Flask cannot import enumerate? UndefinedError: 'enumerate' is undefined

Question

I just write this code in a HTML page.

{% for i, val in enumerate(['a', 'b', 'c']) %}
    <td>
        {{ val }}
    </td>
{% endfor %}

UndefinedError: 'enumerate' is undefined

So, Flask do not support the enumerate?

Answer 1

As Or Duan says, Jinja2 has its own language. Looks like Python but it's not Python. So the Python enumerate built-in function is not part of Jinja2 template engine. There are, however, some alternatives you can use:

If you want to enumerate the items in a list you can use the loop.index0 loop special variable:

>>> from jinja2 import Template  >>> t1 = """ ... {% for val in ['a', 'b', 'c'] %} ...     <td> ...         {{ loop.index0 }} {{ val }} ...     </td> ... {% endfor %} ... """ >>> Template(t1).render() 

Another option is to precalculate the enumerated version of the list:

>>> t2 = """ ... {% for i, val in l %} ...     <td> ...         {{ i }} {{ val }} ...     </td> ... {% endfor %} ... """ >>> Template(t2).render(l=enumerate(['a', 'b', 'c'])) 

And also another one, could be even passing enumerate as a variable too:

>>> t3 = """ ... {% for i, val in enumerate(['a', 'b', 'c']) %} ...     <td> ...         {{ i }} {{ val }} ...     </td> ... {% endfor %} ... """ >>> Template(t3).render(enumerate=enumerate) 

Flask allows injecting variables automatically into the context of a template by means of Context Processors. So if you want enumerate built-in function to be available for all your templates, this could be a nice solution:

@app.context_processor def inject_enumerate():     return dict(enumerate=enumerate) 

Thanks to Sean Vieira for this suggestion.

Answer 2

Flask use Jinja2 to render your template, Jinja2 have a similar python syntax but it's not python.

What you can do? In your python code:

my_dict = enumerate(some_list)

Then when rendering the template send the dict to it:

render_template('page.html', my_dict=my_dict)