add start symbol setter for cfg

Author: bygu4

pyformlang/cfg/cfg.py

@@ -13,7 +13,8 @@
 from .parse_tree import ParseTree
 from .cyk_table import CYKTable
 from .cfg_variable_converter import CFGVariableConverter
-from .utils import remove_nullable_production, get_productions_d
+from .utils import remove_nullable_production, get_productions_d, \
+    is_special_text
 from ..objects.cfg_objects import CFGObject, \
     Variable, Terminal, Epsilon, Production
 from ..objects.cfg_objects.utils import to_variable, to_terminal
@@ -23,13 +24,6 @@
 SUBS_SUFFIX = "#SUBS#"
 
 
-def is_special_text(text: str) -> bool:
-    """ Check if the input is given an explicit type """
-    return len(text) > 5 and \
-        (text[0:5] == '"VAR:' or text[0:5] == '"TER:') and \
-        text[-1] == '"'
-
-
 class CFG(FormalGrammar):
     """ A class representing a context free grammar
 

pyformlang/cfg/formal_grammar.py

@@ -4,6 +4,7 @@
 from abc import abstractmethod
 
 from ..objects.cfg_objects import Variable, Terminal, Production
+from ..objects.cfg_objects.utils import to_variable
 
 GrammarT = TypeVar("GrammarT", bound="FormalGrammar")
 
@@ -66,6 +67,12 @@ def start_symbol(self) -> Optional[Variable]:
         """
         return self._start_symbol
 
+    @start_symbol.setter
+    def start_symbol(self, symbol: Hashable) -> None:
+        """ Sets the start symbol of the grammar """
+        self._start_symbol = to_variable(symbol) \
+            if symbol is not None else None
+
     @abstractmethod
     def copy(self: GrammarT) -> GrammarT:
         """ Copies the grammar """

pyformlang/cfg/utils.py

@@ -5,6 +5,13 @@
 from ..objects.cfg_objects import CFGObject, Variable, Epsilon, Production
 
 
+def is_special_text(text: str) -> bool:
+    """ Check if the input is given an explicit type """
+    return len(text) > 5 and \
+        (text[0:5] == '"VAR:' or text[0:5] == '"TER:') and \
+        text[-1] == '"'
+
+
 def remove_nullable_production_sub(body: List[CFGObject],
                                    nullables: AbstractSet[CFGObject]) \
         -> List[List[CFGObject]]: