Kalign Python Package

Python bindings for Kalign, a fast multiple sequence alignment program for biological sequences (DNA, RNA, protein).

Installation

pip install kalign-python

Optional dependencies for ecosystem integration:

pip install kalign-python[biopython]    # Biopython integration (fmt="biopython", I/O helpers)
pip install kalign-python[skbio]        # scikit-bio integration (fmt="skbio")
pip install kalign-python[io]           # I/O helpers (requires Biopython)
pip install kalign-python[analysis]     # pandas + matplotlib for downstream analysis
pip install kalign-python[all]          # all of the above

Quick Start

import kalign

sequences = [
    "ATCGATCGATCG",
    "ATCGTCGATCG",
    "ATCGATCATCG"
]

# Default mode — single run with consistency anchors (best general-purpose)
aligned = kalign.align(sequences)

# Fast mode — single run, fastest
aligned = kalign.align(sequences, mode="fast")

# Accurate mode — ensemble, highest precision
aligned = kalign.align(sequences, mode="accurate")

Modes

Kalign v3.5 provides four named modes, exposed identically through the Python API and the kalign command-line tool. Presets were derived from NSGA-III multi-objective optimization on BAliBASE v4 (protein) and BRAliBASE (RNA).

Explicit parameters always override mode defaults:

# Fast base + 5 ensemble runs
aligned = kalign.align(sequences, mode="fast", ensemble=5)

# Accurate base + custom gap penalty
aligned = kalign.align(sequences, mode="accurate", gap_open=8.0)

Mode constants are also available: kalign.MODE_FAST, kalign.MODE_DEFAULT, kalign.MODE_RECALL, kalign.MODE_ACCURATE.

Core API

kalign.align()

aligned = kalign.align(
    sequences,              # list of str
    mode=None,              # "fast", "default", "recall", "accurate", or None (= default)
    seq_type="auto",        # "auto", "dna", "rna", "protein", "divergent", "internal"
    gap_open=None,          # positive float, or None for defaults
    gap_extend=None,        # positive float, or None for defaults
    terminal_gap_extend=None,
    n_threads=None,         # int, or None for global default
    refine="none",          # refinement mode: "none", "all", "confident", "inline"
    ensemble=0,             # number of ensemble runs (0 = off, try 3-5)
    min_support=0,          # explicit consensus threshold (0 = auto)
    fmt="plain",            # "plain", "biopython", "skbio"
    ids=None,               # list of str (for biopython/skbio output)
)

Returns a list of aligned strings (default), a Bio.Align.MultipleSeqAlignment (fmt="biopython"), or a skbio.TabularMSA (fmt="skbio").

When ensemble > 0 and fmt="biopython", per-residue confidence is attached as HMMER-style PP letter_annotations["posterior_probability"].

kalign.align_from_file()

Align sequences directly from a FASTA, MSF, or Clustal file:

result = kalign.align_from_file("sequences.fasta", seq_type="protein")
for name, seq in zip(result.names, result.sequences):
    print(f"{name}: {seq}")

Returns an AlignedSequences object with .names, .sequences, and optional confidence fields (see below).

Additional parameters for advanced use:

result = kalign.align_from_file(
    "input.fasta",
    mode="accurate",             # or "fast", "default", "recall"
    ensemble=5,                  # override: 5 runs instead of mode default
    min_support=0,               # consensus threshold (0 = auto)
    save_poar="consensus.poar",  # save POAR table for re-thresholding
    # load_poar="consensus.poar",  # OR load pre-computed POAR
    refine="none",               # refinement mode
)

kalign.compare()

Score a test alignment against a reference using the Sum-of-Pairs (SP) score:

score = kalign.compare("reference.msf", "test.fasta")
print(f"SP score: {score:.1f}")  # 0 (no match) to 100 (identical)

kalign.compare_detailed()

Detailed alignment comparison returning POAR recall/precision/F1/TC:

scores = kalign.compare_detailed("reference.msf", "test.fasta")
print(f"F1: {scores['f1']:.3f}, TC: {scores['tc']:.3f}")

kalign.write_alignment()

Write aligned sequences to a file:

kalign.write_alignment(aligned, "output.fasta", format="fasta", ids=ids)

Supported formats: fasta, clustal, stockholm, phylip (non-FASTA formats require Biopython).

Ensemble Alignment & Confidence Scores

Ensemble mode runs multiple alignments with varied parameters and combines results via POAR (Pairs of Aligned Residues) consensus. The simplest way to use it is mode="accurate" (5-run ensemble) or mode="recall" (recall-tuned ensemble). For more control, set ensemble directly.

import kalign

# Accurate mode: 5-run ensemble — highest precision
result = kalign.align_from_file("proteins.fasta", mode="accurate")

# Or: explicit 5 ensemble runs
result = kalign.align_from_file("proteins.fasta", ensemble=5)

# Per-column confidence: average agreement across ensemble runs [0..1]
print(result.column_confidence[:10])   # e.g. [0.93, 0.87, 1.0, ...]

