[reportlab-users] format error? Boy is that confusing!

[Glenn Linderman]

I changed some code that was generating URLs to feed to <a> tags in a 
Paragraph flowable. I goofed... I left the URL relative instead of 
making it absolute. This resulted in a nearly unintelligible error, 
although because I knew the code I had changed, I didn't spend too long 
battling with the bizarre symptom before I went back to see what was 
different about what I was producing now, versus what I had been 
producing before, and then quickly figured out the problem.

What makes this stack trace so unintelligible, is that it says "format 
not resolved" and then shows a relative URL.  It makes it seem like it 
is trying to interpret the URL as a format code (of some unknown sort), 
and my thoughts immediately went to the possibility of having omitted 
the format code parameter, and supplied the URL as the wrong parameter, 
somewhere along the line, and of course doc.build has so many deferred 
operations that there is little clue as to where in the document this 
actually takes place.

It would be extremely more useful to for the message to say something 
like "URL scheme missing" or "unknown scheme" using the terminology for 
URLs.  If, at the point of error generation, it truly is not known, and 
cannot easily be known, whether this is a "format" (whatever that means) 
or a URL (which is missing its scheme), then having the message say 
"format unresolved or URL scheme missing" would certain be an 
improvement in clarity at little expense of reworking the code (just 
changing a message).

Personally, I have no clue what other cases would cause the message, 
that would make "format not resolved" a useful message, but I'm making 
the assumption that such a case exists.

Here's an abbreviated stack trace:

     doc.build( self.elements )
   File 
"C:\Python34\lib\site-packages\reportlab\platypus\doctemplate.py", line 
916, in build
     self._endBuild()
   File 
"C:\Python34\lib\site-packages\reportlab\platypus\doctemplate.py", line 
858, in _endBuild
     if getattr(self,'_doSave',1): self.canv.save()
   File "C:\Python34\lib\site-packages\reportlab\pdfgen\canvas.py", line 
1209, in save
     self._doc.SaveToFile(self._filename, self)
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 217, in SaveToFile
     data = self.GetPDFData(canvas)
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 242, in GetPDFData
     return self.format()
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 421, in format
     IOf = IO.format(self)
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 864, in format
     fcontent = format(self.content, document, toplevel=1)   # yes this 
is at top level
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 78, in format
     f = element.format(document)
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 1595, in format
     return D.format(document)
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 671, in format
     L = [(format(PDFName(k),document)+b" "+format(dict[k],document)) 
for k in keys]
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 671, in  <listcomp>
     L = [(format(PDFName(k),document)+b" "+format(dict[k],document)) 
for k in keys]
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 78, in format
     f = element.format(document)
   File "C:\Python34\lib\site-packages\reportlab\pdfbase\pdfdoc.py", 
line 1771, in format
     if f is None: raise ValueError("format not resolved %s" % self.name)
ValueError: format not resolved ../../hc/foo.png
  Errorlevel 1 from bixutls.py.  Exiting.

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