✨Allow preservation of existing order of entry fields in writer (#317)

This significantly extends the possibility to change field order when writing entries. Most crucially, it allows to preserve the order in the database (which typically is the order found on parsing).

bibtexparser/bwriter.py

@@ -4,6 +4,8 @@
 
 
 import logging
+from enum import Enum, auto
+from typing import Dict, Callable, Iterable
 from bibtexparser.bibdatabase import (BibDatabase, COMMON_STRINGS,
                                       BibDataString,
                                       BibDataStringExpression)
@@ -14,6 +16,38 @@
 __all__ = ['BibTexWriter']
 
 
+class SortingStrategy(Enum):
+    """
+    Defines different strategies for sorting the entries not defined in :py:attr:`~.BibTexWriter.display_order` and that are added at the end.
+    """
+    ALPHABETICAL_ASC = auto()
+    """
+    Alphabetical sorting in ascending order.
+    """
+    ALPHABETICAL_DESC = auto()
+    """
+    Alphabetical sorting in descending order.
+    """
+    PRESERVE = auto()
+    """
+    Preserves the order of the entries. Entries are not sorted.
+    """
+
+
+def _apply_sorting_strategy(strategy: SortingStrategy, items: Iterable[str]) -> Iterable[str]:
+    """
+    Sorts the items based on the given sorting strategy.
+    """
+    if strategy == SortingStrategy.ALPHABETICAL_ASC:
+        return sorted(items)
+    elif strategy == SortingStrategy.ALPHABETICAL_DESC:
+        return reversed(sorted(items))
+    elif strategy == SortingStrategy.PRESERVE:
+        return items
+    else:
+        raise NotImplementedError(f"The strategy {strategy.name} is not implemented.")
+
+
 def to_bibtex(parsed):
     """
     Convenience function for backwards compatibility.
@@ -65,9 +99,12 @@ def __init__(self, write_common_strings=False):
         self.entry_separator = '\n'
         #: Tuple of fields for ordering BibTeX entries. Set to `None` to disable sorting. Default: BibTeX key `('ID', )`.
         self.order_entries_by = ('ID', )
-        #: Tuple of fields for display order in a single BibTeX entry. Fields not listed here will be displayed
-        #: alphabetically at the end. Set to '[]' for alphabetical order. Default: '[]'
+        #: Tuple of fields for display order in a single BibTeX entry. Fields not listed here will be displayed at the
+        #    end in the order defined by display_order_sorting. Default: '[]'
         self.display_order = []
+        # Sorting strategy for entries not contained in display_order. Entries not defined in display_order are added
+        #    at the end in the order defined by this strategy. Default: SortingStrategy.ALPHABETICAL_ASC
+        self.display_order_sorting: SortingStrategy = SortingStrategy.ALPHABETICAL_ASC
         #: BibTeX syntax allows comma first syntax
         #: (common in functional languages), use this to enable
         #: comma first syntax as the bwriter output
@@ -122,7 +159,7 @@ def _entry_to_bibtex(self, entry):
         # first those keys which are both in self.display_order and in entry.keys
         display_order = [i for i in self.display_order if i in entry]
         # then all the other fields sorted alphabetically
-        display_order += [i for i in sorted(entry) if i not in self.display_order]
+        display_order += [i for i in _apply_sorting_strategy(self.display_order_sorting, entry) if i not in self.display_order]
         if self.comma_first:
             field_fmt = u"\n{indent}, {field:<{field_max_w}} = {value}"
         else:

bibtexparser/tests/test_bibtexwriter.py

@@ -2,7 +2,7 @@
 import tempfile
 import unittest
 import bibtexparser
-from bibtexparser.bwriter import BibTexWriter
+from bibtexparser.bwriter import BibTexWriter, SortingStrategy
 from bibtexparser.bibdatabase import BibDatabase
 
 
@@ -218,6 +218,80 @@ def test_align_multiline_values_with_align(self):
  keyword   = {keyword1, keyword2,
               multiline-keyword1, multiline-keyword2}
 }
+"""
+        self.assertEqual(result, expected)
+
+    def test_display_order_sorting(self):
+        bib_database = BibDatabase()
+        bib_database.entries = [{'ID': 'abc123',
+                                 'ENTRYTYPE': 'book',
+                                 'b': 'test2',
+                                 'a': 'test1',
+                                 'd': 'test4',
+                                 'c': 'test3',
+                                 'e': 'test5'}]
+        # Only 'a' is not ordered. As it's only one element, strategy should not matter.
+        for strategy in SortingStrategy:
+            writer = BibTexWriter()
+            writer.display_order = ['b', 'c', 'd', 'e', 'a']
+            writer.display_order_sorting = strategy
+            result = bibtexparser.dumps(bib_database, writer)
+            expected = \
+"""@book{abc123,
+ b = {test2},
+ c = {test3},
+ d = {test4},
+ e = {test5},
+ a = {test1}
+}
+"""
+            self.assertEqual(result, expected)
+
+        # Test ALPHABETICAL_ASC strategy
+        writer = BibTexWriter()
+        writer.display_order = ['c']
+        writer.display_order_sorting = SortingStrategy.ALPHABETICAL_ASC
+        result = bibtexparser.dumps(bib_database, writer)
+        expected = \
+"""@book{abc123,
+ c = {test3},
+ a = {test1},
+ b = {test2},
+ d = {test4},
+ e = {test5}
+}
+"""
+        self.assertEqual(result, expected)
+
+        # Test ALPHABETICAL_DESC strategy
+        writer = BibTexWriter()
+        writer.display_order = ['c']
+        writer.display_order_sorting = SortingStrategy.ALPHABETICAL_DESC
+        result = bibtexparser.dumps(bib_database, writer)
+        expected = \
+"""@book{abc123,
+ c = {test3},
+ e = {test5},
+ d = {test4},
+ b = {test2},
+ a = {test1}
+}
+"""
+        self.assertEqual(result, expected)
+
+        # Test PRESERVE strategy
+        writer = BibTexWriter()
+        writer.display_order = ['c']
+        writer.display_order_sorting = SortingStrategy.PRESERVE
+        result = bibtexparser.dumps(bib_database, writer)
+        expected = \
+"""@book{abc123,
+ c = {test3},
+ b = {test2},
+ a = {test1},
+ d = {test4},
+ e = {test5}
+}
 """
         self.assertEqual(result, expected)