write-sequences

name	write-sequences
description	Write biological sequences to files (FASTA, FASTQ, GenBank, EMBL) using Biopython Bio.SeqIO. Use when saving sequences, creating new sequence files, or outputting modified records.
tool_type	python
primary_tool	Bio.SeqIO

Version Compatibility

Reference examples tested with: BioPython 1.83+, pysam 0.22+, samtools 1.19+

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show <package> then help(module.function) to check signatures

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Write Sequences

"Write sequences to a file" -> Serialize SeqRecord objects into a formatted sequence file.

• Python: SeqIO.write() (BioPython)
• R: writeXStringSet() (Biostrings)

Write SeqRecord objects to sequence files using Biopython's Bio.SeqIO module.

Required Import

from Bio import SeqIO
from Bio.Seq import Seq
from Bio.SeqRecord import SeqRecord

Core Functions

SeqIO.write() - Write Records to File

Write one or more SeqRecord objects to a file.

SeqIO.write(records, 'output.fasta', 'fasta')

Parameters:

• records - Single SeqRecord, list, or iterator of SeqRecords
• handle - Filename (string) or file handle
• format - Output format string

Returns: Number of records written (integer)

record.format() - Get Formatted String

Get a string representation without writing to file.

formatted = record.format('fasta')
print(formatted)

Creating SeqRecord Objects

Goal: Construct in-memory sequence records suitable for writing to any format.

Approach: Create SeqRecord with at minimum a Seq and id. Add letter_annotations for FASTQ, annotations['molecule_type'] for GenBank/EMBL.

"Create a sequence record from scratch" -> Wrap a Seq string in a SeqRecord with metadata fields.

• Python: SeqRecord(Seq(...), id=...) (BioPython)

Minimal SeqRecord

record = SeqRecord(Seq('ATGCGATCGATCG'), id='seq1')

Full SeqRecord

record = SeqRecord(
    Seq('ATGCGATCGATCG'),
    id='seq1',
    name='sequence_one',
    description='Example sequence for demonstration'
)

With Annotations (for GenBank output)

from Bio.SeqFeature import SeqFeature, FeatureLocation

record = SeqRecord(
    Seq('ATGCGATCGATCG'),
    id='seq1',
    annotations={'molecule_type': 'DNA'}
)
record.features.append(
    SeqFeature(FeatureLocation(0, 9), type='gene', qualifiers={'gene': ['exampleGene']})
)

Common Formats

Format	String	Notes
FASTA	'fasta'	Most universal, sequence + header only
FASTQ	'fastq'	Requires quality scores in letter_annotations
GenBank	'genbank'	Requires annotations and molecule_type
EMBL	'embl'	Similar requirements to GenBank
Tab	'tab'	Simple ID + sequence tabular format

Code Patterns

Write Single Record

record = SeqRecord(Seq('ATGC'), id='my_seq', description='test sequence')
SeqIO.write(record, 'output.fasta', 'fasta')

Write Multiple Records

records = [
    SeqRecord(Seq('ATGC'), id='seq1'),
    SeqRecord(Seq('GCTA'), id='seq2'),
    SeqRecord(Seq('TTAA'), id='seq3')
]
count = SeqIO.write(records, 'output.fasta', 'fasta')
print(f'Wrote {count} records')

Write to File Handle

with open('output.fasta', 'w') as handle:
    SeqIO.write(records, handle, 'fasta')

Write Modified Records

Goal: Transform sequences in-memory and write the modified versions to a new file.

Approach: Parse input, apply transformation via generator, write output. Using a generator avoids loading all records into memory.

"Modify sequences and save" -> Parse records, transform each, write to new file with SeqIO.write().

from Bio.Seq import Seq
from Bio.SeqRecord import SeqRecord

def uppercase_record(rec):
    return SeqRecord(rec.seq.upper(), id=rec.id, description=rec.description)

records = SeqIO.parse('input.fasta', 'fasta')
modified = (uppercase_record(rec) for rec in records)
SeqIO.write(modified, 'output.fasta', 'fasta')

Append to Existing File

with open('output.fasta', 'a') as handle:
    SeqIO.write(new_records, handle, 'fasta')

Write FASTQ with Quality Scores

record = SeqRecord(Seq('ATGCGATCG'), id='read1')
record.letter_annotations['phred_quality'] = [30, 30, 28, 25, 30, 30, 28, 25, 30]
SeqIO.write(record, 'output.fastq', 'fastq')

Write GenBank Format

record = SeqRecord(Seq('ATGCGATCGATCG'), id='SEQ001', name='example')
record.annotations['molecule_type'] = 'DNA'
record.annotations['topology'] = 'linear'
record.annotations['organism'] = 'Example organism'
SeqIO.write(record, 'output.gb', 'genbank')

Common Errors

Error	Cause	Solution
TypeError: SeqRecord expected	Passed raw string/Seq	Wrap in SeqRecord object
ValueError: missing molecule_type	GenBank without annotations	Add record.annotations['molecule_type'] = 'DNA'
ValueError: missing quality scores	FASTQ without phred_quality	Add quality scores to letter_annotations
ValueError: Sequences must all be the same length	PHYLIP with unequal lengths	Pad or trim sequences first

Format-Specific Requirements

FASTQ

Must have quality scores:

record.letter_annotations['phred_quality'] = [30] * len(record.seq)

GenBank/EMBL

Must have molecule_type:

record.annotations['molecule_type'] = 'DNA'  # or 'RNA', 'protein'

PHYLIP

All sequences must be same length. IDs truncated to 10 characters.

Related Skills

• read-sequences - Read sequences before modifying and writing
• format-conversion - Direct format conversion without intermediate processing
• filter-sequences - Filter sequences before writing subset
• sequence-manipulation/seq-objects - Create SeqRecord objects to write
• alignment-files/sam-bam-basics - For SAM/BAM output, use samtools/pysam