Question

When I was trying to access a tuple inside a list in the Django template format, I found out I couldn’t access it with a[ 0 ][ 1 ], instead I had to use a.0.1.

Suppose that a is something like

a = [
    ( 'a', 'apple' ),
    ( 'b', 'bee'   ),
]

Why doesn’t Django template language support a[ 0 ][ 1 ]? In normal Python programming, a.0.1 would give you a syntax error.

Answer 1

The Django docs on the template API explain this nicely:

Dots have a special meaning in template rendering. A dot in a variable name signifies a lookup. Specifically, when the template system encounters a dot in a variable name, it tries the following lookups, in this order:

• Dictionary lookup. Example: foo["bar"]
• Attribute lookup. Example: foo.bar
• List-index lookup. Example: foo[bar]

The template system uses the first lookup type that works. It’s short-circuit logic. Here are a few examples:

>>> from django.template import Context, Template
>>> t = Template("My name is {{ person.first_name }}.")
>>> d = {"person": {"first_name": "Joe", "last_name": "Johnson"}}
>>> t.render(Context(d))
"My name is Joe."

>>> class PersonClass: pass
>>> p = PersonClass()
>>> p.first_name = "Ron"
>>> p.last_name = "Nasty"
>>> t.render(Context({"person": p}))
"My name is Ron."

>>> t = Template("The first stooge in the list is {{ stooges.0 }}.")
>>> c = Context({"stooges": ["Larry", "Curly", "Moe"]})
>>> t.render(c)
"The first stooge in the list is Larry."

Variable._resolve_lookup in django.templates.base appears to be the function responsible for this, and hasn’t changed much since the oldest revision I can find