How To Add Partial Cds Seqfeature In Biopython?

Question

[Leszek]

I'm trying to create partial CDS SeqFeature in BioPython:

from Bio.SeqFeature import SeqFeature, FeatureLocation, AfterPosition, BeforePosition

sf = SeqFeature(FeatureLocation(start,end), strand=1, type='CDS', id='genename')
seq = sf.extract(r.seq)
aa = str(seq.translate(table=gcode))
#mark as partial
if aa[0]!="M":
    sf.location.start.position = BeforePosition(start)
if aa[-1]!="*":
    sf.location.end.position = AfterPosition(end)
r.features.append(sf)

but when I write embl record I got an error:

output.write(r.format('embl'))
File "/usr/lib/pymodules/python2.7/Bio/SeqFeature.py", line 663, in __eq__
    "We can only do comparisons between Biopython Position objects."
AssertionError: We can only do comparisons between Biopython Position objects.

If I first create partial SeqFeature, then I'm unable to translate it:

sf = SeqFeature(FeatureLocation(start,AfterPosition(end)), strand=1, type='CDS', id='genename')
seq = sf.extract(r.seq)
aa = str(seq.translate(table=gcode))

File "/usr/lib/pymodules/python2.7/Bio/SeqFeature.py", line 627, in <lambda>
    fget=lambda self: self._end.position + self._end.extension,
TypeError: unsupported operand type(s) for +: 'AfterPosition' and 'int'

Anyone has an idea how to do it correctly in BioPython?

[Peter]

The test for the first amino acid being M is wrong, you should be checking the first three bases are a start codon. Non-standard start codons become M when used as start codon, but would usually translate as something else (e.g. Valine).

Answer 1

[Leszek]

Ok, I think I have solved it. For some reason, you have to translate the entry first, as partial entries are impossible to translate, and then set it as partial entry:

from Bio.SeqFeature import SeqFeature, FeatureLocation, AfterPosition, BeforePosition

sf = SeqFeature(FeatureLocation(start,end), strand=1, type='CDS', id='genename')
seq = sf.extract(r.seq)
aa = str(seq.translate(table=gcode))
#mark as partial
#both ends partial
if aa[0]!="M" and aa[-1]!="*":
    sf.location = FeatureLocation(BeforePosition(start-1),AfterPosition(end))
#left end partial
elif aa[0]!="M" or aa[-1]!="*" and strand==-1:
    sf.location = FeatureLocation(BeforePosition(start-1),end)
#right end partial
elif aa[-1]!="*" or aa[0]!="M" and strand==-1:
    sf.location = FeatureLocation(start-1,AfterPosition(end))

r.features.append(sf)
output.write(r.format('embl'))

Hope, someone else may benefit from that:)