Version Compatibility

Reference examples tested with: BioPython 1.83+, 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.

Compressed Files

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

"Read a compressed sequence file" → Open a compressed file handle in text mode, then parse with the standard SeqIO interface.

• gzip: gzip.open(path, 'rt') (Python stdlib)
• bzip2: bz2.open(path, 'rt') (Python stdlib)
• BGZF: bgzf.open(path, 'rt') (BioPython) or direct SeqIO.parse(path, fmt)

"Make a compressed file indexable" → Convert to BGZF format. Only BGZF supports SeqIO.index() on compressed data.

Required Imports

python
import gzip
import bz2
from Bio import SeqIO
from Bio import bgzf

Reading Compressed Files

Goal: Parse sequence records from compressed files without decompressing to disk.

Approach: Open a decompression handle in text mode ('rt'), then pass the handle to SeqIO.parse(). The parser works identically to uncompressed input.

Gzip (.gz) (BioPython 1.83+)

python
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' (read text) mode, not 'rb' (read binary).

Bzip2 (.bz2) (BioPython 1.83+)

python
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) (BioPython 1.83+)

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

python
for record in SeqIO.parse('sequences.fasta.bgz', 'fasta'):
    print(record.id)

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

Writing Compressed Files

Goal: Save sequence records directly to compressed files without an intermediate uncompressed step.

Approach: Open a compression handle in text mode ('wt'), then pass it to SeqIO.write().

Gzip (.gz)

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

Bzip2 (.bz2)

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

BGZF (.bgz)

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

BGZF: Indexable Compression

Goal: Enable random access to records in compressed sequence files.

Approach: Write sequences in BGZF (Block GZip Format) — the only compressed format supporting SeqIO.index() and SeqIO.index_db(). BGZF is a gzip variant used by BAM and tabix-indexed files.

Create Indexable Compressed File

python
from Bio import SeqIO, bgzf

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

Index a BGZF File

python
records = SeqIO.index('sequences.fasta.bgz', 'fasta')
seq = records['target_id'].seq
records.close()

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

Convert Gzip to BGZF

"Convert gzip to indexable format" → Parse from gzip handle, write through BGZF handle.

python
from Bio import SeqIO, bgzf
import gzip

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

python
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

python
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

python
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

python
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

python
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

python
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)

python
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)

python
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

| Format | Extension | Indexable | Speed | Compression | |--------|-----------|-----------|-------|-------------| | Gzip | .gz | No | Fast | Good | | BGZF | .bgz | Yes | Fast | Good | | Bzip2 | .bz2 | No | Slow | Better | | LZMA | .xz | No | Slowest | Best |

When to Use Each Format

| Use Case | Recommended Format | |----------|-------------------| | Archive (no random access needed) | gzip or bzip2 | | Need to index compressed file | BGZF | | BAM files and tabix | BGZF (native) | | Maximum compression | bzip2 or xz | | Best speed | gzip or BGZF |

Common Errors

| Error | Cause | Solution | |-------|-------|----------| | TypeError: a bytes-like object is required | Used 'rb' mode | Use 'rt' for text mode | | UnicodeDecodeError | Wrong encoding | Try gzip.open(file, 'rt', encoding='latin-1') | | gzip.BadGzipFile | Not a gzip file | Check file extension matches actual format | | OSError: Not a gzipped file | Corrupt or wrong format | Verify file integrity | | SeqIO.index() fails on .gz | Regular gzip not indexable | Convert to BGZF first |

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