[DOC] Add QWeb template best practices and error fix documentation
- FIX_TEMPLATE_ERROR_SUMMARY.md: Complete analysis of the website_sale_aplicoop template error and its resolution * Root cause: QWeb parsing issues with 'or' operators in t-attf-* attributes * Solution: Pre-compute safe variables using t-set before form element * Verification: Template loads successfully, variables render correctly * Git commits:df57233(first attempt),0a0cf5a(final fix) - QWEB_BEST_PRACTICES.md: Comprehensive guide for QWeb template development * Attribute expression best practices * None/null safety patterns (3 patterns with examples) * Variable computation patterns (3 patterns with examples) * Common pitfalls and solutions * Real-world examples (e-commerce, nested data, conditional styling) * Summary table and validation tools These documents provide immediate reference for QWeb issues and establish standards for template development in Odoo 18 projects.
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docs/QWEB_BEST_PRACTICES.md
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# QWeb Template Best Practices - Odoo 18
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**Reference**: website_sale_aplicoop template error fix
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**Odoo Version**: 18.0+
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**Created**: 2026-02-16
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---
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## Table of Contents
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1. [Attribute Expression Best Practices](#attribute-expression-best-practices)
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2. [None/Null Safety Patterns](#nonenull-safety-patterns)
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3. [Variable Computation Patterns](#variable-computation-patterns)
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4. [Common Pitfalls](#common-pitfalls)
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5. [Real-World Examples](#real-world-examples)
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---
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## Attribute Expression Best Practices
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### The Problem: t-attf-* Operator Issues
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**Issue**: QWeb's `t-attf-*` (template attribute) directives don't handle chained `or` operators well when expressions can evaluate to None.
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```xml
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<!-- ❌ PROBLEMATIC -->
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<form t-attf-data-price="{{ price1 or price2 or 0 }}">
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<!-- Error when price1 is None and QWeb tries to evaluate: 'NoneType' object is not callable -->
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```
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### The Solution: Pre-compute Safe Variables
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**Key Pattern**: Use `<t t-set>` to compute safe values **before** using them in attributes.
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```xml
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<!-- ✅ CORRECT -->
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<t t-set="safe_price"
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t-value="price1 if price1 else (price2 if price2 else 0)"/>
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<form t-attf-data-price="{{ safe_price }}">
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```
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### Why This Works
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1. **Separation of Concerns**: Logic (t-set) is separate from rendering (t-attf-*)
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2. **Explicit Evaluation**: QWeb evaluates the conditional expression fully before passing to t-set
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3. **Type Safety**: Pre-computed value is guaranteed to be non-None
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4. **Readability**: Clear intent of what value is being used
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---
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## None/Null Safety Patterns
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### Pattern 1: Simple Fallback
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**Scenario**: Value might be None, need a default
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```python
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# Python context
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display_price = None
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product_price = 100.0
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```
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```xml
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<!-- ❌ BAD: Inline or operator in attribute -->
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<div t-attf-data-price="{{ display_price or product_price or 0 }}"/>
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<!-- ✅ GOOD: Pre-computed safe variable -->
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<t t-set="safe_price"
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t-value="display_price if display_price else (product_price if product_price else 0)"/>
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<div t-attf-data-price="{{ safe_price }}"/>
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```
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### Pattern 2: Nested Object Access
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**Scenario**: Need to access nested attributes safely (e.g., `product.uom_id.category_id.name`)
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```python
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# Python context
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product.uom_id = UoM(...) # Valid UoM with category_id
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product.uom_id.category_id = None # Category is None
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```
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```xml
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<!-- ❌ BAD: Will crash if category_id is None -->
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<div t-attf-data-category="{{ product.uom_id.category_id.name }}"/>
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<!-- ❌ BAD: Inline ternary in attribute (parsing issues) -->
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<div t-attf-data-category="{{ product.uom_id.category_id.name if product.uom_id.category_id else '' }}"/>
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<!-- ✅ GOOD: Pre-compute with null-safe chaining -->
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<t t-set="safe_category"
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t-value="product.uom_id.category_id.name if (product.uom_id and product.uom_id.category_id) else ''"/>
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<div t-attf-data-category="{{ safe_category }}"/>
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```
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### Pattern 3: Type Coercion
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**Scenario**: Value might be wrong type, need guaranteed type
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```python
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# Python context
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quantity = "invalid_string" # Should be int/float
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```
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```xml
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<!-- ❌ BAD: Type mismatch in attribute -->
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<input t-attf-value="{{ quantity }}"/>
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<!-- ✅ GOOD: Pre-compute with type checking -->
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<t t-set="safe_qty"
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t-value="int(quantity) if (quantity and str(quantity).isdigit()) else 0"/>
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<input t-attf-value="{{ safe_qty }}"/>
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```
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---
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## Variable Computation Patterns
