---
name: biopython
description: Molecular biology toolkit with Biopython — sequence manipulation, FASTA/GenBank/PDB parsing, phylogenetics, BLAST automation, and programmatic NCBI/PubMed access via Bio.Entrez.
category: utility
license: MIT
author: K-Dense Inc. (adapted by Heureka Labs)
attribution: https://github.com/K-Dense-AI/scientific-agent-skills
version: 1.3.0
try-it: pending
tags: [sequences, fasta, genbank, entrez, blast]
allowed-tools: Read, Write, Edit, Bash
verified:
  date: 2026-08-09
  against: biopython 1.88 / Python 3.11
  executed: 84
  unverified: 43
  unverified_reason: >-
    Entrez and BLAST blocks need outbound access to NCBI, which the validating environment
    does not have. Unchanged by the 1.88 update and 1.88's notes record no Entrez or BLAST
    change, but not re-confirmed against it. Re-run from a host with NCBI access.
---
# Biopython: Computational Molecular Biology in Python

## Overview

Biopython is a comprehensive set of freely available Python tools for biological computation. It provides functionality for sequence manipulation, file I/O, database access, structural bioinformatics, phylogenetics, and many other bioinformatics tasks. The current version is **Biopython 1.88** (released 6 August 2026). It supports **Python 3.10-3.14** plus the Python 3.15 release candidate, and PyPy3.10, and requires NumPy; upstream support for Python 3.10 is now deprecated as that version approaches end of life.

Two recent releases were driven by security fixes, and both matter for the parsing workflows below:

- **1.88** hardens the `Bio.Nexus` NEXUS parser. Up to 1.87 it read the `ntax` and `nchar` values out of the file and passed them to Python's built-in `eval`, so those fields were executable and a crafted file could run arbitrary code at parse time. Both call sites are gone in 1.88. `Bio.Phylo` and `Bio.AlignIO` share this parser, so the exposure is not limited to code that imports `Bio.Nexus` directly.
- **1.87** addressed **CVE-2025-68463** in `Bio.Entrez.Parser` when parsing untrusted files.

Prefer **1.88+** for any workflow that parses NEXUS or Entrez XML it did not produce itself.

## When to Use This Skill

Use this skill when:

- Working with biological sequences (DNA, RNA, or protein)
- Reading, writing, or converting biological file formats (FASTA, GenBank, FASTQ, PDB, mmCIF, etc.)
- Accessing NCBI databases (GenBank, PubMed, Protein, Gene, etc.) via Entrez
- Running BLAST searches or parsing BLAST results
- Performing sequence alignments (pairwise or multiple sequence alignments)
- Analyzing protein structures from PDB files
- Creating, manipulating, or visualizing phylogenetic trees
- Finding sequence motifs or analyzing motif patterns
- Calculating sequence statistics (GC content, molecular weight, melting temperature, etc.)
- Performing structural bioinformatics tasks
- Working with population genetics data
- Any other computational molecular biology task

## Core Capabilities

Biopython is organized into modular sub-packages, each addressing specific bioinformatics domains:

1. **Sequence Handling** - Bio.Seq and Bio.SeqIO for sequence manipulation and file I/O
2. **Alignment Analysis** - Bio.Align and Bio.AlignIO for pairwise and multiple sequence alignments
3. **Database Access** - Bio.Entrez for programmatic access to NCBI databases
4. **BLAST Operations** - Bio.Blast for running and parsing BLAST searches
5. **Structural Bioinformatics** - Bio.PDB for working with 3D protein structures
6. **Phylogenetics** - Bio.Phylo for phylogenetic tree manipulation and visualization
7. **Advanced Features** - Motifs, population genetics, sequence utilities, and more

## Installation and Setup

Install the current stable Biopython release with an explicit version pin for reproducibility:

```bash
uv pip install "biopython==1.88"
```

For NCBI database access, always set your email address (required by NCBI). For reusable software, set a stable `Entrez.tool` value and register the tool/email with NCBI. For higher rate limits (10 req/s instead of 3 req/s), read only `NCBI_API_KEY` from the environment — do not hardcode keys or load unrelated environment variables:

```python
import os
from Bio import Entrez

Entrez.email = "your.email@example.com"  # required — use your real email
Entrez.tool = "your_tool_name"  # optional but recommended for reusable software

# Optional: register at https://www.ncbi.nlm.nih.gov/account/settings/
if api_key := os.environ.get("NCBI_API_KEY"):
    Entrez.api_key = api_key
```

