Call a python function from jinja2

I am using jinja2, and I want to call a python function as a helper, using a similar syntax as if I were calling a macro. jinja2 seems intent on preventing me from making a function call, and insists I repeat myself by copying the function into a template as a macro.

Is there any straightforward way to do this? And, is there any way to import a whole set of python functions and have them accessible from jinja2, without going through a whole lot of rigamarole (such as writing an extension)?

For those using Flask, put this in your __init__.py:

def clever_function():
    return u'HELLO'

app.jinja_env.globals.update(clever_function=clever_function)

and in your template call it with {{ clever_function() }}

Note: This is Flask specific!

I know this post is quite old, but there are better methods of doing this in the newer versions of Flask using context processors.

Variables can easily be created:

@app.context_processor
def example():
    return dict(myexample='This is an example')

The above can be used in a Jinja2 template with Flask like so:

{{ myexample }}

(Which outputs This is an example)

As well as full fledged functions:

@app.context_processor
def utility_processor():
    def format_price(amount, currency=u'€'):
        return u'{0:.2f}{1}'.format(amount, currency)
    return dict(format_price=format_price)

The above when used like so:

{{ format_price(0.33) }}

(Which outputs the input price with the currency symbol)

Alternatively, you can use jinja filters, baked into Flask. E.g. using decorators:

@app.template_filter('reverse')
def reverse_filter(s):
    return s[::-1]

Or, without decorators, and manually registering the function:

def reverse_filter(s):
    return s[::-1]
app.jinja_env.filters['reverse'] = reverse_filter

Filters applied with the above two methods can be used like this:

{% for x in mylist | reverse %}
{% endfor %}

I think jinja deliberately makes it difficult to run 'arbitrary' python within a template. It tries to enforce the opinion that less logic in templates is a good thing.

You can manipulate the global namespace within an Environment instance to add references to your functions. It must be done before you load any templates. For example:

from jinja2 import Environment, FileSystemLoader

def clever_function(a, b):
    return u''.join([b, a])

env = Environment(loader=FileSystemLoader('/path/to/templates'))
env.globals['clever_function'] = clever_function

from jinja2 import Template

def custom_function(a):
    return a.replace('o', 'ay')

template = Template('Hey, my name is {{ custom_function(first_name) }} {{ func2(last_name) }}')
template.globals['custom_function'] = custom_function

You can also give the function in the fields as per Matroskin's answer

fields = {'first_name': 'Jo', 'last_name': 'Ko', 'func2': custom_function}
print template.render(**fields)

Will output:

Hey, my name is Jay Kay

Works with Jinja2 version 2.7.3

And if you want a decorator to ease defining functions on template.globals check out Bruno Bronosky's answer

I like @AJP's answer. I used it verbatim until I ended up with a lot of functions. Then I switched to a Python function decorator.

from jinja2 import Template

template = '''
Hi, my name is {{ custom_function1(first_name) }}
My name is {{ custom_function2(first_name) }}
My name is {{ custom_function3(first_name) }}
'''
jinga_html_template = Template(template)

def template_function(func):
    jinga_html_template.globals[func.__name__] = func
    return func

@template_function
def custom_function1(a):
    return a.replace('o', 'ay')

@template_function
def custom_function2(a):
    return a.replace('o', 'ill')

@template_function
def custom_function3(a):
    return 'Slim Shady'

fields = {'first_name': 'Jo'}
print(jinga_html_template.render(**fields))

Good thing functions have a __name__!

Never saw such simple way at official docs or at stack overflow, but i was amazed when found this:

# jinja2.__version__ == 2.8
from jinja2 import Template

def calcName(n, i):
    return ' '.join([n] * i)

template = Template("Hello {{ calcName('Gandalf', 2) }}")

template.render(calcName=calcName)
# or
template.render({'calcName': calcName})

There's a much simpler decision.

@app.route('/x')
def x():
    return render_template('test.html', foo=y)

def y(text):
    return text

Then, in test.html:

{{ foo('hi') }}

To call a python function from Jinja2, you can use custom filters which work similarly as the globals.

It's quite simple and useful. In a file myTemplate.txt, I wrote:

{{ data | pythonFct }}

And in a python script:

import jinja2

def pythonFct(data):
    return "This is my data: {0}".format(data)
    
input="my custom filter works!"
  
loader = jinja2.FileSystemLoader(path or './')
env = jinja2.Environment(loader=loader)
env.filters['pythonFct'] = pythonFct
result = env.get_template("myTemplate.txt").render(data=input)
print(result)

Use a lambda to connect the template to your main code

return render_template("clever_template", clever_function=lambda x: clever_function x)

Then you can seamlessly call the function in the template

{{clever_function(value)}}

is there any way to import a whole set of python functions and have them accessible from jinja2 ?

Yes there is, In addition to the other answers above, this works for me.

Create a class and populate it with the associated methods e.g

class Test_jinja_object:

    def __init__(self):
	    self.myvar = 'sample_var'

    def clever_function (self):
	    return 'hello' 

Then create an instance of your class in your view function and pass the resultant object to your template as a parameter for the render_template function

my_obj = Test_jinja_object()

Now in your template, you can call the class methods in jinja like so

{{ my_obj.clever_function () }}

To import all the builtin functions you can use:

app.jinja_env.globals.update(__builtins__)

Add .__dict__ after __builtins__ if this doesn't work.

Based on John32323's answer.

If you are doing it with Django, you can just pass the function with the context:

context = {
    'title':'My title',
    'str': str,
}
...
return render(request, 'index.html', context)

Now you will be able to use the str function in jinja2 template

@John32323 's answer is a very clean solution.

Here is the same one, but save into a seperate file, maybe more cleaner.

Create helper file

app\helper.py

from app import app

def clever_function_1():
    return u'HELLO'

def clever_function_2(a, b):
    return a + b



app.jinja_env.globals.update(
    clever_function_1=clever_function_1,
    clever_function_2=clever_function_2,
)

Import from app

app.py

from app import routes
from app import helper   # add this one

Use like this

app\templates\some.html

{{ clever_function_1() }}
{{ clever_function_2(a, b) }}

For those using FastApi, put this in your __init__.py:

from fastapi.templating import Jinja2Templates
templates = Jinja2Templates(directory="templates")

def clever_function():
    return u'HELLO'

templates.env.globals.update(clever_function=clever_function)

and in your template call it with {{ clever_function() }}