Compressed Files

Handle gzip, bzip2, and BGZF compressed sequence files with Biopython.

Required Imports

import gzip
import bz2
from Bio import SeqIO
from Bio import bgzf  # For BGZF (indexable compression)

Reading Compressed Files

Gzip (.gz)

with gzip.open('sequences.fasta.gz', 'rt') as handle:
    for record in SeqIO.parse(handle, 'fasta'):
        print(record.id, len(record.seq))

Important: Use

'rt'

'rb'

Bzip2 (.bz2)

with bz2.open('sequences.fasta.bz2', 'rt') as handle:
    for record in SeqIO.parse(handle, 'fasta'):
        print(record.id, len(record.seq))

BGZF (Block Gzip)

BGZF files can be read like regular gzip, but also support indexing:

# Read like normal gzip (auto-detected)
for record in SeqIO.parse('sequences.fasta.bgz', 'fasta'):
    print(record.id)

# Or explicitly with bgzf module
with bgzf.open('sequences.fasta.bgz', 'rt') as handle:
    for record in SeqIO.parse(handle, 'fasta'):
        print(record.id)

Writing Compressed Files

Gzip (.gz)

with gzip.open('output.fasta.gz', 'wt') as handle:
    SeqIO.write(records, handle, 'fasta')

Bzip2 (.bz2)

with bz2.open('output.fasta.bz2', 'wt') as handle:
    SeqIO.write(records, handle, 'fasta')

BGZF (.bgz)

with bgzf.open('output.fasta.bgz', 'wt') as handle:
    SeqIO.write(records, handle, 'fasta')

BGZF: Indexable Compression

BGZF is the only compressed format that supports

SeqIO.index()

SeqIO.index_db()

BGZF (Block GZip Format) is a variant of gzip that allows random access. It's used by BAM files and tabix-indexed files.

Create Indexable Compressed File

from Bio import SeqIO, bgzf

# Write as BGZF (can be indexed later)
records = SeqIO.parse('input.fasta', 'fasta')
with bgzf.open('output.fasta.bgz', 'wt') as handle:
    SeqIO.write(records, handle, 'fasta')

Index a BGZF File

# SeqIO.index() works with BGZF!
records = SeqIO.index('sequences.fasta.bgz', 'fasta')
seq = records['target_id'].seq
records.close()

# SeqIO.index_db() also works
records = SeqIO.index_db('index.sqlite', 'sequences.fasta.bgz', 'fasta')

Convert Gzip to BGZF

from Bio import SeqIO, bgzf
import gzip

# Read from gzip, write to BGZF
with gzip.open('input.fasta.gz', 'rt') as in_handle:
    with bgzf.open('output.fasta.bgz', 'wt') as out_handle:
        SeqIO.write(SeqIO.parse(in_handle, 'fasta'), out_handle, 'fasta')

Code Patterns

Read Gzipped FASTQ

with gzip.open('reads.fastq.gz', 'rt') as handle:
    records = list(SeqIO.parse(handle, 'fastq'))
print(f'Loaded {len(records)} reads')

Count Records in Gzipped File

with gzip.open('sequences.fasta.gz', 'rt') as handle:
    count = sum(1 for _ in SeqIO.parse(handle, 'fasta'))
print(f'{count} sequences')

Fast Count with Low-Level Parser

from Bio.SeqIO.FastaIO import SimpleFastaParser
import gzip

with gzip.open('sequences.fasta.gz', 'rt') as handle:
    count = sum(1 for _ in SimpleFastaParser(handle))

Convert Compressed to Uncompressed

with gzip.open('input.fasta.gz', 'rt') as in_handle:
    records = SeqIO.parse(in_handle, 'fasta')
    SeqIO.write(records, 'output.fasta', 'fasta')

Convert Uncompressed to Compressed

records = SeqIO.parse('input.fasta', 'fasta')
with gzip.open('output.fasta.gz', 'wt') as out_handle:
    SeqIO.write(records, out_handle, 'fasta')

Auto-Detect Compression

from pathlib import Path
from Bio import SeqIO, bgzf
import gzip
import bz2

def open_sequence_file(filepath, format):
    filepath = Path(filepath)
    suffix = filepath.suffix.lower()
    if suffix == '.gz':
        # Could be gzip or bgzf - bgzf handles both
        handle = bgzf.open(filepath, 'rt')
    elif suffix == '.bgz':
        handle = bgzf.open(filepath, 'rt')
    elif suffix == '.bz2':
        handle = bz2.open(filepath, 'rt')
    else:
        handle = open(filepath, 'r')
    return SeqIO.parse(handle, format)

Process Large Gzipped File (Memory Efficient)

with gzip.open('large.fastq.gz', 'rt') as handle:
    for record in SeqIO.parse(handle, 'fastq'):
        if len(record.seq) >= 100:
            process(record)

Compress Existing File (Raw Copy)

import shutil

with open('sequences.fasta', 'rb') as f_in:
    with gzip.open('sequences.fasta.gz', 'wb') as f_out:
        shutil.copyfileobj(f_in, f_out)

Compression Comparison

.gz

.bgz

.bz2

.xz

When to Use Each Format

Common Errors

TypeError: a bytes-like object is required

UnicodeDecodeError

gzip.open(file, 'rt', encoding='latin-1')

gzip.BadGzipFile

OSError: Not a gzipped file

SeqIO.index() fails on .gz

Decision Tree

Working with compressed sequence files?
├── Just reading sequentially?
│   └── Use gzip.open() or bz2.open() with 'rt' mode
├── Need to index the compressed file?
│   └── Convert to BGZF, then use SeqIO.index()
├── Writing compressed output?
│   ├── Will need to index later? → Use bgzf.open()
│   └── Just archiving? → Use gzip.open() or bz2.open()
└── Converting between formats?
    └── Parse with SeqIO, write to new handle

Related Skills

• read-sequences - Core parsing functions used with compressed handles
• write-sequences - Write to compressed output files
• batch-processing - Process multiple compressed files
• alignment-files - BAM files use BGZF natively; samtools handles compression