Reuse parsed value enclosures by default

bibtexparser/middlewares/enclosing.py

@@ -148,7 +148,12 @@ def _enclose(
         if demanded_enclosing is not None:
             enclosing = demanded_enclosing
         elif self._reuse_previous_enclosing and metadata_enclosing is not None:
-            enclosing = metadata_enclosing
+            # Parser metadata describes the source value, not necessarily its
+            # current edited value. Reusing a bare integer's metadata for new
+            # ordinary text would emit invalid BibTeX, so fall back to the
+            # configured default unless the bare representation remains safe.
+            if metadata_enclosing != "no-enclosing" or str(value).isdigit():
+                enclosing = metadata_enclosing
         elif apply_int_rule and not self._enclose_integers and str(value).isdigit():
             return str(value)
 

bibtexparser/middlewares/parsestack.py

@@ -19,7 +19,7 @@ def default_unparse_stack(allow_inplace_modification: bool = False) -> list[Midd
         AddEnclosingMiddleware(
             allow_inplace_modification=allow_inplace_modification,
             default_enclosing="{",
-            reuse_previous_enclosing=False,
+            reuse_previous_enclosing=True,
             enclose_integers=True,
         )
     ]

docs/source/customize.rst

@@ -55,6 +55,13 @@ The default write stack consists of the following layers:
 
 * :class:`bibtexparser.middlewares.AddEnclosingMiddleware`: Encloses values in curly braces where needed.
 
+For parsed values, the default writer reuses the quote, brace, or safe
+unenclosed style recorded by
+:class:`bibtexparser.middlewares.RemoveEnclosingMiddleware`. New programmatic
+values without prior enclosure metadata use curly braces. If a formerly bare
+integer is edited to ordinary text, the writer also falls back to braces rather
+than emitting invalid bare text.
+
 When specifying their own stack, user get to chose if they want to add to or overwrite the default stack
 by selecting the corresponding argument when calling :code:`bibtexparser.parse` or :code:`bibtexparser.write`:
 

tests/middleware_tests/test_enclosing.py

@@ -166,7 +166,10 @@ def test_addition_of_enclosing_on_entry(
     used_enclosing = _figure_out_added_enclosing(changed_value, value)
 
     # Assert correct enclosing was added
-    if reuse_previous_enclosing and metadata_enclosing is not None:
+    can_safely_reuse_metadata = metadata_enclosing != "no-enclosing" or (
+        isinstance(value, int) or value.isdigit()
+    )
+    if reuse_previous_enclosing and metadata_enclosing is not None and can_safely_reuse_metadata:
         expected_enclosing = metadata_enclosing
     elif (isinstance(value, int) or value.isdigit()) and not enclose_ints:
         expected_enclosing = "no-enclosing"
@@ -417,8 +420,53 @@ def test_string_reference_roundtrip():
 
     assert "month = jan" in written
     assert 'pages = intro # "--" # outro' in written
-    # Ints are still enclosed by the default unparse stack
-    assert "year = {2019}" in written
+    # The default unparse stack also retains the integer's bare source style.
+    assert "year = 2019" in written
+
+
+def test_default_roundtrip_reuses_parsed_value_enclosures():
+    """Ordinary writing retains quote, brace, and safe bare source choices."""
+    library = bibtexparser.parse_string(
+        '@article{key, quoted="Quoted", braced={Braced}, year=2019}'
+    )
+
+    written = bibtexparser.write_string(library)
+
+    assert 'quoted = "Quoted"' in written
+    assert "braced = {Braced}" in written
+    assert "year = 2019" in written
+
+
+def test_edit_reuses_quoted_and_braced_enclosures():
+    """Changing a value must not also replace its established safe enclosure style."""
+    library = bibtexparser.parse_string('@article{key, quoted="Before", braced={Before}}')
+    library.entries[0]["quoted"] = "After"
+    library.entries[0]["braced"] = "After"
+
+    written = bibtexparser.write_string(library)
+
+    assert 'quoted = "After"' in written
+    assert "braced = {After}" in written
+
+
+def test_editing_bare_integer_to_text_falls_back_to_braces():
+    """Stale bare metadata cannot make an edited ordinary string invalid BibTeX."""
+    library = bibtexparser.parse_string("@article{key, year=2019}")
+    library.entries[0]["year"] = "forthcoming soon"
+
+    written = bibtexparser.write_string(library)
+
+    assert "year = {forthcoming soon}" in written
+
+
+def test_new_field_without_enclosure_metadata_uses_braces():
+    """Source-style reuse does not change the safe default for programmatic values."""
+    library = bibtexparser.parse_string('@article{key, title="Existing"}')
+    library.entries[0]["note"] = "New value"
+
+    written = bibtexparser.write_string(library)
+
+    assert "note = {New value}" in written
 
 
 # TODO round-trip tests (removal -> addition -> removal)