fix a bug of string interop

src/vm.cpp

@@ -398,7 +398,11 @@ PyObject* VM::format(Str spec, PyObject* obj){
     int width, precision;
     try{
         if(dot >= 0){
-            width = Number::stoi(spec.substr(0, dot).str());
+            if(dot == 0){
+                width = -1;
+            }else{
+                width = Number::stoi(spec.substr(0, dot).str());
+            }
             precision = Number::stoi(spec.substr(dot+1).str());
         }else{
             width = Number::stoi(spec.str());
@@ -424,7 +428,7 @@ PyObject* VM::format(Str spec, PyObject* obj){
     }else{
         ret = CAST(Str&, py_str(obj));
     }
-    if(width > ret.length()){
+    if(width != -1 && width > ret.length()){
         int pad = width - ret.length();
         std::string padding(pad, pad_c);
         if(align == '>') ret = padding.c_str() + ret;

tests/25_rawstring.py

@@ -42,6 +42,8 @@ assert f'{a:010}' == '0000000010'
 assert f'{a:010d}' == '0000000010'
 assert f'{a:010f}' == '010.000000'
 assert f'{a:010.2f}' == '0000010.00'
+assert f'{a:.2f}' == '10.00'
+assert f'{a:.5f}' == '10.00000'
 
 b = '123'
 assert f'{b:10}' == '123       '