Add .save() method to Template class

[Huggies23]

Motivation
A standalone html object with embedded output from widget states can be created using the .save(embed = True) method for a panel object.

When creating app which uses a template (Template class) to structure the app, this functionality is not available and the app can only be served on a live instance.

What I would like
An equivalent method of .save() for Template apps which would allow the user to customise the format of the app using a jinja template but also export a standalone html version of the app with fully embedded widget state outputs.

For example, in using the standard panel app creation I can do:

import panel as pn
pn.extension()

select = pn.widgets.Select(value = 'Option 1', options = ['Option 1', 'Option 2', 'Option 3'])

@pn.depends(select.param.value)
def print_selection(selected):
    return f'<h2>You selected: {selected}'

dashboard = pn.Row(select, print_selection)

dashboard.save(filename = 'Test_embed.html', embed = True)

And what I would like to be able to do the same using the Template class:

import panel as pn
pn.extension()

select = pn.widgets.Select(value = 'Option 1', options = ['Option 1', 'Option 2', 'Option 3'])

@pn.depends(select.param.value)
def print_selection(selected):
    return f'<h2>You selected: {selected}'

template = """
{% extends base %}

<!-- goes in body -->
{% block postamble %}
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css">
{% endblock %}

<!-- goes in body -->
{% block contents %}
<p>This is a Panel app with a custom template allowing us to compose multiple Panel objects into a single HTML document.</p>
<br>
<div class="container">
  <div class="row">
    <div class="col-sm">
      {{ embed(roots.widget) }}
    </div>
    <div class="col-sm">
      {{ embed(roots.output) }}
    </div>
  </div>
</div>
{% endblock %}
"""

dashboard = pn.Template(template)

dashboard.add_panel('widget', select)
dashboard.add_panel('output', print_selection)
 
# Want this:
dashboard.save(filename = 'template_embed.html', embed = True)

[Huggies23]

@philippjfr Has this enhancement been implemented through panel.save() using the template and template_variables arguments?

def save(panel, filename, title=None, resources=None, template=None,
         template_variables=None, embed=False, max_states=1000,
         max_opts=3, embed_json=False, json_prefix='', save_path='./',
         load_path=None, progress=True)

If so, would it be possible to provide a brief example of its usage? Thanks!

[philippjfr]

It hasn't afaik.

[Huggies23]

Ah ok, thanks for your swift reply. What are these new args for?

[MarcSkovMadsen]

I would also like to be able to .save for example apps based on the FastListTemplate and FastGridTemplate.

So does a user here https://discourse.holoviz.org/t/save-panel-template-app-as-a-html/2453

[bryanwhiting]

+1 - i'd like to save a template class!

[bryanwhiting]

is this closed per #1224?

[itsgifnotjiff]

This is still not the case for something like

template = pn.template.MaterialTemplate(
    site="Exp",
    title="Save Experiment",
    main=[pn.panel(ds.HR.sel(lon=slice(-179,179)).hvplot.quadmesh(rasterize=True, geo=True, coastline=True))],
)
template.save("SaveExp.html")

even if you add ressources=INLINE or embed=True you still get at best

---------------------------------------------------------------------------
UndefinedError                            Traceback (most recent call last)
Cell In[49], line 2
      1 # from bokeh.resources import INLINE
----> 2 template.save("SaveExp.html")

File PATH/viz/lib/python3.10/site-packages/panel/template/base.py:473, in BaseTemplate.save(self, filename, title, resources, embed, max_states, max_opts, embed_json, json_prefix, save_path, load_path)
    470 if embed:
    471     raise ValueError("Embedding is not yet supported on Template.")
--> 473 return save(
    474     self, filename, title, resources, self.template,
    475     self._render_variables, embed, max_states, max_opts,
    476     embed_json, json_prefix, save_path, load_path
    477 )

File PATH/viz/lib/python3.10/site-packages/panel/io/save.py:290, in save(panel, filename, title, resources, template, template_variables, embed, max_states, max_opts, embed_json, json_prefix, save_path, load_path, progress, embed_states, as_png, **kwargs)
    288 # Set resource mode
    289 with set_resource_mode(resources):
--> 290     html = file_html(doc, resources, title, **kwargs)
    291 if hasattr(filename, 'write'):
    292     if isinstance(filename, io.BytesIO):

File PATH/viz/lib/python3.10/site-packages/panel/io/save.py:164, in file_html(models, resources, title, template, template_variables, theme, _always_new)
    162 title = _title_from_models(models_seq, title)
    163 bundle = bundle_resources(models_seq, resources)
--> 164 return html_page_for_render_items(
    165     bundle, docs_json, render_items, title=title, template=template,
    166     template_variables=template_variables
    167 )

File PATH/viz/lib/python3.10/site-packages/bokeh/embed/elements.py:150, in html_page_for_render_items(bundle, docs_json, render_items, title, template, template_variables)
    147 elif isinstance(template, str):
    148     template = get_env().from_string("{% extends base %}\n" + template)
--> 150 html = template.render(context)
    151 return html

File PATH/viz/lib/python3.10/site-packages/jinja2/environment.py:1301, in Template.render(self, *args, **kwargs)
   1299     return self.environment.concat(self.root_render_func(ctx))  # type: ignore
   1300 except Exception:
-> 1301     self.environment.handle_exception()

File PATH/viz/lib/python3.10/site-packages/jinja2/environment.py:936, in Environment.handle_exception(self, source)
    931 """Exception handling helper.  This is used internally to either raise
    932 rewritten exceptions or return a rendered traceback for the template.
    933 """
    934 from .debug import rewrite_traceback_stack
--> 936 raise rewrite_traceback_stack(source=source)

File PATH/viz/lib/python3.10/site-packages/panel/template/material/material.html:1, in top-level template code()
----> 1 {% extends "base/base.html" %}
      2 
      3 {% block custom_css %}

File PATH/viz/lib/python3.10/site-packages/panel/template/base/base.html:1, in top-level template code()
----> 1 {% extends "base.html" %}
      2 
      3 <!-- goes in body -->

File PATH/viz/lib/python3.10/site-packages/panel/_templates/base.html:20, in top-level template code()
     18 <!DOCTYPE html>
     19 <html lang="en" {{ html_attrs | default("", true) }}>
---> 20   {% block head %}
     21   <head>
     22   {% block inner_head %}

File PATH/viz/lib/python3.10/site-packages/panel/_templates/base.html:22, in block 'head'()
     20 {% block head %}
     21 <head>
---> 22 {% block inner_head %}
     23   <meta charset="utf-8">
     24   <title>{% block title %}{{ title | e if title else "Panel App" }}{% endblock %}</title>

File PATH/viz/lib/python3.10/site-packages/panel/_templates/base.html:47, in block 'inner_head'()
     45 {%-  endblock js_resources %}
     46 {%  endblock resources %}
---> 47 {%  block postamble %}{% endblock %}
     48 {% endblock inner_head %}
     49 </head>

File PATH/viz/lib/python3.10/site-packages/panel/template/base/base.html:41, in block 'postamble'()
     39 {% block postamble %}
     40     <!-- Template CSS -->
---> 41     {% for css in template_resources['css'].values() %}
     42     <link rel="stylesheet" href="{{ css }}">
     43     {% endfor %}

File PATH/viz/lib/python3.10/site-packages/jinja2/environment.py:466, in Environment.getitem(self, obj, argument)
    464 """Get an item or attribute of an object but prefer the item."""
    465 try:
--> 466     return obj[argument]
    467 except (AttributeError, TypeError, LookupError):
    468     if isinstance(argument, str):

UndefinedError: 'template_resources' is undefined