# Per-residue confidence: per-sequence, per-position agreement [0..1]
print(result.residue_confidence[0][:10])  # e.g. [0.95, 0.90, 1.0, ...]

POAR Save/Load

Save the POAR consensus table to avoid re-running the ensemble when experimenting with thresholds:

# First run: compute ensemble and save POAR
kalign.align_file_to_file("input.fa", "output.fa", ensemble=5,
                          save_poar="consensus.poar")

# Later: instant re-threshold from saved POAR (no re-alignment)
kalign.align_file_to_file("input.fa", "output2.fa",
                          load_poar="consensus.poar", min_support=3)

Stockholm Output with Confidence

Write confidence annotations in Stockholm format (#=GR PP per-residue, #=GC PP_cons per-column):

result = kalign.align_from_file("input.fasta", ensemble=5)
kalign.write_alignment(
    result.sequences, "output.sto", format="stockholm",
    ids=result.names,
    column_confidence=result.column_confidence,
    residue_confidence=result.residue_confidence,
)

Confidence uses HMMER-style PP encoding: *=95%+, 9=85-95%, ..., 0=0-5%, .=gap.

Biopython Per-Residue PP

When using fmt="biopython" with ensemble, per-residue confidence is attached as letter_annotations["posterior_probability"]:

aln = kalign.align(seqs, ensemble=3, fmt="biopython", ids=ids)
print(aln[0].letter_annotations["posterior_probability"])  # e.g. "998.76*9..."

Threading

import kalign

kalign.set_num_threads(4)        # set global default
n = kalign.get_num_threads()     # query current default

# or override per call
aligned = kalign.align(sequences, n_threads=8)

Thread settings are thread-local, so different threads can use different defaults.

Utilities (kalign.utils)

Requires only NumPy (installed automatically):

import kalign

aligned = kalign.align(sequences)

arr = kalign.utils.to_array(aligned)                          # numpy array
stats = kalign.utils.alignment_stats(aligned)                 # dict with gap_fraction, conservation, identity
consensus = kalign.utils.consensus_sequence(aligned, threshold=0.7)
matrix = kalign.utils.pairwise_identity_matrix(aligned)       # numpy array
trimmed = kalign.utils.remove_gap_columns(aligned)
region = kalign.utils.trim_alignment(aligned, start=2, end=10)

Biopython Integration

Requires pip install kalign-python[biopython].

import kalign

# Return a Biopython MultipleSeqAlignment
aln = kalign.align(sequences, fmt="biopython", ids=["s1", "s2", "s3"])
print(aln.get_alignment_length())

# Write in various formats via Biopython
from Bio import AlignIO
AlignIO.write(aln, "output.clustal", "clustal")

I/O helpers (kalign.io)

sequences = kalign.io.read_fasta("input.fasta")
sequences, ids = kalign.io.read_sequences("input.fasta")

aligned = kalign.align(sequences)
kalign.io.write_fasta(aligned, "output.fasta", ids=ids)
kalign.io.write_clustal(aligned, "output.aln", ids=ids)
kalign.io.write_stockholm(aligned, "output.sto", ids=ids)
kalign.io.write_phylip(aligned, "output.phy", ids=ids)

scikit-bio Integration

Requires pip install kalign-python[skbio].

import kalign

# Returns a TabularMSA of DNA, RNA, or Protein depending on seq_type
aln = kalign.align(sequences, seq_type="dna", fmt="skbio")
print(type(aln))  # <class 'skbio.alignment._tabular_msa.TabularMSA'>

Constants

Sequence Types

Refinement Modes

Command-line Interface

# Modes
kalign-py -i sequences.fasta -o aligned.fasta                    # default mode
kalign-py --mode fast -i sequences.fasta -o aligned.fasta        # fast mode
kalign-py --mode recall -i sequences.fasta -o aligned.fasta      # recall mode
kalign-py --mode accurate -i sequences.fasta -o aligned.fasta    # accurate mode

# I/O options
kalign-py -i sequences.fasta -o - --format clustal               # stdout
cat input.fa | kalign-py -i - -o aligned.fasta                   # stdin
kalign-py -i sequences.fasta -o aligned.fasta --type protein     # explicit type

# Ensemble with POAR save/load
kalign-py -i input.fa -o output.fa --ensemble 5 --save-poar consensus.poar
kalign-py -i input.fa -o output.fa --load-poar consensus.poar --min-support 3

kalign-py --version

Development

git clone https://github.com/TimoLassmann/kalign.git
cd kalign
uv pip install -e .
uv run pytest tests/python/ -v

Requirements: Python 3.9+, CMake 3.18+, C++11 compiler, NumPy.

Citation

If you use Kalign in your research, please cite:

Lassmann, T. (2020). Kalign 3: multiple sequence alignment of large data sets. Bioinformatics, 36(6), 1928-1929. doi:10.1093/bioinformatics/btz795

License

Apache License, Version 2.0. See COPYING.