,21 +522,19 @@ def __init__(
         if validation:
             for ff_file_name in preprocessed_files:
                 Validator(ff_file_name, debug)
-        try:
-            super(Forcefield, self).__init__(*preprocessed_files)
-        finally:
-            for ff_file_name in preprocessed_files:
-                os.remove(ff_file_name)
+        super(Forcefield, self).__init__(*preprocessed_files)
 
-        if isinstance(forcefield_files, str):
-            self._version = self._parse_version_number(forcefield_files)
-            self._name = self._parse_name(forcefield_files)
-        elif isinstance(forcefield_files, list):
+        if len(preprocessed_files) == 1:
+            self._version = self._parse_version_number(preprocessed_files[0])
+            self._name = self._parse_name(preprocessed_files[0])
+        elif len(preprocessed_files) > 1:
             self._version = [
-                self._parse_version_number(f) for f in forcefield_files
+                self._parse_version_number(f) for f in preprocessed_files
             ]
-            self._name = [self._parse_name(f) for f in forcefield_files]
+            self._name = [self._parse_name(f) for f in preprocessed_files]
 
+        for fp in preprocessed_files:
+            os.remove(fp)
         self.parser = smarts.SMARTS(self.non_element_types)
         self._system_data = None
 

foyer/tests/test_forcefield.py

@@ -2,13 +2,14 @@
 import glob
 import itertools as it
 import os
+from typing import List
 
 import parmed as pmd
 import pytest
 from lxml import etree as ET
 from pkg_resources import resource_filename
 
-from foyer import Forcefield
+from foyer import Forcefield, forcefields
 from foyer.exceptions import FoyerError, ValidationWarning
 from foyer.forcefield import (
     _check_independent_residues,
@@ -639,6 +640,30 @@ def test_load_metadata(self):
         assert lj_ff.version == ["0.4.1", "4.8.2"]
         assert lj_ff.name == ["LJ", "JL"]
 
+    def test_load_metadata_single_xml(self):
+        from_xml_ff = Forcefield(forcefield_files=get_fn("lj.xml"))
+        assert from_xml_ff.version == "0.4.1"
+        assert from_xml_ff.name == "LJ"
+
+    def test_load_metadata_list_xml(self):
+        from_xml_ff = Forcefield(
+            forcefield_files=[get_fn("lj.xml"), get_fn("lj2.xml")]
+        )
+        assert isinstance(from_xml_ff.version, List)
+        assert isinstance(from_xml_ff.name, List)
+        assert all([x in from_xml_ff.version for x in ["0.4.1", "4.8.2"]])
+        assert all([x in from_xml_ff.name for x in ["JL", "LJ"]])
+
+    def test_load_metadata_from_internal_forcefield_plugin_loader(self):
+        from_xml_ff = forcefields.load_OPLSAA()
+        assert from_xml_ff.version == "0.0.1"
+        assert from_xml_ff.name == "OPLS-AA"
+
+    def test_load_metadata_from_internal_name(self):
+        from_xml_ff = Forcefield(name="oplsaa")
+        assert from_xml_ff.version == "0.0.1"
+        assert from_xml_ff.name == "OPLS-AA"
+
     @pytest.mark.skipif(not has_mbuild, reason="mbuild is not installed")
     def test_no_overlap_residue_atom_overlap(self):
         import mbuild as mb