## Using This Skill

This skill provides comprehensive documentation organized by functionality area. When working on a task, consult the relevant reference documentation:

### 1. Sequence Handling (Bio.Seq & Bio.SeqIO)

**Reference:** `references/sequence_io.md`

Use for:
- Creating and manipulating biological sequences
- Reading and writing sequence files (FASTA, GenBank, FASTQ, etc.)
- Converting between file formats
- Extracting sequences from large files
- Sequence translation, transcription, and reverse complement
- Working with SeqRecord objects

**Quick example:**
```python
from Bio import SeqIO

# Read sequences from FASTA file
for record in SeqIO.parse("sequences.fasta", "fasta"):
    print(f"{record.id}: {len(record.seq)} bp")

# Convert GenBank to FASTA
SeqIO.convert("input.gb", "genbank", "output.fasta", "fasta")
```

### 2. Alignment Analysis (Bio.Align & Bio.AlignIO)

**Reference:** `references/alignment.md`

Use for:
- Pairwise sequence alignment (global and local)
- Reading and writing multiple sequence alignments
- Using substitution matrices (BLOSUM, PAM)
- Calculating alignment statistics
- Customizing alignment parameters

**Quick example:**
```python
from Bio import Align

# Pairwise alignment
aligner = Align.PairwiseAligner()
aligner.mode = 'global'
alignments = aligner.align("ACCGGT", "ACGGT")
print(alignments[0])
```

### 3. Database Access (Bio.Entrez)

**Reference:** `references/databases.md`

Use for:
- Searching NCBI databases (PubMed, GenBank, Protein, Gene, etc.)
- Downloading sequences and records
- Fetching publication information
- Finding related records across databases
- Batch downloading with proper rate limiting

**Quick example:**
```python
from Bio import Entrez
Entrez.email = "your.email@example.com"

# Search PubMed
handle = Entrez.esearch(db="pubmed", term="biopython", retmax=10)
results = Entrez.read(handle)
handle.close()
print(f"Found {results['Count']} results")
```

### 4. BLAST Operations (Bio.Blast)

**Reference:** `references/blast.md`

Use for:
- Running BLAST searches via NCBI web services
- Running local BLAST searches
- Parsing BLAST XML output
- Filtering results by E-value or identity
- Extracting hit sequences

**Quick example:**
```python
from Bio.Blast import NCBIWWW, NCBIXML

# Run BLAST search
result_handle = NCBIWWW.qblast("blastn", "nt", "ATCGATCGATCG")
blast_record = NCBIXML.read(result_handle)

# Display top hits
for alignment in blast_record.alignments[:5]:
    print(f"{alignment.title}: E-value={alignment.hsps[0].expect}")
```

### 5. Structural Bioinformatics (Bio.PDB)

**Reference:** `references/structure.md`

Use for:
- Parsing PDB and mmCIF structure files
- Navigating protein structure hierarchy (SMCRA: Structure/Model/Chain/Residue/Atom)
- Calculating distances, angles, and dihedrals
- Secondary structure assignment (DSSP)
- Structure superimposition and RMSD calculation
- Extracting sequences from structures

**Quick example:**
```python
from Bio.PDB import PDBParser

# Parse structure
parser = PDBParser(QUIET=True)
structure = parser.get_structure("1crn", "1crn.pdb")

# Calculate distance between alpha carbons
chain = structure[0]["A"]
distance = chain[10]["CA"] - chain[20]["CA"]
print(f"Distance: {distance:.2f} Å")
```

### 6. Phylogenetics (Bio.Phylo)

**Reference:** `references/phylogenetics.md`

Use for:
- Reading and writing phylogenetic trees (Newick, NEXUS, phyloXML)
- Building trees from distance matrices or alignments
- Tree manipulation (pruning, rerooting, ladderizing)
- Calculating phylogenetic distances
- Creating consensus trees
- Visualizing trees

**Quick example:**
```python
from Bio import Phylo

# Read and visualize tree
tree = Phylo.read("tree.nwk", "newick")
Phylo.draw_ascii(tree)

# Calculate distance
distance = tree.distance("Species_A", "Species_B")
print(f"Distance: {distance:.3f}")
```

