Basic error wrapping.

3 files changed, 81 insertions, 4 deletions

diff --git a/scss/__init__.py b/scss/__init__.py
index 540fcfb..3cad57c 100644
--- a/scss/__init__.py
+++ b/scss/__init__.py
@@ -65,6 +65,7 @@ from scss.cssdefs import (
     _spaces_re, _expand_rules_space_re, _collapse_properties_space_re,
     _strings_re, _prop_split_re,
 )
+from scss.errors import SassError
 from scss.expression import Calculator
 from scss.functions import ALL_BUILTINS_LIBRARY
 from scss.functions.compass.sprites import sprite_map
@@ -514,6 +515,16 @@ class Scss(object):
 
     @print_timing(4)
     def manage_children(self, rule, p_children, scope):
+        try:
+            return self._manage_children_impl(rule, p_children, scope)
+        except SassError, e:
+            e.set_rule(rule)
+            raise
+        except Exception, e:
+            raise SassError(e, rule=rule)
+
+
+    def _manage_children_impl(self, rule, p_children, scope):
         # A rule that has already returned should not end up here
         assert rule.retval is None
 
diff --git a/scss/errors.py b/scss/errors.py
new file mode 100644
index 0000000..6e946e5
--- /dev/null
+++ b/scss/errors.py
@@ -0,0 +1,61 @@
+import sys
+import traceback
+
+class SassError(Exception):
+    """Error class that wraps another exception and attempts to bolt on some
+    useful context.
+    """
+    def __init__(self, exc, rule=None, expression=None):
+        self.exc = exc
+        self.rule = rule
+        self.expression = expression
+        _, _, self.original_traceback = sys.exc_info()
+
+    def set_rule(self, rule):
+        """Set the current rule.
+
+        If this error already has a rule, it's left alone; this is because
+        exceptions propagate outwards and so the first rule seen is the
+        "innermost".
+        """
+        if not self.rule:
+            self.rule = rule
+
+    def format_prefix(self):
+        # TODO pointer, yadda
+        # TODO newlines, yadda
+        if self.rule:
+            return "Error parsing block:\n" + "    " + self.rule.unparsed_contents
+        else:
+            return "Unknown error"
+
+    def __str__(self):
+        prefix = self.format_prefix()
+
+        ret = [prefix, "\n\n"]
+
+        if self.rule:
+            ret.extend(("From ", self.rule.file_and_line, "\n\n"))
+
+        ret.append("Traceback:\n")
+        ret.extend(traceback.format_tb(self.original_traceback))
+        ret.extend((type(self.exc).__name__, ": ", str(self.exc), "\n"))
+        return ''.join(ret)
+
+class SassParseError(SassError):
+    """Error raised when parsing a Sass expression fails."""
+
+    def format_prefix(self):
+        # TODO pointer
+        # TODO deal with newlines, etc
+        return """Error parsing expression:\n    %s""" % (self.expression,)
+
+
+class SassEvaluationError(SassError):
+    """Error raised when evaluating a parsed expression fails."""
+
+    def format_prefix(self):
+        # TODO pointer
+        # TODO ast needs to know where each node came from for that to work  :(
+        # TODO deal with newlines, etc
+        return """Error evaluating expression:\n    %s""" % (self.expression,)
diff --git a/scss/expression.py b/scss/expression.py
index 869550a..5c6dd15 100644
--- a/scss/expression.py
+++ b/scss/expression.py
@@ -9,6 +9,7 @@ import six
 
 import scss.config as config
 from scss.cssdefs import COLOR_NAMES, is_builtin_css_function, _expr_glob_re, _interpolate_re, _variable_re
+from scss.errors import SassEvaluationError, SassParseError
 from scss.rule import Namespace
 from scss.types import Boolean, Color, List, Map, Null, Number, ParserValue, String, Undefined
 from scss.util import dequote, normalize_var
@@ -137,14 +138,18 @@ class Calculator(object):
             try:
                 parser = SassExpression(SassExpressionScanner(expr))
                 ast = parser.goal()
-            except SyntaxError:
+            except SyntaxError, e:
                 if config.DEBUG:
-                    raise
-                return None
+                    raise SassParseError(e, expression=expr)
+                else:
+                    return None
             else:
                 ast_cache[expr] = ast
 
-        return ast.evaluate(self, divide=divide)
+        try:
+            return ast.evaluate(self, divide=divide)
+        except Exception, e:
+            raise SassEvaluationError(e, expression=expr)
 
     def parse_expression(self, expr, target='goal'):
         ast_cache = self.get_ast_cache(target)