Why doesn't my condition logic work as expected in Jinja2/CherryPy?

问题

{% if bCat2 == True %}
    <div>True</div>
{% else %}
    <div>False</div>

Returns <div>False</div> even when bCat2 is True. Thanks, Andrew

回答1:

This part of documentation can help you:

The special constants true, false and none are indeed lowercase. Because that caused confusion in the past, when writing True expands to an undefined variable that is considered false, all three of them can be written in title case too (True, False, and None). However for consistency (all Jinja identifiers are lowercase) you should use the lowercase versions.

Source: http://jinja.pocoo.org/docs/templates/

Try that code:

{% if bCat2 == true %}
<div>True</div>
{% else %}
<div>False</div>
{% endif %}

回答2:

To test a Boolean variable in a template, convert it to a string in Python

str(bCat2)

and then compared it to a string in the template

{% if bCat2 == 'True' %}
    <div>True</div>
{% else %}
    <div>False</div>

回答3:

The proper way to do this in Jinja2 is:

{% if bCat2 is sameas true %}
    <div>True</div>
{% elif bCat2 is sameas false %}
    <div>False</div>
{% endif %}

The reason why you cannot do

{% if bCat2 == true %}

is that if bCat2 == 1 or bCat2 == 1.0 it will also be considered True.

回答4:

Option 1: Most common solution

Solve that like python do.

Check if variable is true

{% if bCat2 %}
    <div>True</div>
{% else %}
    <div>False</div>

Check if variable is false

{% if not bCat2 %}
    <div>False</div>
{% else %}
    <div>True</div>

Jinja2 If documentation

Option 2: Jinja2 sameas solution

Solve like jinja2 do. Becareful with boolean lowercase.

Check if variable is true

{% if bCat2 is sameas true %}
    <div>True</div>
{% endif %}

Check if variable is false

{% if bCat2 is sameas false %}
    <div>False</div>
{% endif %}

Jinja2 sameas documentation

回答5:

I would like to add that if your logic is a bit more complex you might want to read about scopes.

As mentioned in official documentation:

As of version 2.10 more complex use cases can be handled using namespace objects which allow propagating of changes across scopes:

{% set ns = namespace(found=false) %}
{% for item in items %}
    {% if item.check_something() %}
        {% set ns.found = true %}
    {% endif %}
    * {{ item.title }}
{% endfor %}
Found item having something: {{ ns.found }}

来源：https://stackoverflow.com/questions/8433450/why-doesnt-my-condition-logic-work-as-expected-in-jinja2-cherrypy