If you pass csv.reader() a filename as its first argument:

  csv.reader('filename')

instead of a file object like you're supposed to, you don't get an error.  
You instead get a reader object which returns the characters which make up 
the filename.

Technically, this is not a bug, since the documentation says, "csvfile can 
be any object which supports the iterator protocol and returns a string 
each time its next method is called", and a string meets that definition.  
Still, this is unexpected behavior, and is almost certainly not what the 
user intended.  It would be useful if a way could be devised to catch this 
kind of mistake.