Web2Py Error - TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

I am getting an error when I try to run the code for Web2Py.

Basically we have to write a function that would query the database and show the flights available.

The function is defined as below:

def searchFlights():
    return dict()

def show():
    receivedFlights = request.vars.originCity+','+request.vars.destCity+','+request.vars.type+','+request.vars.deptDate+','+request.vars.arrivalDate+','+request.vars.vlassType+','+request.vars.noOfAdults+','+request.vars.noOfChildren+','+request.vars.noOfInfants
    return dict(txt1=recievedflights, flights=db().select(db.Flight.request.originCity+','+request.vars.destCity+','+request.vars.type+','+request.vars.deptDate+','+request.vars.arrivalDate+','+request.vars.classType+','+request.vars.noOfAdults+','+request.vars.noOfChildren+','+request.vars.noOfInfants))

The error we are getting right now is:

  File "K:/wdd-softw/web2py/applications/Assignment/controllers/default.py", line 106, in show
    receivedFlights = request.vars.originCity+','+request.vars.destCity+','+request.vars.type+','+request.vars.deptDate+','+request.vars.arrivalDate+','+request.vars.vlassType+','+request.vars.noOfAdults+','+request.vars.noOfChildren+','+request.vars.noOfInfants
  TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Solution

The first problem is that when the page with the form first loads, nothing has been submitted, so request.vars is empty and therefore each request.vars.some_var returns None. Instead, you could do something like this:

receivedFlights = ', '.join(request.vars.values()) if request.vars else 'Empty'

You'll have the same problem with your database select -- you should only run the query when request.vars exists. Furthermore, your query is incorrect -- you cannot simply concatenate all the values into a single string and put that in the select(). I suggest you read the chapter on the DAL for proper query syntax. It should look more like:

db((db.Flight.originCity == request.vars.originCity) &
   (db.Flight.destCity == request.vars.destCity) &
   ...).select()

Finally, you can simplify the query construction, as suggested here:

query = reduce(lambda a, b: (a & b),
    (db.mytable[v] == request.vars[v] for v in request.vars if v != None))