<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def best_assemblyfile(self):
 """ Determine whether the contigs.fasta output file from the assembler is present. If not, set the .bestassembly attribute to 'NA' """

        for sample in self.metadata:
            try:
                # Set the name of the filtered assembly file
                filtered_outputfile = os.path.join(self.path, 'raw_assemblies', '{}.fasta'.format(sample.name))
                # Set the name of the unfiltered spades assembly output file
                if os.path.isfile(sample.general.assemblyfile):
                    size = os.path.getsize(sample.general.assemblyfile)
                    # Ensure that the assembly isn't just an empty file
                    if size == 0:
                        sample.general.bestassemblyfile = 'NA'
                    else:
                        sample.general.bestassemblyfile = sample.general.assemblyfile
                        shutil.copyfile(sample.general.bestassemblyfile, filtered_outputfile)
                else:
                    sample.general.bestassemblyfile = 'NA'
                # Add the name and path of the filtered file to the metadata
                sample.general.filteredfile = filtered_outputfile
            except AttributeError:
                sample.general.assemblyfile = 'NA'
                sample.general.bestassemblyfile = 'NA'