[web2py] How to check if attribute exists

[Omi Chiba]

I have a simple upload form. It works fine but it shows the following error 
when user submit it without selecting file. How can I check 
if request.vars.csvfile.file attribute exists ?

 File "C:/trunk/web2py/applications/tedi/controllers/default.py", line 110, 
in customer_upload
    if request.vars.csvfile != None:
AttributeError: 'str' object has no attribute 'file'

*view*
{{extend 'layout.html'}}
{{=FORM(INPUT(_type='file',_name='csvfile'),INPUT(_type='hidden',_value='EDXTKF00',_name='table'),INPUT(_type='submit',_class='btn',_value='Upload'))}}

*controller*
def customer_upload():
    response.title = 'Upload'
    response.subtitle = 'Customer Code'
    
    if request.vars.csvfile != None: 
       # set values
       table = db[request.vars.table]
       file = request.vars.csvfile.file
        
       # import csv file
       table.import_from_csv_file(file)
        
       # update who imported
       query = db.EDXTKF00.TKUPFL==""
       db(query).update(TKUPFL="1")
        
       session.flash = 'Data Uploaded'
       redirect(URL('customer_index'))
    
    return dict()