fix: swap '_' for leading or trailing apostrophes in worksheet names

- make xls exports more excel-compatible
- fix failing export when form names start or end with apostrophe

src/formpack/utils/string.py

@@ -75,10 +75,14 @@ def ellipsize(s, max_len, ellipsis='...'):
 
 def unique_name_for_xls(sheet_name, other_sheet_names, base_ellipsis='...'):
     r"""
-    Return a sheet name that does not collide with any string in the iterable
-    `other_sheet_names` and does not exceed the Excel sheet name length limit.
-    Characters that are not allowed in sheet names are replaced with
-    underscores.
+    Return a unique Excel-compatible worksheet name that does not collide
+    with a sheet name in the iterable `other_sheet_names`
+      1. Apply substitutions for worksheet names
+        a. Use '_' for disallowed characters ([]:*?/\)
+        b. Use '_' for leading or trailing apostrophes (')
+      2. Limit worksheet name length to <= 31 characters, truncate with
+         base_ellipsis
+      3. Ensure uniqueness with an incrementing parenthesized integer (n)
     :Example:
         >>> unique_name_for_xls(
         ...     'This string has more than 31 characters!',
@@ -90,6 +94,7 @@ def unique_name_for_xls(sheet_name, other_sheet_names, base_ellipsis='...'):
     sheet_name = sheet_name.translate(
         {ord(c): '_' for c in EXCEL_FORBIDDEN_WORKSHEET_NAME_CHARACTERS}
     )
+    sheet_name = re.sub(r"(^'|'$)", '_', sheet_name)
 
     candidate = ellipsize(
         sheet_name, EXCEL_SHEET_NAME_SIZE_LIMIT, base_ellipsis

tests/test_utils_string.py

@@ -1,5 +1,5 @@
 # coding: utf-8
-from formpack.utils.string import orderable_with_none
+from formpack.utils.string import orderable_with_none, unique_name_for_xls
 
 
 def test_sort_list_with_none():
@@ -10,3 +10,33 @@ def test_sort_list_with_none():
     assert orderable_with_none(None).__class__.__name__ == '__OrderableNone'
     assert isinstance(orderable_with_none('foo'), str)
     assert sorted_list == expected_list
+
+def test_excel_compatible_worksheet_names():
+
+    # Truncate (with '...' by default)
+    length_limit = [ # to <= 31 characters
+        '123456789_123456789_123456789_12',
+        '123456789_123456789_12345678...',
+    ]
+    assert unique_name_for_xls(length_limit[0], []) == length_limit[1]
+
+    # Replace disallowed characters ([]:*?/\) with '_'
+    char_safety = [
+        '[hi]: *nice*? ok "/_o,o_\\"',
+        '_hi__ _nice__ ok "__o,o__"',
+    ]
+    assert unique_name_for_xls(char_safety[0], []) == char_safety[1]
+
+    # Replace leading or trailing apostrophes with '_'
+    leading_trailing_apostrophes = [
+        [ "'both'", '_both_' ],
+        [ "'leading", '_leading'],
+        [ "trailing'", 'trailing_'],
+        [
+            "'_'mixed'''",
+            "__'mixed''_",
+        ]
+    ]
+    for test in leading_trailing_apostrophes:
+        assert unique_name_for_xls(test[0], []) == test[1]
+