### 7. Advanced Features

**Reference:** `references/advanced.md`

Use for:
- **Sequence motifs** (Bio.motifs) - Finding and analyzing motif patterns
- **Population genetics** (Bio.PopGen) - GenePop files, Fst calculations, Hardy-Weinberg tests
- **Sequence utilities** (Bio.SeqUtils) - GC content, melting temperature, molecular weight, protein analysis
- **Restriction analysis** (Bio.Restriction) - Finding restriction enzyme sites
- **Clustering** (Bio.Cluster) - K-means and hierarchical clustering
- **Genome diagrams** (GenomeDiagram) - Visualizing genomic features

**Quick example:**
```python
from Bio.SeqUtils import gc_fraction, molecular_weight
from Bio.Seq import Seq

seq = Seq("ATCGATCGATCG")
print(f"GC content: {gc_fraction(seq):.2%}")
print(f"Molecular weight: {molecular_weight(seq, seq_type='DNA'):.2f} g/mol")
```

## General Workflow Guidelines

### Reading Documentation

When a user asks about a specific Biopython task:

1. **Identify the relevant module** based on the task description
2. **Read the appropriate reference file** using the Read tool
3. **Extract relevant code patterns** and adapt them to the user's specific needs
4. **Combine multiple modules** when the task requires it

Example search patterns for reference files:
```bash
# Find information about specific functions
rg -n "SeqIO.parse" references/sequence_io.md

# Find examples of specific tasks
rg -n "BLAST" references/blast.md

# Find information about specific concepts
rg -n "alignment" references/alignment.md
```

### Writing Biopython Code

Follow these principles when writing Biopython code:

1. **Import modules explicitly**
   ```python
   from Bio import SeqIO, Entrez
   from Bio.Seq import Seq
   ```

2. **Set Entrez email** when using NCBI databases; load only `NCBI_API_KEY` from the environment if present
   ```python
   import os
   from Bio import Entrez

   Entrez.email = "your.email@example.com"
   Entrez.tool = "your_tool_name"
   if api_key := os.environ.get("NCBI_API_KEY"):
       Entrez.api_key = api_key
   ```

3. **Use appropriate file formats** - Check which format best suits the task
   ```python
   # Common formats: "fasta", "genbank", "fastq", "clustal", "phylip"
   ```

4. **Handle files properly** - Close handles after use or use context managers
   ```python
   with open("file.fasta") as handle:
       records = SeqIO.parse(handle, "fasta")
   ```

5. **Use iterators for large files** - Avoid loading everything into memory
   ```python
   for record in SeqIO.parse("large_file.fasta", "fasta"):
       # Process one record at a time
       print(record.id, len(record.seq))
   ```

6. **Handle errors gracefully** - Network operations and file parsing can fail
   ```python
   from urllib.error import HTTPError

   try:
       handle = Entrez.efetch(db="nucleotide", id=accession)
   except HTTPError as e:
       print(f"Error: {e}")
   ```

## Common Patterns

### Pattern 1: Fetch Sequence from GenBank

```python
from Bio import Entrez, SeqIO

Entrez.email = "your.email@example.com"

# Fetch sequence
handle = Entrez.efetch(db="nucleotide", id="EU490707", rettype="gb", retmode="text")
record = SeqIO.read(handle, "genbank")
handle.close()

print(f"Description: {record.description}")
print(f"Sequence length: {len(record.seq)}")
```

### Pattern 2: Sequence Analysis Pipeline

```python
from Bio import SeqIO
from Bio.SeqUtils import gc_fraction

for record in SeqIO.parse("sequences.fasta", "fasta"):
    # Calculate statistics
    gc = gc_fraction(record.seq)
    length = len(record.seq)

    # Find ORFs, translate, etc.
    protein = record.seq.translate()

    print(f"{record.id}: {length} bp, GC={gc:.2%}")
```

### Pattern 3: BLAST and Fetch Top Hits

```python
from Bio.Blast import NCBIWWW, NCBIXML
from Bio import Entrez, SeqIO

Entrez.email = "your.email@example.com"

# Run BLAST
result_handle = NCBIWWW.qblast("blastn", "nt", sequence)
blast_record = NCBIXML.read(result_handle)

# Get top hit accessions
accessions = [aln.accession for aln in blast_record.alignments[:5]]

# Fetch sequences
for acc in accessions:
    handle = Entrez.efetch(db="nucleotide", id=acc, rettype="fasta", retmode="text")
    record = SeqIO.read(handle, "fasta")
    handle.close()
    print(f">{record.description}")
```

