Debugging Pelican templates

The problem

When building websites with Pelican, template errors can be frustratingly cryptic. You might encounter blank pages, missing content, or error messages that don't clearly indicate the source of the problem. Without proper debugging techniques, you could spend hours searching for a simple syntax error or template variable issue.

Why it happens

Pelican uses Jinja2 as its templating engine, which processes templates at build time. When errors occur, the default error reporting might not provide enough context to identify the specific issue, especially for beginners. Common causes include:

Missing or misspelled template variables
Incorrect template inheritance
Syntax errors in Jinja2 expressions
Problems with template paths or static file references
Errors in the underlying Python code

Solution approaches

Pelican provides several debugging methods that can help you quickly identify and fix template issues.

Enable the Jinja2 debug extension

Add this to your pelicanconf.py file:

JINJA_ENVIRONMENT = {
    'extensions': ['jinja2.ext.debug']
}

This enables the use of {% debug %} tags in your templates, which display all variables available at that point in the template.

Use template tracing

TEMPLATE_PAGES = {'path/to/template.html': 'output.html'}
TEMPLATE_DEBUG = True

Setting TEMPLATE_DEBUG provides detailed information about how Pelican processes templates, showing each template as it's loaded and rendered.

Inspect variables directly in templates

When you need to see the content of a specific variable, use these techniques within your templates:

{{ variable|pprint }}
{{ dump(variable) }}

These filters display the full structure and content of variables, helping you understand what data is available.

Command-line debugging options

Run Pelican with verbose flags to see more information during the build process:

pelican content -D --print-settings
pelican content -vv

The -D flag shows all settings, while -vv enables verbose output that includes template processing details.

Use the development server with auto-reload

pelican --listen --autoreload --debug

This starts a development server that automatically rebuilds your site when files change and shows errors in real-time, making the debugging process much faster.

Implementation

Let's tackle a common issue: a template that isn't displaying article tags correctly.

First, identify where the problem might be occurring by adding debugging tags:

{% if article.tags %}
  {% debug %}
  <ul class="tags">
    {% for tag in article.tags %}
      <li><a href="{{ SITEURL }}/{{ tag.url }}">{{ tag }}</a></li>
    {% endfor %}
  </ul>
{% endif %}

Run Pelican with verbose output to see if there are any errors:
```
pelican content -vv
```
Check the structure of the article.tags variable:
```
{{ article.tags|pprint }}
```
If needed, enable template tracing to see how Pelican processes the template:
```
# In pelicanconf.py
TEMPLATE_DEBUG = True
```

Results

After applying these debugging techniques, you'll often discover that the issue is simple to fix once properly identified. Common solutions include:

Correcting variable names or attributes
Fixing template inheritance paths
Ensuring proper Jinja2 syntax
Adjusting template settings in pelicanconf.py

Note

For particularly complex issues, you can use Python's debugging tools by adding `import pdb; pdb.set_trace()` to your Pelican plugins or theme code.

With these techniques, you'll be able to quickly identify and resolve most Pelican template issues, making your theme development process much more efficient.