preprocess_premailer.diff

diff --git a/commands/preprocess_cssless.py b/commands/preprocess_cssless.py
index 0c29d96..94edc3f 100644
--- a/commands/preprocess_cssless.py
+++ b/commands/preprocess_cssless.py
@@ -19,0 +20 @@ import functools
+import inspect
@@ -28 +28,0 @@ from lxml.etree import strip_elements
-import premailer
@@ -31,2 +31,21 @@ from premailer import Premailer
-premailer_version = premailer.__version__.split('.')
-premailer_is_new=len(premailer_version) >= 2 and int(premailer_version[0]) >= 3 and int(premailer_version[1]) >= 9
+
+def premailer_options(src_path):
+    options = {
+        'base_url': src_path,
+        'disable_link_rewrites': True,
+        'remove_classes': True,
+    }
+    parameters = inspect.signature(Premailer.__init__).parameters
+    if 'allow_loading_external_files' in parameters:
+        options['allow_loading_external_files'] = True
+    if 'disable_leftover_css' in parameters:
+        options['disable_leftover_css'] = True
+    return options
+
+
+class CsslessPremailer(Premailer):
+    def _css_rules_to_string(self, rules):
+        # Leftover CSS is stripped after inlining. Premailer 3.7 can crash
+        # serializing nested @media leftovers, so keep the cssless pass empty.
+        return ''
+
@@ -78,7 +97 @@ def preprocess_html_merge_css(root, src_path):
-        if premailer_is_new:
-            premailer = Premailer(root, base_url=src_path,
-                                  disable_link_rewrites=True, remove_classes=True,
-                                  allow_loading_external_files=True)
-        else:
-            premailer = Premailer(root, base_url=src_path,
-                                  disable_link_rewrites=True, remove_classes=True)
+        premailer = CsslessPremailer(root, **premailer_options(src_path))
diff --git a/preprocess_qch.py b/preprocess_qch.py
index 856afb5..ea97aba 100755
--- a/preprocess_qch.py
+++ b/preprocess_qch.py
@@ -66,0 +67 @@ def main():
+            src_path, dst_path = paths_list[i]
@@ -68,2 +69,7 @@ def main():
-                    i, len(paths_list), paths_list[i][1]))
-            output = future.result()
+                    i, len(paths_list), dst_path))
+            try:
+                output = future.result()
+            except Exception as exc:
+                raise RuntimeError(
+                    'failed to preprocess {} -> {}'.format(
+                        src_path, dst_path)) from exc