Correctly track line numbers for multi-line code blocks

Improved the line-number tracking for source lines inside of Python  ``<%
... %>`` blocks, such that text- and HTML-formatted exception traces such
as that of  :func:`.html_error_template` now report the correct source line
inside the block, rather than the first line of the block itself.
Exceptions in ``<%! ... %>`` blocks which get raised while loading the
module are still not reported correctly, as these are handled before the
Mako code is generated.  Pull request courtesy Martin von Gagern.

Closes: #297
Pull-request: https://github.com/sqlalchemy/mako/pull/297
Pull-request-sha: 60ad749604f8de0a9b8133430995045a93d0939c

Change-Id: I2086e4370e8e2db7099de01743840286f0160efc

2 files changed, 8 insertions, 6 deletions

diff --git a/mako/codegen.py b/mako/codegen.py
index 733d264..1acc5e6 100644
--- a/mako/codegen.py
+++ b/mako/codegen.py
@@ -354,8 +354,7 @@ class _GenerateRenderMethod(object):
         """write module-level template code, i.e. that which
         is enclosed in <%! %> tags in the template."""
         for n in module_code:
-            self.printer.start_source(n.lineno)
-            self.printer.write_indented_block(n.text)
+            self.printer.write_indented_block(n.text, starting_lineno=n.lineno)
 
     def write_inherit(self, node):
         """write the module-level inheritance-determination callable."""
@@ -896,8 +895,9 @@ class _GenerateRenderMethod(object):
 
     def visitCode(self, node):
         if not node.ismodule:
-            self.printer.start_source(node.lineno)
-            self.printer.write_indented_block(node.text)
+            self.printer.write_indented_block(
+                node.text, starting_lineno=node.lineno
+            )
 
             if not self.in_def and len(self.identifiers.locally_assigned) > 0:
                 # if we are the "template" def, fudge locally
diff --git a/mako/pygen.py b/mako/pygen.py
index 265c0b0..603676d 100644
--- a/mako/pygen.py
+++ b/mako/pygen.py
@@ -54,14 +54,16 @@ class PythonPrinter(object):
         self.stream.write("\n" * num)
         self._update_lineno(num)
 
-    def write_indented_block(self, block):
+    def write_indented_block(self, block, starting_lineno=None):
         """print a line or lines of python which already contain indentation.
 
         The indentation of the total block of lines will be adjusted to that of
         the current indent level."""
         self.in_indent_lines = False
-        for l in re.split(r"\r?\n", block):
+        for i, l in enumerate(re.split(r"\r?\n", block)):
             self.line_buffer.append(l)
+            if starting_lineno is not None:
+                self.start_source(starting_lineno + i)
             self._update_lineno(1)
 
     def writelines(self, *lines):