With FastHTML I'm writing a route to process a list of same-named checkboxes representing filenames in a Pico CSS Dropdown passed in a query string by HTMX:

@rt.get("/files")
def files_selected(my_files: list[pathlib.Path]):
   print("\n".join(f.name for f in my_files))

This works nicely most of the time:

• HTMX makes a request for /files?my_files=foo.txt&my_files=bar.txt
• FastHTML looks at the method signature and ~magically converts this into a list of pathlib.Path instances for me and exposes as the local variable my_files.

However, when no checkboxes are passed—when HTXML requests just /files with no query string—FastHTML throws HTTP 400 with the response Missing required field: my_files.

I tried switching the function signature to…

def files_selected(my_files: list[pathlib.Path] | None = None):

…but that breaks the magic; passing a single file results in
my_files=['f', 'o', 'o', '.', 't', 'x', 't'].

I've tried changing the function to…

def files_selected(req: fh.starlette.Request):
    my_files = req.query_params["my_files"]

…but this only returns one file (as a string) when multiple are supplied.

What's the right way to accept optional parameters in the query string?

With FastHTML I'm writing a route to process a list of same-named checkboxes representing filenames in a Pico CSS Dropdown passed in a query string by HTMX:

@rt.get("/files")
def files_selected(my_files: list[pathlib.Path]):
   print("\n".join(f.name for f in my_files))

This works nicely most of the time:

• HTMX makes a request for /files?my_files=foo.txt&my_files=bar.txt
• FastHTML looks at the method signature and ~magically converts this into a list of pathlib.Path instances for me and exposes as the local variable my_files.

However, when no checkboxes are passed—when HTXML requests just /files with no query string—FastHTML throws HTTP 400 with the response Missing required field: my_files.

I tried switching the function signature to…

def files_selected(my_files: list[pathlib.Path] | None = None):

…but that breaks the magic; passing a single file results in
my_files=['f', 'o', 'o', '.', 't', 'x', 't'].

I've tried changing the function to…

def files_selected(req: fh.starlette.Request):
    my_files = req.query_params["my_files"]

…but this only returns one file (as a string) when multiple are supplied.

What's the right way to accept optional parameters in the query string?

• python
• htmx
• starlette
• fasthtml

1 Answer 1

The answer is simple:

@rt.get("/files")
def files_selected(my_files: list[pathlib.Path] = []):
   print("\n".join(f.name for f in my_files))

Provide a default value for the list.