Python Django Templates and testing if a variable is null or empty string

Question

I am pretty new to django, but have many years experience coding in the java world, so I feel ridiculous asking this question - I am sure the answer is obvious and I am just missing it. I can't seem to find the right way to query this in google or something... I have searched through the django docs and it either isn't there or I am just not seeing it. All I want to do is in a template test if the var is not null OR an empty string OR just a bunch of spaces. I have an issue where spaces are getting introduced into my field - another issue I have to, and will, work out... but, I want my logic to work regardless. Right now, because my string contains just spaces simply doing this: {% if lesson.assignment %} always passes even though I don't want it to. I have looked for a trim type functionality that would work between {% %}, but I can't seem to find anything. I have tried strip, but it doesn't work between {% %}. Can someone please point me in the direction of the answer... some docs I might have missed... something?

Thanks a ton in advance!

Shawn Chin · Accepted Answer · 2011-07-09 20:43:21Z

64

{% if lesson.assignment and lesson.assignment.strip %}

The .strip calls str.strip() so you can handle whitespace-only strings as empty, while the preceding check makes sure we weed out None first (which would not have the .strip() method)

Proof that it works (in ./manage.py shell):

>>> import django
>>> from django.template import Template, Context
>>> t = Template("{% if x and x.strip %}OK{% else %}Empty{% endif %}")
>>> t.render(Context({"x": "ola"}))
u'OK'
>>> t.render(Context({"x": "   "}))
u'Empty'
>>> t.render(Context({"x": ""}))
u'Empty'
>>> t.render(Context({"x": None}))
u'Empty'

edited Jul 9, 2011 at 20:43

answered Jul 9, 2011 at 20:38

Shawn Chin

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1 Comment

Mike DeSimone Over a year ago

@PaulP1975: You might have missed it because using and in an if is a recent addition to Django.

Mike DeSimone · Accepted Answer · 2011-07-09 20:21:13Z

6

If lesson.assignment is a model field, you could define a helper function in your model and use it:

class Lesson(models.Model):
    assignment = models.CharField(..., null=True, ...)
    # ...

    @property
    def has_assignment(self):
        return self.assignment is not None and self.assignment.strip() != ""

Then use {% if lesson.has_assignment %} in your template.

answered Jul 9, 2011 at 20:21

Mike DeSimone

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PaulP1975 Over a year ago

This is an interesting option... I very much like how it cleans up the logic in the template. I am very new to python and django, so I am still learning the ropes. Thank you for this suggestion!

Peter H · Accepted Answer · 2016-05-18 19:57:06Z

6

This may not have been available when the question was posted but one option could be using the built in template filter default_if_none (Django Documentation Link).

For example: {{ lesson.assignment|default_if_none:"Empty" }}

answered May 18, 2016 at 19:57

Peter H

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Comments

Ignacio Vazquez-Abrams · Accepted Answer · 2011-07-09 20:13:43Z

1

You can simulate it by using the cut filter to remove all spaces. But you should probably find out why it contains all spaces in the first place.

answered Jul 9, 2011 at 20:13

Ignacio Vazquez-Abrams

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PaulP1975 Over a year ago

Yes, I intend to fix that fact that it has all the spaces. However, I wanted it to work either way and couldn't get the strip function to work (Shawn's code above worked).

Daniel Roseman · Accepted Answer · 2011-07-09 20:20:32Z

1

You can call any built-in methods anywhere in a Django template variable. For example, you can call the Python string method strip. So this will work:

{% if lesson.assignment.strip %}

answered Jul 9, 2011 at 20:20

Daniel Roseman

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4 Comments

Mike DeSimone Over a year ago

Problem: his CharField allows NULLs, so a NULL field will cause an exception since None doesn't have a strip method.

PaulP1975 Over a year ago

I do not know why, but the first time I tried to do this it failed... I might have hit the exceptional condition (null)... I can't remember.

bershika Over a year ago

Try {% if lesson.assignment and lesson.assignment.strip %}

Dennis Over a year ago

For a foreign key you can use {% if lesson.foreign_key_field is Null %}

Arham Aalam Ansari · Accepted Answer · 2021-08-24 10:14:02Z

1

Here is a simple way to do this, from Django version 3.2

{{ lesson.assignment|default:"nothing" }}

This works for both cases (empty string and None object).

ref link: https://docs.djangoproject.com/en/3.2/ref/templates/builtins/#std:templatefilter-default

answered Aug 24, 2021 at 10:14

Arham Aalam Ansari

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