fix custom 404 page error in multi page application

[Divyessh]

Start with a description of this PR. Then edit the list below to the items that make sense for your PR scope, and check off the boxes as you go!

Problem Faced

The issue comes up when trying to set up a custom not_found_404 page in a multi-page setup where the pages folder is not a direct subdirectory in the application root folder. In the example below, the dash will not use not_found_404.py as the custom 404 page.

Example

The file structure is as follows

.
|-- application.py
|-- requirements.txt
`-- src
    |-- __init__.py
    |-- assets
    |   |-- favicon.ico
    |-- pages
    |   |-- callbacks
    |   |   `-- sample_callback.py
    |   |-- layouts
    |   |   `-- sample_layout.py
    |   |-- not_found_404.py
    |   `-- procs
    |       `-- sample_proc
    |           `-- sample_proc.py

application.py

app = dash.Dash(
    __name__,
    server=server,
    use_pages=True,
    pages_folder='src/pages',
    assets_folder='src/assets',
    suppress_callback_exceptions=True,
)

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • task 1 - fix custom 404 page error in multi page application
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[alexcjohnson]

@Divyessh thanks for catching this bug, and for opening this PR! I wonder whether we can be even more flexible and pick up a 404 page wherever it's registered, as long as it's called not_found_404? Something like:

for module, page in _pages.PAGE_REGISTRY.items():
    if module.split(".")[-1] == "not_found_404":
        layout = page["layout"]
        title = page["title"]
        break
else:
    layout = html.H1("404 - Page not found")
    title = self.title

@AnnMarieW any thoughts?

[AnnMarieW]

@alexcjohnson - oh of course- that would be the ideal solution. It would pick it up even if there was no pages folder.

@Divyessh Nice catch and thanks again for the PR!

[Divyessh]

@alexcjohnson @AnnMarieW
Thank You!
Sure, Let me implement these changes to the PR and add a commit.

[alexcjohnson]

💃 Excellent, thanks again @Divyessh!