Use Django's standard field rendering. Create filter for custom attrs.

core/templatetags/bootstrap_field.py

@@ -1,10 +1,10 @@
-from django.template import loader, Context, Library
-
-
+from django.template import Library
+ 
 register = Library()
-
-
-@register.simple_tag
-def bootstrap_field(field):
-    template = loader.get_template('core/templatetags/bootstrap_field.html')
-    return template.render(Context({'field': field}))
+ 
+@register.filter(name='bootstrap_field')
+def bootstrap_field(field, class_attr):
+    return field.as_widget(attrs={
+        'placeholder': field.label,
+        'class': class_attr
+    })

pinry/templates/includes/field.html

@@ -1,5 +1,7 @@
+{% load bootstrap_field %}
+
 <div class="form-group{% if field.errors %} has-error{% endif %}">
-    <input type="{% if field.html_name == "password" %}password{% else %}text{% endif %}" id="id_{{ field.html_name }}" name="{{ field.html_name }}" class="form-control input-lg" placeholder="{{ field.label }}"/>
+    {{ field|bootstrap_field:"form-control input-lg" }}
     {% if field.errors %}
         <span class="help-block">
             {% if field.errors %}