Debugging the Mako Template Engine

Debugging — Software Engineer

Problem Overview

This debugging exercise asks you to locate and correct defects in a Python templating-library repository. You will be given a repository containing a deliberately altered copy of the Mako template engine. Use the test suite along with your debugging approach to discover and describe the defects.

This assessment evaluates how well you can explore an unfamiliar project, rapidly reason about nontrivial code, apply debugging tools, and clearly narrate your reasoning.

Repository: You will receive access to the repository for this exercise.

Assessment Format

• Duration: Around 45–60 minutes
• Environment: Clone the supplied repository, work on your local machine, and share your screen
• Language: Python
• Interaction: The interviewer watches your debugging workflow, so explain your thinking as you proceed

Background: The Mako Template Library

Mako is a popular Python templating system that turns templates into Python modules to improve runtime performance. Its major capabilities include:

• Embedded Python: Python snippets may appear in templates through <%... %> blocks
• Variable Interpolation: Insert a value with ${variable}
• Template Inheritance: A template can derive from a parent template with <%inherit>
• Reusable Components: Template functions can be declared with <%def> elements
• Control Flow: Regular Python flow constructs, including loops and conditionals, can be used directly

Example Template:

<%inherit file="layout.html"/>

<%def name="welcome(person)">
    Welcome, ${person}!
</%def>

% for product in products:
    <li>${product}</li>
% endfor

You are not expected to know Mako beforehand; the project’s extensive tests should guide you toward the broken behavior.

Getting Started

Setup Instructions

1. Copy the repository locally and change into the debug-mako directory.
2. Install the required packages with pip install -r bugsquash-requirements.txt.
3. Execute the tests using python -m pytest test/.
4. Review the failing tests, then start tracing the cause.

Codebase Structure

The repository provides a complete implementation of a template library:

mako/
├── template.py # Primary Template type and render behavior
├── lexer.py # Breaks template text into tokens
├── codegen.py # Builds Python code from parsed templates
├── ast.py # AST-node declarations for template constructs
├── _ast_util.py # Helpers for working with ASTs
├── lookup.py # Finds template files and manages caching
├── runtime.py # Runtime context and helper functionality
├── parsetree.py # Parse-tree node declarations
├── pygen.py # Produces Python source code
├── pyparser.py # Parses Python expressions
├── filters.py # Functions that filter rendered output
├── cache.py # Template-cache implementation
├── exceptions.py # Library-specific exception types
├── cmd.py # Command-line entry point
├── compat.py # Compatibility support across Python versions
└── util.py # Shared utility helpers

test/
├── test_template.py # Tests for fundamental template rendering
├── test_lexer.py # Tests for lexing and tokenization
├── test_lookup.py # Tests for locating and loading files
├── test_ast.py # Tests for AST processing
└──... # Other test modules

Debugging Strategy

Recommended Approach

1. Start With the Entire Test Suite: First, see the full set of failures.

   python -m pytest test/ -v
   

2. Narrow Down Individual Failures: Re-run a failing test by itself to get more focused output.

   python -m pytest test/test_lookup.py::TestClass::test_name -v
   

3. Inspect the Tests: Determine the behavior that each test is asserting.

4. Follow the Execution Path: Trace processing from the test’s input through to the point where it fails.

5. Use Print Statements or pdb: Add breakpoints when you need to examine program state.

   import pdb; pdb.set_trace()
   

6. Review Suspicious Modifications: Examine code regions that look unusual or internally inconsistent.

What to Look For

Defects in this repository commonly fit the following patterns:

• Incorrect Conditionals: Boolean expressions or comparison operations differ from the required behavior
• Missing Edge Case Handling: Certain states or input kinds are not handled by a code path
• AST/Parse Tree Issues: Nodes or transformations generate the wrong result
• String Processing Errors: Template-syntax text is processed incorrectly