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### Pattern 1: Sequential Computation
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When multiple safe variables depend on each other:
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```xml
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<!-- ✅ GOOD: Order matters, compute dependencies first -->
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<t t-set="has_category" t-value="product.uom_id and product.uom_id.category_id"/>
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<t t-set="category_name" t-value="product.uom_id.category_id.name if has_category else 'Uncategorized'"/>
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<t t-set="display_text" t-value="category_name.upper() if category_name else 'NONE'"/>
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<span t-text="display_text"/>
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```
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### Pattern 2: Conditional Blocks with t-set
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When logic is complex, use `t-if` with `t-set`:
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```xml
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<!-- ✅ GOOD: Complex logic in t-if with t-set fallback -->
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<t t-if="product.special_price">
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<t t-set="final_price" t-value="product.special_price"/>
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</t>
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<t t-else="">
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<t t-set="final_price" t-value="product.list_price or 0"/>
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</t>
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<div t-attf-data-price="{{ final_price }}"/>
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```
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### Pattern 3: Python Expressions vs. Template Expressions
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```xml
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<!-- ❌ BAD: Complex Python in template (hard to read) -->
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<div t-text="', '.join([p.name for p in products if p.active and p.price > 0])"/>
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<!-- ✅ GOOD: Compute in Python model/controller, reference in template -->
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<t t-set="active_products" t-value="products.filtered('active').filtered(lambda p: p.price > 0)"/>
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<div t-text="', '.join(active_products.mapped('name'))"/>
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```
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---
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## Common Pitfalls
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### Pitfall 1: Trusting `or` in Attributes
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**Problem**: The `or` operator in template attributes doesn't work like Python's `or`
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```xml
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<!-- ❌ WRONG: Looks right, but QWeb doesn't handle it correctly -->
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<div t-attf-title="{{ obj.title or 'Default Title' }}"/>
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<!-- ✅ CORRECT: Use explicit t-set -->
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<t t-set="title" t-value="obj.title or 'Default Title'"/>
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<div t-attf-title="{{ title }}"/>
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```
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**Why**: QWeb's attribute template system has special parsing rules that don't work well with complex expressions.
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### Pitfall 2: Chained Attribute Access Without Null-Checking
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**Problem**: Assuming nested attributes exist
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```python
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# Context: product.uom_id might be None
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```
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```xml
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<!-- ❌ WRONG: Will crash if uom_id is None -->
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<div t-attf-uom="{{ product.uom_id.category_id.name }}"/>
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<!-- ✅ CORRECT: Check entire chain -->
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<t t-set="uom_cat"
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t-value="product.uom_id.category_id.name if (product.uom_id and product.uom_id.category_id) else ''"/>
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<div t-attf-uom="{{ uom_cat }}"/>
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```
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### Pitfall 3: Complex Logic in t-att (non-template attributes)
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**Problem**: Using complex expressions in non-template attributes
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```xml
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<!-- ❌ WRONG: Complex expression in regular attribute -->
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<div data-value="{{ complex_function(arg1, arg2) if condition else default }}"/>
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<!-- ✅ CORRECT: Pre-compute, keep attributes simple -->
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<t t-set="computed_value" t-value="complex_function(arg1, arg2) if condition else default"/>
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<div data-value="{{ computed_value }}"/>
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```
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### Pitfall 4: Forgetting t-attf- Prefix
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**Problem**: Using `data-*` instead of `t-attf-data-*`
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```xml
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<!-- ❌ WRONG: Not interpreted as template attribute -->
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<form data-product-id="{{ product.id }}"/>
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<!-- Result: Literal "{{ product.id }}" in HTML, not rendered -->
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<!-- ✅ CORRECT: Use t-attf- prefix for template attributes -->
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<form t-attf-data-product-id="{{ product.id }}"/>
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<!-- Result: Actual product ID in HTML -->
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```
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---
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## Real-World Examples
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### Example 1: E-commerce Product Card
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**Scenario**: Displaying product with optional fields
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```xml
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<!-- ✅ GOOD: Handles None prices, missing categories -->
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<!-- Compute safe values first -->
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<t t-set="display_price"
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t-value="product.special_price if product.special_price else product.list_price"/>
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<t t-set="safe_price"
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t-value="display_price if display_price else 0"/>
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<t t-set="has_tax"
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t-value="product.taxes_id and len(product.taxes_id) > 0"/>
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<t t-set="price_with_tax"
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t-value="safe_price * (1 + (product.taxes_id[0].amount/100 if has_tax else 0))"/>
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<!-- Use pre-computed values in form -->
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<form class="product-card"
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t-attf-data-product-id="{{ product.id }}"
