Testing whether a reader class is enabled *before* calling it, otherwise the test is never reached.

For example, if Markdown hasn't been installed, and you attempt read_file('foo.md') then an exception is raised in MarkdownReader.__init__ as it tries to call Markdown, which is False.  The class field "enabled" was not checked before attempting to instantiate MarkdownReader.  Instead, there is a later test on the instance, which won't be reached.

pelican/readers.py

@@ -347,15 +347,16 @@ def read_file(path, fmt=None, settings=None):
     if settings is None:
         settings = {}
 
-    reader = EXTENSIONS[fmt](settings)
+    cls = EXTENSIONS[fmt]
+    if not cls.enabled:
+        raise ValueError("Missing dependencies for %s" % fmt)
+
+    reader = cls(settings)
     settings_key = '%s_EXTENSIONS' % fmt.upper()
 
     if settings and settings_key in settings:
         reader.extensions = settings[settings_key]
 
-    if not reader.enabled:
-        raise ValueError("Missing dependencies for %s" % fmt)
-
     metadata = parse_path_metadata(
         path=path, settings=settings, process=reader.process_metadata)
     content, reader_metadata = reader.read(path)