How list indexing works in Jinja

Issue

I have a simple flask application where I need to show 2 images in a html page, all the images are stored in a static folder

While rendering the html page I am passing image file names as a list and on html page I am using jinja logic to show the images

img_lst = [filename1, filename2]
return render_template("index.html", img_lst=img_lst)

I can easily loop over the list and show images using below logic

{% for image in img_lst %}
    <img src="{{url_for('static', filename=image)}}">
{% endfor %}

but if I index a particulr file from a list I am getting an error

jinja2.exceptions.UndefinedError: 'img_lst' is undefined

<img src=" {{url_for('static', filename=img_lst[0])}}"> 

I want to individually access filenames from list instead of looping over

Solution

i don’t see any problem with your code above, but i’m guessing you need to force flask to reload templates, so jinja2 will take in consideration your variable img_lst the next reload.

add this config below and let me know if it works:

app.config["TEMPLATES_AUTO_RELOAD"] = True

Update

Your code works and i didn’t get that error, so my guess probably you are working on other view/template and you forgot to pass img_lst in the context in render function.

Answered By – cizario

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