How to implement python style indentation in yacc or bison? -

i have been trying implement python style indentation in bison grammar, insights or ideas implementation nice.

the best way have lexer track indentation level , insert indent/unindent tokens token stream appropriately. here's flex code have that:

%x linestart %s normal %{ static std::stack<int> indent; static int indent_depth(const char *); %}  %%  <initial>.*|\n          { yyless(0); begin(linestart); indent.push(0); } <linestart>[ \t]*       { int depth = indent_depth(yytext);                           if (depth < indent.top()) {                               indent.pop();                               yyless(0);                               return unindent; }                           begin(normal);                           if (depth > indent.top()) {                               indent.push(depth);                               return indent; } } <linestart>.            { yyless(0);                           if (indent.top() > 0) {                               indent.pop();                               return unindent; }                           begin(normal); } <linestart><<eof>>      { if (indent.top() > 0) {                               indent.pop();                               return unindent; }                           begin(normal); } <linestart>[ \t]*\n     { lineno++; } <linestart>[ \t]*#.*\n  { lineno++; } [[({]                   { parens++; return *yytext; } [])}]                   { if (--parens < 0) parens = 0;                           return *yytext; } \n                      { lineno++;                           if (parens == 0) begin(linestart); } 

this code tricky special cases -- example, need ignore blank lines , lines comments, , want ignore indentation within unbalanced parenthesis (which above does).