Jinja2邮件模板渲染问题：表达式因无值无法求值求助

Let's break down your problem and walk through possible fixes—this error usually stems from either template syntax missteps or out-of-order processing between your SQL placeholder replacement and Jinja2 rendering.

First: Fix Your Jinja Variable Definitions #

Looking at your code, you’ve wrapped your {% set %} statements inside HTML comments:

<!-- {% set partner_shortname = '@@SHORTNAME' %} -->
<!-- {% set partner_name = '@@PARTNER' %} -->

Jinja2 ignores code inside HTML comments—those variable assignments aren’t being executed at all. That means partner_name and partner_shortname never get defined in your template context, which directly causes the "Expression not evaluating due to no value" error when you try to render {{ partner_name }}.

Fix: Remove the HTML comment tags so Jinja can process the assignments:

{% set partner_shortname = '@@SHORTNAME' %}
{% set partner_name = '@@PARTNER' %}

Second: Check the Order of SQL Replacement & Jinja Rendering #

You noted that SQL should assign values to @@-formatted variables—your workflow order here is critical.

If your current process is:

1. Jinja2 renders the template first
2. Then SQL replaces the @@ placeholders

That’s backwards. By the time Jinja tries to evaluate {{ partner_name }}, the value is still the raw string '@@PARTNER'—which your system likely doesn’t recognize as a valid value, triggering the error.

Fix: Rearrange your workflow to run SQL placeholder replacement before passing the content to Jinja2. Let SQL replace @@SHORTNAME and @@PARTNER with their actual values first, so Jinja receives a template with real, usable data.

Third: Validate SQL Placeholder Replacement Logic #

Even with the right order, double-check that your SQL replacement is working as intended:

• Are placeholder names (SHORTNAME, PARTNER) spelled identically in both the template and your SQL logic?
• Does your SQL have valid values assigned to these placeholders? A missing assignment will leave @@PARTNER untouched, leading to the same error.
• Is the replacement case-sensitive? For example, if SQL looks for @@partner but your template uses @@PARTNER, it won’t replace anything.

Quick Safety Net: Add Default Values #

To avoid this error even if variables are missing or unprocessed, use Jinja2’s default filter to fall back to a friendly placeholder:

<p>PARTNER NAME: {{ partner_name | default('No Partner Specified') }}</p>

This will render the fallback text instead of throwing an error if partner_name is undefined or empty.

内容的提问来源于stack exchange，提问作者FROSTYSMOOTH