Hi I am trying to learnt python3 and Biopython,

I am trying to check imported fasta file before processing so far using:

sequences = SeqIO.parse("fake_fasta_file", "fasta")

if not any(sequa):

works but as soon as I enter a '>' in my fake_fasta_file , as an example

, Békibpç
, mnkn


>  ftfyfiuuogpigèi

any(sequences) returns True even if:

for seq_record in sequences:

doesnt print anything

Any better way to check for real fasta file in Biopython ?

Or maybe it my fault using any() on the iterator instead of creating

a list of seq_record and checking on that ?


1 Answer 1


any() loops over the iterator until the first truthy item is found or the end is reached. The iterator is not reset to the start after this. Using a list will change that behaviour since a list is not exhausted like a iterator. So, remove your any() call and you will see a sequence record printed. Your 'fake_fasta_file.fa' is actually recognised by Biopython as valid fasta:

>>> SeqIO.read('fake_fasta_file.fa', 'fasta')
SeqRecord(seq=Seq(''), id='ftfyfiuuogpigèi', name='ftfyfiuuogpigèi', description='  ftfyfiuuogpigèi', dbxrefs=[])

Biopython ignores lines before the first line beginning with '>' (as possible comment lines) and then starts collecting records, in this case an empty sequence record is still a record. This behaviour is deliberate from Biopython, although you could argue if it's best or not, it's unlikely to change for historical reasons.

What is real fasta? (philosophical pause) If you want to validate a particular definition of FASTA format then you can write your own parser depending on the exact specification and rules you want.

Edit, e.g.

from Bio import SeqIO

records = SeqIO.parse("myfile.fa", "fasta")
first = next(records, None)
if first is None:
    print("No fasta records")
elif first.seq == "":
    print("Empty sequence")
elif all(c not in "ATCGN" for c in first.seq.upper()):
    print("Non ATCGN characters")


  • $\begingroup$ thank You very much indeed for the very informative and detailed answer. Given I am really a newbie is there any chance you could point me to pre-made fasta validator I could borrow for my script ? (i was thinking about valid if >header than sequence on one or multiple lines and a way to check the characters used in the sequences) $\endgroup$
    – pippo1980
    Dec 13, 2020 at 17:29
  • $\begingroup$ @pippo1980 see edit (untested) $\endgroup$ Dec 13, 2020 at 17:58
  • $\begingroup$ cheers !!!!!!!!! $\endgroup$
    – pippo1980
    Dec 13, 2020 at 18:20

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