FIX: Update escaping rules in connection string parser and builder (#364)

### Work Item / Issue Reference  

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> GitHub Issue: #363

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### Summary   
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FIX: The escaping rules for the parser and the builder

The parser was treating {{ as an escaped string and was parsing it as {
and then the builder was escaping { to {{
However the ODBC driver doesn't require escaping of the opening brace,
if its already wrapped in {}. Only closing brace values inside curlies
require escaping. Hence a literal pw}d needs to be sent as {pw}}d}

Fixed the parser and the builder to take care of this nuance.

Fixes #363

Author: saurabh500

mssql_python/connection_string_builder.py

@@ -77,10 +77,9 @@ def _escape_value(self, value: str) -> str:
         """
         Escape a parameter value if it contains special characters.
 
-        Per MS-ODBCSTR specification:
         - Values containing ';', '{', '}', '=', or spaces should be braced for safety
         - '}' inside braced values is escaped as '}}'
-        - '{' inside braced values is escaped as '{{'
+        - '{' does not need to be escaped
 
         Args:
             value: Parameter value to escape
@@ -95,7 +94,7 @@ def _escape_value(self, value: str) -> str:
             >>> builder._escape_value("local;host")
             '{local;host}'
             >>> builder._escape_value("p}w{d")
-            '{p}}w{{d}'
+            '{p}}w{d}'
             >>> builder._escape_value("ODBC Driver 18 for SQL Server")
             '{ODBC Driver 18 for SQL Server}'
         """
@@ -107,8 +106,9 @@ def _escape_value(self, value: str) -> str:
         needs_braces = any(ch in value for ch in ";{}= ")
 
         if needs_braces:
-            # Escape existing braces by doubling them
-            escaped = value.replace("}", "}}").replace("{", "{{")
+            # Escape closing braces by doubling them (ODBC requirement)
+            # Opening braces do not need to be escaped
+            escaped = value.replace("}", "}}")
             return f"{{{escaped}}}"
         else:
             return value

mssql_python/connection_string_parser.py

@@ -7,7 +7,7 @@
 Handles ODBC-specific syntax per MS-ODBCSTR specification:
 - Semicolon-separated key=value pairs
 - Braced values: {value}
-- Escaped braces: }} → }, {{ → {
+- Escaped braces: }} → } (only closing braces need escaping)
 
 Parser behavior:
 - Validates all key=value pairs
@@ -331,7 +331,7 @@ def _parse_braced_value(self, connection_str: str, start_pos: int) -> Tuple[str,
         Braced values:
         - Start with '{' and end with '}'
         - '}' inside the value is escaped as '}}'
-        - '{' inside the value is escaped as '{{'
+        - '{' inside the value does not need escaping
         - Can contain semicolons and other special characters
 
         Args:
@@ -366,18 +366,8 @@ def _parse_braced_value(self, connection_str: str, start_pos: int) -> Tuple[str,
                     # Single '}' means end of braced value
                     start_pos += 1
                     return "".join(value), start_pos
-            elif ch == "{":
-                # Check if it's an escaped left brace
-                if start_pos + 1 < str_len and connection_str[start_pos + 1] == "{":
-                    # Escaped left brace: '{{' → '{'
-                    value.append("{")
-                    start_pos += 2
-                else:
-                    # Single '{' inside braced value - keep it as is
-                    value.append(ch)
-                    start_pos += 1
             else:
-                # Regular character
+                # Regular character (including '{' which doesn't need escaping per ODBC spec)
                 value.append(ch)
                 start_pos += 1
 

tests/test_010_connection_string_parser.py

@@ -61,13 +61,13 @@ def test_parse_braced_value_with_escaped_right_brace(self):
         """Test parsing braced values with escaped }}."""
         parser = _ConnectionStringParser()
         result = parser._parse("PWD={p}}w{{d}")
-        assert result == {"pwd": "p}w{d"}
+        assert result == {"pwd": "p}w{{d"}
 
     def test_parse_braced_value_with_all_escapes(self):
-        """Test parsing braced values with both {{ and }} escapes."""
+        """Test parsing braced values with }} escape ({{ not an escape sequence)."""
         parser = _ConnectionStringParser()
         result = parser._parse("Value={test}}{{escape}")
-        assert result == {"value": "test}{escape"}
+        assert result == {"value": "test}{{escape"}
 
     def test_parse_empty_value(self):
         """Test that empty value raises error."""
@@ -146,10 +146,10 @@ def test_parse_braced_value_with_left_brace(self):
         assert result == {"value": "test{value"}
 
     def test_parse_braced_value_double_left_brace(self):
-        """Test parsing braced value with escaped {{ (left brace)."""
+        """Test parsing braced value with {{ (not an escape sequence)."""
         parser = _ConnectionStringParser()
         result = parser._parse("Value={test{{value}")
-        assert result == {"value": "test{value"}
+        assert result == {"value": "test{{value"}
 
     def test_parse_unicode_characters(self):
         """Test parsing values with unicode characters."""
@@ -197,7 +197,7 @@ def test_parse_special_characters_in_braced_values(self):
 
         # Multiple special chars including braces
         result = parser._parse("Token={Bearer: abc123; Expires={{2024-01-01}}}")
-        assert result == {"token": "Bearer: abc123; Expires={2024-01-01}"}
+        assert result == {"token": "Bearer: abc123; Expires={{2024-01-01}"}
 
     def test_parse_numbers_and_symbols_in_passwords(self):
         """Test parsing passwords with various numbers and symbols."""