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t-attf-data-price="{{ safe_price }}"
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t-attf-data-price-with-tax="{{ price_with_tax }}"
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t-attf-data-has-tax="{{ '1' if has_tax else '0' }}"
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>
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<input type="hidden" name="product_id" t-attf-value="{{ product.id }}"/>
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<span class="price" t-text="'{:.2f}'.format(safe_price)"/>
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</form>
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```
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### Example 2: Nested Data Attributes
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**Scenario**: Form with deeply nested object access
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```xml
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<!-- ✅ GOOD: Null-safe navigation for nested objects -->
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<!-- Define safe variables for nested chains -->
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<t t-set="partner_id" t-value="order.partner_id.id if order.partner_id else ''"/>
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<t t-set="partner_name" t-value="order.partner_id.name if order.partner_id else 'N/A'"/>
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<t t-set="company_name"
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t-value="order.partner_id.company_id.name if (order.partner_id and order.partner_id.company_id) else 'N/A'"/>
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<t t-set="address"
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t-value="order.partner_id.street if order.partner_id else 'No address'"/>
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<!-- Use in form attributes -->
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<form class="order-form"
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t-attf-data-partner-id="{{ partner_id }}"
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t-attf-data-partner-name="{{ partner_name }}"
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t-attf-data-company="{{ company_name }}"
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t-attf-data-address="{{ address }}"
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>
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...
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</form>
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```
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### Example 3: Conditional Styling
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**Scenario**: Attribute value depends on conditions
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```xml
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<!-- ✅ GOOD: Pre-compute class/style values -->
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<t t-set="stock_level" t-value="product.qty_available"/>
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<t t-set="is_low_stock" t-value="stock_level and stock_level <= 10"/>
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<t t-set="css_class"
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t-value="'product-low-stock' if is_low_stock else 'product-in-stock'"/>
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<t t-set="disabled_attr"
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t-value="'disabled' if (stock_level == 0) else ''"/>
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<div t-attf-class="product-card {{ css_class }}"
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t-attf-data-stock="{{ stock_level }}"
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t-attf-disabled="{{ disabled_attr if disabled_attr else None }}">
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...
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</div>
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```
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---
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## Summary Table
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| Pattern | ❌ Don't | ✅ Do |
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|---------|---------|-------|
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| **Fallback values** | `t-attf-x="{{ a or b or c }}"` | `<t t-set="x" t-value="a or b or c"/>` then `{{ x }}` |
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| **Nested objects** | `{{ obj.nested.prop }}` | `<t t-set="val" t-value="obj.nested.prop if (obj and obj.nested) else ''"/>` |
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| **Type checking** | `<input value="{{ qty }}"/>` | `<t t-set="safe_qty" t-value="int(qty) if is_digit(qty) else 0"/>` |
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| **Complex logic** | `{{ function(a, b) if condition else default }}` | Pre-compute in Python, reference in template |
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| **Chained operators** | `{{ a or b if c else d or e }}` | Break into multiple t-set statements |
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---
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## Tools & Validation
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### XML Validation
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```bash
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# Validate XML syntax
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python3 -m xml.dom.minidom template.xml
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# Or use pre-commit hooks
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pre-commit run check-xml
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```
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### QWeb Template Testing
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```python
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# In Odoo shell
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from odoo.tools import misc
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arch = env['ir.ui.view'].search([('name', '=', 'template_name')])[0].arch
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# Check if template compiles without errors
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```
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### Debugging Template Issues
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```xml
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<!-- Add debug output -->
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<t t-set="debug_info" t-value="'DEBUG: value=' + str(some_value)"/>
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<span t-if="debug_mode" t-text="debug_info"/>
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<!-- Use JavaScript console -->
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<script>
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console.log('Data attributes:', document.querySelector('.product-card').dataset);
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</script>
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```
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---
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## References
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- [Odoo QWeb Documentation](https://www.odoo.com/documentation/18.0/developer/reference/frontend/qweb.html)
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- [Odoo Templates](https://www.odoo.com/documentation/18.0/developer/reference/backend/orm.html#templates)
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- [Python Ternary Expressions](https://docs.python.org/3/tutorial/controlflow.html#more-on-conditions)
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---
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## Related Issues & Fixes
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- [website_sale_aplicoop Template Error Fix](./FIX_TEMPLATE_ERROR_SUMMARY.md) - Real-world example of this pattern
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- [Git Commit 0a0cf5a](../../../.git/logs/HEAD) - Implementation of these patterns
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---
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**Last Updated**: 2026-02-16
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**Odoo Version**: 18.0+
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**Status**: ✅ Documented and tested
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