File download - disable link when no files are available

[P T]

Hello All,

Currently using Web2Py 2.8.2-stable+timestamp.2013.11.28.13.54.07

I have a simple page for announcing seminars and for downloading presentation files.

Here are definitions for table, controller and view :

db.define_table("seminars",
                Field('sem_date', 'date', label='Date'),
                Field('Sem_Time', 'string', label='Time'),
                Field('room', 'string', label='Location'),
                Field('person', 'string', label='Presenter'),
                Field('title', 'string', label='Seminar Title'),
                Field('sem_file', 'upload', label='Files', requires=False)
                )
db.seminars.sem_file.represent = lambda value,row:A('Download', _href=URL('download', args=value))

----------

def index():
    db.seminars.id.readable=False
    grid = SQLFORM.grid(db.seminars, maxtextlength=150,
                        details=False, deletable=False, searchable=False, csv=False,  editargs=dict(id=False),  formstyle='bootstrap')
    grid.element('.web2py_counter', replace=None)
    return dict(grid=grid)

----------

{{extend 'seminar_layout.html'}}

{{=grid}}

It works fine as intended. But, I want to replace "Download" link with "No Files Available" and disable the link when there are there are no files available. Any suggestion or help is appreciated.

Thanks,
PT

[Leonel Câmara]

Hey, 

How about this:

db.seminars.sem_file.represent = lambda value,row:A('Download', _href=URL('download', args=value)) if value else "No Files Available"

[P T]

Thanks Leonel, it works great. But, I am stumped by the syntax.

[Leonel Câmara]

The syntax is just regular python, what I've used is called a conditional expression which is python's version of the ternary operator.

It has the form:

a if test else b

Which is equivalent to this in many languages:

test ? a : b;

So what's happening here is that the value is None if there's no file, None evaluates to False when used as a boolean. So what the syntax actually means is - return the link if there's a value otherwise return "No Files Available", which coincidentally is exactly what you wanted.

[P T]

Thank you Leonel for taking the time to explain the syntax!

Best Regards,
PT

On Friday, February 27, 2015 at 11:45:03 AM UTC-6, P T wrote: