CODEWITHSUNDEEP
djangodjango-templates
Travis D.
Travis D.

Reputation: 342

Boolean logic in Django template system

If you set a context variable (eg. 'woot') as None or just leave it undefined....

{% if woot %} Yeah! {% endif %}

Does what you'd expect (nothing). But if you do:

{% if woot == True %} Yeah! {% endif %}

It will print "Yeah!" even though woot is None / undefined. That seems very non-intuitive. Obviously, I can work around this... but I'd like to understand the root cause. Any ideas why it happens....?

Proof:

from django.template import Context, Template

x = Template("{% if woot %}Yeah!{% endif %}")
y = Template("{% if woot == True %}Yeah!{% endif %}")

x.render( Context( {} ))  # => u''
y.render( Context( {} ))  # => u'Yeah!'

x.render( Context( {'woot':None} ))  # => u''
y.render( Context( {'woot':None} ))  # => u'Yeah!'

This is on Django 1.4.3

Upvotes: 4

Views: 1264

Answers (1)

Alasdair
Alasdair

Reputation: 309109

In Django 1.5 (release notes), the template engine interprets True, False and None as the corresponding Python objects, so {% if woot == True %} will evaluate to False.

In earlier versions of Django, neither the woot nor True variables exist in the template context. The expression None == None evaluates to True, so Yeah! is displayed

Upvotes: 5

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