Filter Sequences by Criteria

Hard rules

1. No fabricated citations. Every cited work must resolve to a verifiable
2. No claim without provenance. Every quantitative or factual claim
3. No silent failure. Every script invocation, API call, or tool use must declare its exit status and what to do on non-zero. A skill that silently swallows errors is a violation.

When to use

• Selecting reads/contigs by length, GC, N count, or complexity.
• Subsetting FASTA by header pattern (e.g., keep only chr[1-9XY], drop chrUn_*).
• Removing low-complexity reads (e.g., poly-A runs) before assembly.
• Filtering paired FASTQ — see the paired-end skill for sync filtering.

When NOT to use

• Adapter/quality trimming → use fastp (single-tool, much faster).
• Host/contaminant screening → use kraken2, minimap2 against a host DB, or nf-core modules.
• For UMI dedup → umi-tools dedup or fab are better than custom code.

Prerequisites

• biopython>=1.83, regex>=2024.5 (better Unicode support than re).
• For high-throughput filtering: seqkit (C) is ~50x faster.

Core workflow

1. Define your predicates as composable Python functions returning bool.
2. Wrap them in a generator chain so the file is read once.
3. Stream the survivors to disk with SeqIO.write.
4. Always log how many records you kept vs dropped — for reproducibility.

Code patterns

Length filter (FASTA)

from Bio import SeqIO

def length_filter(records, min_len=500, max_len=10_000_000):
    for rec in records:
        if min_len <= len(rec.seq) <= max_len:
            yield rec

with open("filtered.fasta", "w") as out:
    n_in = n_out = 0
    for rec in SeqIO.parse("input.fasta", "fasta"):
        n_in += 1
        if min_len := 500 <= len(rec.seq):
            SeqIO.write(rec, out, "fasta")
            n_out += 1
print(f"Kept {n_out}/{n_in} records")

(Prefer the explicit form for clarity — the example above shows the structural pattern.)

Identity filter (regex on header)

import regex
from Bio import SeqIO

keep_pattern = regex.compile(r"^chr(1[0-9]|2[0-2]|[1-9]|X|Y|M)$")

def keep_chrom(rec, pat=keep_pattern):
    return bool(pat.match(rec.id))

with open("primary.fasta", "w") as out:
    SeqIO.write(r for r in SeqIO.parse("genome.fasta", "fasta") if keep_chrom(r)),
                out, "fasta")

GC window filter

from Bio import SeqIO
from Bio.SeqUtils import gc_fraction

def gc_window(rec, lo=0.30, hi=0.70):
    return lo <= gc_fraction(rec.seq) <= hi

SeqIO.write((r for r in SeqIO.parse("asm.fasta", "fasta") if gc_window(r)),
            "asm_gc_filtered.fasta", "fasta")

N content filter

def max_n(rec, max_frac=0.05):
    s = str(rec.seq).upper()
    if not s:
        return False
    return (s.count("N") / len(s)) <= max_frac

Quality filter (FASTQ)

def min_mean_quality(rec, min_q=30):
    q = rec.letter_annotations["phred_quality"]
    return (sum(q) / len(q)) >= min_q

Complexity filter (Shannon entropy over a window)

Useful for removing low-complexity reads that escape length filters:

import math
from collections import Counter

def entropy(s, k=4):
    """k-mer Shannon entropy; > ~2 bits usually means 'not junk' for k=4."""
    if len(s) < k:
        return 0.0
    counts = Counter(s[i:i+k] for i in range(len(s) - k + 1))
    total = sum(counts.values())
    return -sum((c/total) * math.log2(c/total) for c in counts.values())

def complex_enough(rec, min_entropy=1.8, k=4):
    return entropy(str(rec.seq).upper(), k=k) >= min_entropy

Composition predicates (composition)

from functools import reduce

def keep(rec, preds):
    return all(p(rec) for p in preds)

predicates = [lambda r: min_length(r, 1000), lambda r: max_n(r, 0.01), gc_window]
SeqIO.write((r for r in SeqIO.parse("in.fasta", "fasta") if keep(r, predicates)),
            "out.fasta", "fasta")

Drop empty records after trim

def non_empty(rec):
    return len(rec.seq) > 0

Common pitfalls

• Forgetting the file was modified in place. Filter chains should always be pure functions of rec -> bool.
• GC skew from Ns. Use Bio.SeqUtils.gc_fraction (excludes Ns) or normalize manually.
• Regex on headers is brittle. ^chr(\d+|X|Y|M)$ matches chr1 but not CHR1; decide and be explicit.
• Streaming into a list defeats the purpose. Use generator expressions inside SeqIO.write.
• Length filter on FASTQ before QC trim is wrong order. Trim first, then filter.

Validation

• After every filter, the input count ≥ output count.
• For paired filtering, mate counts in R1 and R2 match.
• Spot-check: a few records from the output satisfy every predicate.
• For complexity, an obvious junk read (e.g., AAAAAAAAAA...) has entropy ~0 and is dropped.

Open alternatives

References

• Biopython sequence utilities: https://biopython.org/docs/latest/api/Bio.SeqUtils.html
• regex package: https://pypi.org/project/regex/
• Companion: ors-bioinformatics-sequence-fastq-quality-scores, ors-bioinformatics-sequence-paired-end-fastq.

Changelog

• 1.0.0 (2026-06-10): Initial adaptation by Pradyumna Jayaram from bio-filter-sequences (bioSkills-main/sequence-io/filter-sequences).

Cross-references

Other skills in this category:

• batch-processing
• bowtie2-alignment
• bwa-alignment
• bwa-mem2-alignment
• codon-usage
• compressed-sequence-files
• fastq-quality-scores
• format-conversion
• hisat2-alignment
• motif-search
• paired-end-fastq
• pysam-genomics
• read-write-sequences
• reverse-complement
• sam-bam-basics
• samtools-bam-processing
• seq-objects
• sequence-properties
• sequence-slicing
• sequence-statistics
• star-alignment
• transcription-translation