fix: get_writer errors if filename is none

package/CHANGELOG

@@ -15,10 +15,11 @@ The rules for this file:
 ------------------------------------------------------------------------------
 
 ??/??/?? IAlibay, pgbarletta, mglagolev, hmacdope, manuel.nuno.melo, chrispfae, 
-         ooprathamm, MeetB7, BFedder, v-parmar, MoSchaeffler, jbarnoud
+         ooprathamm, MeetB7, BFedder, v-parmar, MoSchaeffler, jbarnoud, jandom
  * 2.5.0
 
 Fixes
+  * Fix uninitialized `format` variable issue when calling `selections.get_writer` directly (PR #4043)
   * Fix tests should use results.rmsf to avoid DeprecationWarning (Issue #3695)
   * Fix EDRReader failing when parsing single-frame EDR files (Issue #3999)
   * Fix element parsing from PSF files tests read via Parmed (Issue #4015)

package/MDAnalysis/selections/__init__.py

@@ -48,14 +48,15 @@
 
 from .. import _SELECTION_WRITERS
 
+from . import base
 from . import vmd
 from . import pymol
 from . import gromacs
 from . import charmm
 from . import jmol
 
 
-def get_writer(filename, defaultformat):
+def get_writer(filename: str, defaultformat: str) -> base.SelectionWriterBase:
     """Return a SelectionWriter for `filename` or a `defaultformat`.
 
     Parameters
@@ -67,15 +68,15 @@ def get_writer(filename, defaultformat):
 
     Returns
     -------
-    SelectionWriter : `type`
+    SelectionWriterBase : `type`
         the writer *class* for the detected format
 
     Raises
     ------
     :exc:`NotImplementedError`
         for any format that is not defined
     """
-
+    format = None
     if filename:
         format = os.path.splitext(filename)[1][1:]  # strip initial dot!
     format = format or defaultformat  # use default if no fmt from fn

testsuite/MDAnalysisTests/utils/test_selections.py

@@ -25,6 +25,7 @@
 # use StringIO and NamedStream to write to memory instead to temp files
 import pytest
 from io import StringIO
+from unittest.mock import patch
 
 import re
 
@@ -34,6 +35,7 @@
 from MDAnalysis.tests.datafiles import PSF, DCD
 
 import MDAnalysis
+import MDAnalysis.selections as selections
 from MDAnalysis.lib.util import NamedStream
 
 
@@ -196,3 +198,10 @@ def _assert_selectionstring(self, namedfile):
                      err_msg="SPT file has wrong selection name")
         assert_array_equal(indices, self.ref_indices,
                            err_msg="SPT indices were not written correctly")
+
+
+class TestSelections:
+    @patch.object(selections, "_SELECTION_WRITERS", {"FOO": "BAR"})
+    def test_get_writer_valid(self):
+        writer = selections.get_writer(None, "FOO")
+        assert writer == "BAR"