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I'm writing some Jinja2 templates that I'd like to be able to reuse as painlessly as possible in tangentially related projects. I have a set of custom convenience filters that I'd like the templates to "carry around" with them. Is there a Jinja2 syntax for embedding filter definitions into a template itself? Or a mechanism for embedding any kind of pure Python function into a Jinja2 template that can act on variables passed into the template? I used to use mako, and there it was trivial to do this, but templating LaTeX in mako is painful because of the lack of custom syntax, so I had to make the switch.

dreftymac
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Daisy Sophia Hollman
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2 Answers2

38

There is NO way you can embed python directly into a Jinja2 Template, the way that I know of is to define in your application and add them to your Jinja2 environment instance. Like the following example taken from https://jinja.palletsprojects.com/en/2.11.x/api/#writing-filters.

import jinja2

loader = jinja2.FileSystemLoader('/tmp')
env = jinja2.Environment(autoescape=True, loader=loader)

def upperstring(input):
    """Custom filter"""
    return input.upper()

env.filters['upperstring'] = upperstring
temp = env.get_template('template.html')
temp.render(name="testing")

Here the Template I am using

{{ name | upperstring }}

Result is this

TESTING
flazzarini
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  • Any idea on how to remove all the old filters? – Tanmay Baranwal Feb 27 '19 at 08:39
  • If you mean removing all existing _default_ filters you could simple replace `env.filters` with an empty dictionary by doing `env.filters = {}`. Not sure however why would want to do that. – flazzarini Feb 27 '19 at 20:09
  • Sorry, I meant all the old "custom" filters. But I found that Jinja environment sets up the basic filters explicitly, so can't assign to empty dictionary. – Tanmay Baranwal Mar 01 '19 at 09:39
  • @flazzarini How do you call upperstring and feed in the variable "input"? If I wanted a custom filter for boolean, how do I return the variable true/false that I pass in? – superloopnetworks Nov 12 '20 at 11:17
  • Not sure if I understand your question correctly, but If you would like to define a filter that would return a boolean type you could write something [here](https://repl.it/@FrankLazzarini/Custom-Jinja2-Filters) to transform any given strings into boolean values using a jinja2 filter. – flazzarini Nov 12 '20 at 13:55
18

There is an easy way to add custom filters in jinja2 template. FILTERS is the dictionary containing all filters that we can use to render the template. However, we can add more filters to it.

Here is a quick example to add new filters to it.


from jinja2 import Template
from jinja2.filters import FILTERS, environmentfilter


// Remove the decorator in the next line if using version 3.1.0 or higher, read below
@environmentfilter
def do_reverse_by_word(environment, value, attribute=None):
    """
    custom max calculation logic
    """
    if attribute:
        return [list(reversed(i.get(attribute).split())) for i in value]

    return list(reversed(value.split()))


FILTERS["reverse_by_word"] = do_reverse_by_word
print(Template("{{ name | reverse_by_word }}").render({"name": "Stack Overflow"}))
print(Template("{{ names | reverse_by_word(attribute='name') }}").render({"names": [{"name": "Stack Overflow"}, {"name": "Stack Exchange"}]}))

Outputs

['Overflow', 'Stack']
[['Overflow', 'Stack'], ['Exchange', 'Stack']]

please comment below in case you've more complex use cases. I'll be happy to answer all your queries.

Update (tested with version 3.1.2): The decorator environmentfilter is no longer needed (and does not exist anymore). Source

Marc Sances
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vikas soni
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