### Pattern 4: Build Phylogenetic Tree from Sequences

```python
from Bio import AlignIO, Phylo
from Bio.Phylo.TreeConstruction import DistanceCalculator, DistanceTreeConstructor

# Read alignment
alignment = AlignIO.read("alignment.fasta", "fasta")

# Calculate distances
calculator = DistanceCalculator("identity")
dm = calculator.get_distance(alignment)

# Build tree
constructor = DistanceTreeConstructor()
tree = constructor.nj(dm)

# Visualize
Phylo.draw_ascii(tree)
```

## Best Practices

1. **Always read relevant reference documentation** before writing code
2. **Use grep to search reference files** for specific functions or examples
3. **Validate file formats** before parsing
4. **Handle missing data gracefully** - Not all records have all fields
5. **Cache downloaded data** - Don't repeatedly download the same sequences
6. **Respect NCBI rate limits** - Use API keys, registered tool/email values for reusable software, and Entrez history/batching for large jobs
7. **Test with small datasets** before processing large files
8. **Keep Biopython updated** to get the latest features, bug fixes, and security fixes — 1.87 and 1.88 were both driven by parser vulnerabilities
9. **Use appropriate genetic code tables** for translation
10. **Document analysis parameters** for reproducibility

## Troubleshooting Common Issues

### Issue: "No handlers could be found for logger 'Bio.Entrez'"
**Solution:** This is just a warning. Set Entrez.email to suppress it.

### Issue: "HTTP Error 400" from NCBI
**Solution:** Check that IDs/accessions are valid and properly formatted.

### Issue: "ValueError: EOF" when parsing files
**Solution:** Verify file format matches the specified format string.

### Issue: Alignment fails with "sequences are not the same length"
**Solution:** Ensure sequences are aligned before using AlignIO or MultipleSeqAlignment.

### Issue: BLAST searches are slow
**Solution:** Use local BLAST for large-scale searches, or cache results.

### Issue: PDB parser warnings
**Solution:** Use `PDBParser(QUIET=True)` to suppress warnings, or investigate structure quality.

### Issue: ImportError for Bio.HMM, Bio.MarkovModel, or Bio.Application
**Solution:** These modules were removed in Biopython 1.86. Use [hmmlearn](https://pypi.org/project/hmmlearn/) for HMMs and the standard library `subprocess` module instead of `Bio.Application` CLI wrappers.

### Issue: Reading a NEXUS file from a source you do not control
**Solution:** Use Biopython 1.88 or later. Up to 1.87 the NEXUS parser passed the file's own `ntax` and `nchar` fields to Python's built-in `eval`, so reading a crafted file could run arbitrary code. Every NEXUS entry point shares that parser — `Phylo.read(path, "nexus")`, `AlignIO.read(path, "nexus")` and `Bio.Nexus.Nexus` alike — so upgrading, rather than switching entry point, is the fix.

### Issue: PairwiseAligner returns fewer alignments after upgrading to 1.86+
**Solution:** The default gap score changed from 0 to -1 in 1.86, eliminating trivial tie alignments. Set `aligner.gap_score = 0` to restore the old behavior if needed (see `references/alignment.md`).

## Additional Resources

- **Official Documentation**: https://biopython.org/docs/latest/
- **Tutorial**: https://biopython.org/docs/latest/Tutorial/
- **Cookbook**: https://biopython.org/docs/latest/Tutorial/ (advanced examples)
- **GitHub**: https://github.com/biopython/biopython
- **Release notes**: https://github.com/biopython/biopython/blob/master/NEWS.rst
- **Deprecated APIs**: https://github.com/biopython/biopython/blob/master/DEPRECATED.rst
- **Mailing List**: biopython@biopython.org

## Quick Reference

To locate information in reference files, use these search patterns:

```bash
# Search for specific functions
rg -n "function_name" references/*.md

# Find examples of specific tasks
rg -n "example" references/sequence_io.md

# Find all occurrences of a module
rg -n "Bio.Seq" references/*.md
```

## Summary

Biopython provides comprehensive tools for computational molecular biology. When using this skill:

1. **Identify the task domain** (sequences, alignments, databases, BLAST, structures, phylogenetics, or advanced)
2. **Consult the appropriate reference file** in the `references/` directory
3. **Adapt code examples** to the specific use case
4. **Combine multiple modules** when needed for complex workflows
5. **Follow best practices** for file handling, error checking, and data management

The modular reference documentation ensures detailed, searchable information for every major Biopython capability.