Agent skill

bio-compressed-files

Read and write compressed sequence files (gzip, bzip2, BGZF) using Biopython. Use when working with .gz or .bz2 sequence files. Use BGZF for indexable compressed files.

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npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/compressed-files

SKILL.md

Compressed Files

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

Required Imports

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

Reading Compressed Files

Gzip (.gz)

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)

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)

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

python
# 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)

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

BGZF is the only compressed format that supports SeqIO.index() and 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

python
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

python
# 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

python
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

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

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