Why is "if fileitem.file:" (almost) never false?

Peter peter at monicol.co.uk
Thu Nov 17 18:44:33 CET 2005

I think:

   form = cgi.FieldStorage()
   fileitem = form["userfile"]
   if fileitem.filename:
         data = form.getfirst("userfile")
         if data:
              #do somethinfg with it

FieldStorage objects always have a filename attribute which is  the
filename that the user used to upload the file or '' (maybe None) when
the FieldStorage object was not created for  a File upload. The
getfirst call returns the data of the file, but you need to access the
FieldStorage object directly forthe filename (as you did).



mark wrote:
> How do I detect that a particular form element is a file upload or if
> the file upload has worked?
> In the Python cgi module documentation I found suggested code...
> form = cgi.FieldStorage()
> fileitem = form["userfile"]
> if fileitem.file:
>     # It's an uploaded file; count lines
> I've tried something like this in a test script and fileitem.file
> evaluates to true when form element "userfile" is just a text b
ox in
> the web page. When "userfile" is a type="file" it evaluates true when I
> select a real file to upload and when I manually type in a false path
> to a file that doesn't exist. When I do upload a file, the script
> correctly prints out the file contents (fileitem.value) so the upload
> does work OK.
> It seems that provided there's a form element called "userfile" then
> fileitem.file evaluates to true provided the form has
> enctype="multipart/form-data" (it doesn't matter if the file upload
> works or if the form element is not type="file").
> When I tried different forms without the enctype then fileitem.file
> evaluated to false when userfile was either type="text" or type="file".
> My conclusion is that if fileitem.file: detects enctype of form rather
> than if a particular element is a file upload. How do I detect that a
> particular form element is a file upload or if the file upload has
> worked?
> My best suggestion (but I'd like to hear your views) is to use:
> if fileitem.filename and len(fileitem.value):
>    # It's an uploaded file
> When the form element userfile is just a text box or the form lacks the
> enctype then the filename attribute evaluates to false. When I supply
> an incorrect file path to the file form element the upload fails so
> len(fileitem.value) evaluates to false. When I upload a real file, both
> fileitem.filename and len(fileitem.value) evaluate to true.
> If I upload a real file that's empty then fileitem.value is an empty
> string so len(fileitem.value) evaluates to false which is not quite
> right (an empty file is an obscure test but it is a valid file to
> upload).
> Any suggestions for improvements would be gratefully received.
> Thanks
> Mark
> PS if it's relevant, I'm using Windows2000, Python 2.3.2, the shebang
> includes -u, Apache 2.0.42 and Firefox 1.0.4.

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