use ast.literal_eval instead of eval (#73)

* Make syntax check for bool constant stricter

* Use ast.literal_eval()

* Use ternary expression to return True/False

src/genmsg/msg_loader.py

@@ -39,6 +39,7 @@
 possible layouts.
 """
 
+import ast
 import os
 import sys
 
@@ -181,8 +182,7 @@ def convert_constant_value(field_type, val):
             raise InvalidMsgSpec("cannot coerce [%s] to %s (out of bounds)"%(val, field_type))
         return val
     elif field_type == 'bool':
-        # TODO: need to nail down constant spec for bool
-        return True if eval(val) else False
+        return True if ast.literal_eval(val) else False
     raise InvalidMsgSpec("invalid constant type: [%s]"%field_type)
 
 def _load_constant_line(orig_line):