Support showing compilation problems in-browser. #150

3 files changed, 107 insertions, 18 deletions

diff --git a/scss/__init__.py b/scss/__init__.py
index b0487be..f2c6c80 100644
--- a/scss/__init__.py
+++ b/scss/__init__.py
@@ -293,7 +293,10 @@ class SourceFile(object):
 
 
 class Scss(object):
-    def __init__(self, scss_vars=None, scss_opts=None, scss_files=None, super_selector=None, library=ALL_BUILTINS_LIBRARY, func_registry=None, search_paths=None):
+    def __init__(self,
+            scss_vars=None, scss_opts=None, scss_files=None, super_selector=None,
+            live_errors=False, library=ALL_BUILTINS_LIBRARY, func_registry=None, search_paths=None):
+
         if super_selector:
             self.super_selector = super_selector + ' '
         else:
@@ -319,6 +322,9 @@ class Scss(object):
         self._library = func_registry or library
         self._search_paths = search_paths
 
+        # If true, swallow compile errors and embed them in the output instead
+        self.live_errors = live_errors
+
         self.reset()
 
     def get_scss_constants(self):
@@ -434,7 +440,15 @@ class Scss(object):
 
         return final_cont
 
-    compile = Compilation
+    def compile(self, *args, **kwargs):
+        try:
+            return self.Compilation(*args, **kwargs)
+        except SassError as e:
+            if self.live_errors:
+                # TODO should this setting also capture and display warnings?
+                return e.to_css()
+            else:
+                raise
 
     def parse_selectors(self, raw_selectors):
         """
diff --git a/scss/errors.py b/scss/errors.py
index c898500..52a8f5b 100644
--- a/scss/errors.py
+++ b/scss/errors.py
@@ -2,6 +2,28 @@ import sys
 import traceback
 
 
+BROWSER_ERROR_TEMPLATE = """\
+body:before {{
+    content: {0};
+
+    display: block;
+    position: fixed;
+    top: 0;
+    left: 0;
+    right: 0;
+
+    font-size: 14px;
+    margin: 1em;
+    padding: 1em;
+    border: 3px double red;
+
+    white-space: pre;
+    font-family: monospace;
+    background: #fcebeb;
+    color: black;
+}}
+"""
+
 def add_error_marker(text, position, start_line=1):
     """Add a caret marking a given position in a string of input.
 
@@ -47,34 +69,80 @@ class SassError(Exception):
         self.rule_stack.append(rule)
 
     def format_prefix(self):
+        """Return the general name of the error and the contents of the rule or
+        property that caused the failure.  This is the initial part of the
+        error message and should be error-specific.
+        """
         # TODO this contains NULs and line numbers; could be much prettier
         if self.rule_stack:
             return "Error parsing block:\n" + "    " + self.rule_stack[0].unparsed_contents
         else:
             return "Unknown error"
 
-    def __str__(self):
-        prefix = self.format_prefix()
+    def format_sass_stack(self):
+        """Return a "traceback" of Sass imports."""
+        if not self.rule_stack:
+            return ""
 
-        ret = [prefix, "\n\n"]
+        ret = ["From ", self.rule_stack[0].file_and_line, "\n"]
+        last_file = self.rule_stack[0].source_file
 
-        if self.rule_stack:
-            # TODO this looks very neat-o but doesn't actually work because
-            # manage_children doesn't recurse for imports  :)
-            ret.extend(("From ", self.rule_stack[0].file_and_line, "\n"))
-            last_file = self.rule_stack[0].source_file
+        # TODO this could go away if rules knew their import chains...
+        for rule in self.rule_stack[1:]:
+            if rule.source_file is not last_file:
+                ret.extend(("...imported from ", rule.file_and_line, "\n"))
+            last_file = rule.source_file
 
-            for rule in self.rule_stack[1:]:
-                if rule.source_file is not last_file:
-                    ret.extend(("...imported from ", rule.file_and_line, "\n"))
-                last_file = rule.source_file
+        ret.append("\n")
 
-            ret.append("\n")
+        return "".join(ret)
 
-        ret.append("Traceback:\n")
+    def format_python_stack(self):
+        """Return a traceback of Python frames, from where the error occurred
+        to where it was first caught and wrapped.
+        """
+        ret = ["Traceback:\n"]
         ret.extend(traceback.format_tb(self.original_traceback))
-        ret.extend((type(self.exc).__name__, ": ", str(self.exc), "\n"))
-        return ''.join(ret)
+        return "".join(ret)
+
+    def format_original_error(self):
+        """Return the typical "TypeError: blah blah" for the original wrapped
+        error.
+        """
+        # TODO eventually we'll have sass-specific errors that will want nicer
+        # "names" in browser display and stderr
+        return "".join((type(self.exc).__name__, ": ", str(self.exc), "\n"))
+
+    def __str__(self):
+        try:
+            prefix = self.format_prefix()
+            sass_stack = self.format_sass_stack()
+            python_stack = self.format_python_stack()
+            original_error = self.format_original_error()
+
+            return prefix + sass_stack + python_stack + original_error
+        except Exception:
+            # "unprintable error" is not helpful
+            return str(self.exc)
+
+    def to_css(self):
+        """Return a stylesheet that will show the wrapped error at the top of
+        the browser window.
+        """
+        # TODO should this include the traceback?  any security concerns?
+        prefix = self.format_prefix()
+        original_error = self.format_original_error()
+        sass_stack = self.format_sass_stack()
+
+        message = prefix + original_error + sass_stack
+
+        # Super simple escaping: only quotes and newlines are illegal in css
+        # strings
+        message = message.replace('\\', '\\\\')
+        message = message.replace('"', '\\"')
+        message = message.replace('\n', '\\A')
+
+        return BROWSER_ERROR_TEMPLATE.format('"' + message + '"')
 
 
 class SassParseError(SassError):
diff --git a/scss/tests/test_misc.py b/scss/tests/test_misc.py
index 9cf2cb3..d19ab18 100644
--- a/scss/tests/test_misc.py
+++ b/scss/tests/test_misc.py
@@ -57,6 +57,13 @@ table {
     assert expected == output
 
 
+def test_live_errors():
+    compiler = Scss(live_errors=True)
+    output = compiler.compile("""$foo: unitless(one);""")
+    assert "body:before" in output
+    assert "TypeError: Expected" in output
+
+
 def test_extend_across_files():
     compiler = Scss(scss_opts=dict(compress=0))
     compiler._scss_files = {}