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

4 files changed, 66 insertions, 6 deletions

diff --git a/doc/build/unreleased/codeerr.rst b/doc/build/unreleased/codeerr.rst
new file mode 100644
index 0000000..2497896
--- /dev/null
+++ b/doc/build/unreleased/codeerr.rst
@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, exceptions
+
+    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.
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):
diff --git a/test/test_exceptions.py b/test/test_exceptions.py
index c904d65..46fbcdd 100644
--- a/test/test_exceptions.py
+++ b/test/test_exceptions.py
@@ -383,3 +383,51 @@ ${foobar}
             "local variable &#39;y&#39; referenced before assignment"
             in html_error
         )
+
+    def test_code_block_line_number(self):
+        l = TemplateLookup()
+        l.put_string(
+            "foo.html",
+            """
+<%
+msg = "Something went wrong."
+raise RuntimeError(msg)  # This is the line.
+%>
+            """,
+        )
+        t = l.get_template("foo.html")
+        try:
+            t.render()
+            assert False
+        except:
+            text_error = exceptions.text_error_template().render_unicode()
+            assert 'File "foo_html", line 4, in render_body' in text_error
+            assert "raise RuntimeError(msg)  # This is the line." in text_error
+        else:
+            assert False
+
+    def test_module_block_line_number(self):
+        l = TemplateLookup()
+        l.put_string(
+            "foo.html",
+            """
+<%!
+def foo():
+    msg = "Something went wrong."
+    raise RuntimeError(msg)  # This is the line.
+%>
+${foo()}
+            """,
+        )
+        t = l.get_template("foo.html")
+        try:
+            t.render()
+            assert False
+        except:
+            text_error = exceptions.text_error_template().render_unicode()
+            sys.stderr.write(text_error)
+            assert 'File "foo_html", line 7, in render_body' in text_error
+            assert 'File "foo_html", line 5, in foo' in text_error
+            assert "raise RuntimeError(msg)  # This is the line." in text_error
+        else:
+            assert False