Debug Three Planted Bugs in the Mako Python Template Engine

You are given a working copy of Mako, an open-source Python template engine, into which bugs have been planted. Find each bug, fix it with the smallest correct change to the library, and add a regression test for it.

Background: Mako compiles every template into a Python module. It parses the template into a tree of nodes (plain text, ${...} expressions, % control lines such as % if and % endif, ## comments, and tags such as <%def>), parses the embedded Python with the standard ast module, and generates the module's source code from both.

Three bugs are reported. Assume the failing tests show the symptoms described in the three parts below.

Constraints and Clarifications

• Fix the library, never the tests. You can run the test suite and any snippet you like.
• Treat each symptom as the result of one planted bug, and fix them one at a time.
• Mako can show the Python source it generates for a template; you may use it while debugging.

Clarifying Questions

• Am I expected to add regression tests, or only to make the existing tests pass?
• When a requested template name matches a directory in one search directory and a file in a later one, should the lookup return the later file?
• Should a control block whose body is only comments render nothing, or should the engine reject it with a clear error?
• Which Python versions must the fixes support? The ast module's node types differ between versions.

Part 1 — Parameters disappear from regenerated code

When Mako turns a parsed Python expression back into source code, the parameter names of a lambda are lost: lambda item: item.name comes back as a lambda with no parameters, lambda : item.name. A template function whose signature has such a default value still compiles, but calling the default fails when the template renders:

<%def name="listing(items, key=lambda item: item.name)">
...
</%def>

What This Part Should Cover

• How the missing text was traced to the code that handles one kind of ast node
• The root cause and the minimal fix
• A regression test that fails before the fix and passes after it

Part 2 — A directory is taken for a template

A template lookup is configured with several search directories. When the requested template name matches a directory inside one of them, the lookup tries to open that directory as a template and fails with an operating-system error such as IsADirectoryError, instead of moving on to the next search directory or raising the lookup's own "template not found" error.

What This Part Should Cover

• The check that accepts the wrong candidate, and the corrected check
• Behavior with several search directories, and when no directory has the template
• Related cases: symbolic links, and a file that disappears between the check and the open

Part 3 — A comment-only block fails to compile

This template fails to compile with a Python syntax error, although a block whose body is only a comment should simply produce no output:

% if show_footer:
 ## the footer is disabled for now
% endif

What This Part Should Cover

• Why a body made only of comments produces invalid Python
• The exact rule for when an empty body needs special handling, including % elif and % else branches
• Regression tests for each variant