fallback to default HTML template if custom template does not exist (#604)

* fallback to default HTML template if custom template does not exist

* add docs

docs/source/html-templating.rst

@@ -18,6 +18,12 @@ The default pygeoapi configuration has ``server.templates`` commented out and de
 
 Your templates folder should mimic the same file names and structure of the default pygeoapi templates. Otherwise, you will need to modify ``api.py`` accordingly.
 
+Note that you need only copy and edit the templates you are interested in updating.  For example,
+if you are only interested in updating the ``landing_page.html`` template, then create your own version
+of the only that same file.  When pygeoapi detects that a custom HTML template is being used,
+it will look for the custom template in ``server.templates.path``.  If it does not exists, pygeoapi
+will render the default HTML template for the given endpoint/requuest.
+
 Linking to a static file in your HTML templates can be done using Jinja syntax and the exposed ``config['server']['url']``:
 
 .. code-block:: html

pygeoapi/util.py

@@ -44,6 +44,7 @@ from urllib.parse import urlparse
 
 import dateutil.parser
 from jinja2 import Environment, FileSystemLoader
+from jinja2.exceptions import TemplateNotFound
 import yaml
 
 from pygeoapi import __version__
@@ -265,9 +266,11 @@ def render_j2_template(config, template, data):
     :returns: string of rendered template
     """
 
+    custom_templates = False
     try:
         templates_path = config['server']['templates']['path']
         env = Environment(loader=FileSystemLoader(templates_path))
+        custom_templates = True
         LOGGER.debug('using custom templates: {}'.format(templates_path))
     except (KeyError, TypeError):
         env = Environment(loader=FileSystemLoader(TEMPLATES))
@@ -287,7 +290,17 @@ def render_j2_template(config, template, data):
     env.filters['filter_dict_by_key_value'] = filter_dict_by_key_value
     env.globals.update(filter_dict_by_key_value=filter_dict_by_key_value)
 
-    template = env.get_template(template)
+    try:
+        template = env.get_template(template)
+    except TemplateNotFound as err:
+        if custom_templates:
+            LOGGER.debug(err)
+            LOGGER.debug('Custom template not found; using default')
+            env = Environment(loader=FileSystemLoader(TEMPLATES))
+            template = env.get_template(template)
+        else:
+            raise
+
     return template.render(config=config, data=data, version=__version__)