pallets
explain associativity and precedence between operators
The doc of jinja lacks of descriptions about the followings, if I'm not mistaken.
- associative properties of operators
- precedence between operators
Therefore, there are some cases users may not be able to expect how expressions will be evaluated beforehand, like the following examples, IMHO.
- 1 + 2 * 3
- 1 + "2" | int
- xs | length > 0 (where xs is a list, e.g. [1, 2])
- xs | length > 0 is true
To resolve this, I would like to suggest to enhance the doc (the section "Template Designer Documentation" should be suitable for this purpose, I think), to add some notes or a section to explain about associativity of, and precedence between operators like the followings.
- Operators are left associative basically, that is, grouped from left to right. For example,
- {{ 1 + 2 - 3 }} will be evaluated as {{ (1 + 2) - 3 }}
- {{ 3 ** 2 ** 3 }} will be evaluated as {{ (3 ** 2) ** 3 }} unlike python evaluates it as {{ 3 ** (2 ** 3) }}
- {{ xs | list | length }} will be evaluated as {{ (xs | list) | length }}
- Operators have precedence if similar named functions exist in python. For example,
- {{ 1 + 2 * 3 }} will be evaluated as {{ 1 + (2 * 3) }} and return 7
- Some operators may have higher priority than other operators. For example,
- {{ 1 + "2" | int }} will be evaluated as {{ 1 + ("2" | int) }} and return 3
- {{ [3] + [2, 1] | sort }} will be evaluated to {{ [3] + ([2, 1] | sort) }} and return [3, 1, 2]
- Precedence between operators are ... (explanation) ...
BTW, this is related to #379 and may close it.
Excuse me. I made a mistake that filters are always applied with '|' operator, AFAIK. So that it may be enough to add notes about associativity and precedence between operators.
To avoid unnecessary confusion, I update the title and description of this issue.