writer: if string starts with '-' followed by digit, write in quotes

to distinguish unquoted `23` (an int) or `-3.45` (float) from quoted strings `"23"` and `"-3.45"`

src/openstep_plist/writer.pyx

@@ -35,14 +35,19 @@ cdef Py_UNICODE *DICT_ITEM_SEP_NO_INDENT = [c';', c' ']
 
 
 cdef inline bint is_valid_unquoted_string(const Py_UNICODE *a, Py_ssize_t length):
-    cdef Py_ssize_t i
-
-    # if string starts with digit, always write it within quotes
-    # to distinguish it from an actual integer or float number,
-    # which are always written without quotes
-    if c'0' <= a[0] <= c'9':
+    cdef Py_UNICODE ch = a[0]
+    # if string starts with digit, or with a '-' followed by a digit, always
+    # write it within quotes to distinguish it from an actual (signed) integer
+    # or float number, which are always written without quotes
+    if c'0' <= ch <= c'9':
         return False
+    elif ch == c'-':
+        if length > 1:
+            ch = a[1]
+            if c'0' <= ch <= c'9':
+                return False
 
+    cdef Py_ssize_t i
     for i in range(length):
         if not is_valid_unquoted_string_char(a[i]):
             return False

tests/test_writer.py

@@ -45,7 +45,9 @@ def test_unquoted_string(self):
             # if string starts with digit, always quote it to distinguish from number
             ("1", '"1"'),
             ("1.1", '"1.1"'),
+            ("-23", '"-23"'),
             ("1zzz", '"1zzz"'),  # ... even if it's not actually a number
+            ("-23yyy", '"-23yyy"'),
         ],
     )
     def test_quoted_string(self, string, expected):