Template syntax error. Could not parse the remainder (Django)

 Can someone point me to the right direction? Below is what I have been doing

    
{{article.snippet}}

    
{{article.date}}

    
{{article.retrieve_by_category("web")}}

I have no problems getting the date or the snippet but I keep getting the following error when trying to run article.retireve_by_category.

Could not parse the remainder: '("web")' from 'article.retrieve_by_category("web")'

Below is the code for the model:

class Article(models.Model):
    category = models.CharField(max_length=100)
    title = models.CharField(max_length=100)
    slug = models.SlugField()
    body = models.TextField()
    date = models.DateTimeField(auto_now_add=True)
    #add in thumbnail
    def __str__(self):
        return self.title
    def snippet(self):
        return self.body[:500] + "..."
    def retrieve_by_category(self,category):
        """retrieves blogs based on their category"""
        if (str(self.category) == category):
            return self.category
        return ''