Bio Paired End Fastq

Version Compatibility

Reference examples tested with: BioPython 1.83+

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.

Paired-End FASTQ

"Work with my paired-end FASTQ files" -> Iterate R1/R2 pairs in sync, filter both mates together (routing orphans out), interleave/deinterleave files, and match mates by read name.

• Python: SeqIO.parse() with zip() iteration (BioPython)

The Governing Principle: R1 and R2 Are Parallel Streams

Aligners (bwa mem, bowtie2, STAR) consume R1 and R2 as two parallel streams and pair the i-th record of each file: same order, same count. They assume the k-th read in R1 is the mate of the k-th read in R2.

This makes independent per-mate processing the #1 paired-end correctness trap. Filtering or trimming ONE mate without the other DESYNCS the files:

• Best case: the aligner detects a read-name mismatch and crashes loudly.
• Worst case: it silently pairs the wrong R1 with the wrong R2 -> mismapping, corrupt insert sizes, no error at all.

Governing rule: never filter, trim, sort, or subsample one mate independently. Process both mates as a unit. When a read fails but its mate passes, route the survivor to a separate singleton/orphan file rather than leaving a gap that desyncs the stream. Proper paired trimmers (Trimmomatic PE, fastp, cutadapt -p) do exactly this: synchronized paired output plus separate orphan files.

Required Import

from Bio import SeqIO

Read-Name Conventions: How Mates Are Matched

Two distinct naming layers exist. File naming (which file is R1 vs R2) is separate from read naming (how a tool decides two records are mates).

File naming patterns

• sample_R1.fastq / sample_R2.fastq
• sample_1.fastq / sample_2.fastq
• sample_R1_001.fastq / sample_R2_001.fastq (Illumina bcl2fastq)
• sample.R1.fastq.gz / sample.R2.fastq.gz

Read naming: mate matched by shared ID up to the first whitespace

In CASAVA 1.8+ the second field is <read>:<is_filtered>:<control>:<index> -> read=1 or 2 (mate number), is_filtered=Y (failed chastity) or N, control=0 normally, index=barcode.

Most tools (and Biopython) take the read ID as everything up to the first whitespace. Biopython puts that token in record.id and the full header line in record.description. So for 1.8+ data, r1.id == r2.id directly; the 1:/2: distinction is only visible in record.description. For pre-1.8 data, strip the /1//2 suffix before comparing.

A normalizer that handles both eras:

def mate_key(record):
    return record.id.rsplit('/', 1)[0]

record.id already excludes anything after the first space, so this single rsplit covers both the space-format (1.8+) and the slash-suffix (pre-1.8) conventions.

Iterate Pairs Together

Basic Paired Iteration

r1_records = SeqIO.parse('reads_R1.fastq', 'fastq')
r2_records = SeqIO.parse('reads_R2.fastq', 'fastq')

for r1, r2 in zip(r1_records, r2_records):
    print(f'R1: {r1.id}, R2: {r2.id}')
    print(f'Lengths: {len(r1.seq)}, {len(r2.seq)}')

zip stops at the shorter iterator. If R1 has 1000 reads and R2 has 998, zip silently processes 998 and drops the tail with no warning. Verify counts match (see Paired Statistics) before trusting a zip loop on files of unknown provenance.

Verify Pair Matching

def iterate_pairs(r1_file, r2_file, format='fastq'):
    r1_records = SeqIO.parse(r1_file, format)
    r2_records = SeqIO.parse(r2_file, format)

    for r1, r2 in zip(r1_records, r2_records):
        if mate_key(r1) != mate_key(r2):
            raise ValueError(f'Pair mismatch: {r1.id} vs {r2.id}')
        yield r1, r2

for r1, r2 in iterate_pairs('reads_R1.fastq', 'reads_R2.fastq'):
    process_pair(r1, r2)

Filter Pairs Together (Synchronized, With Orphan Routing)

Goal: Quality-filter paired reads so that R1 and R2 stay in lockstep, and reads whose mate was discarded are routed to orphan files instead of silently desyncing the stream.

Approach: Stream both files together with zip. Evaluate both mates. If both pass, write to the paired outputs. If exactly one passes, write the survivor to its orphan file. This mirrors the four-output behavior of Trimmomatic PE (paired R1, paired R2, orphan R1, orphan R2).

Reference (BioPython 1.83+):

def filter_pairs_synced(r1_in, r2_in, r1_out, r2_out, r1_orphan, r2_orphan, min_qual=25):
    '''Keep a pair only if both mates pass; route lone survivors to orphan files.'''
    r1_records = SeqIO.parse(r1_in, 'fastq')
    r2_records = SeqIO.parse(r2_in, 'fastq')

    counts = {'paired': 0, 'r1_orphan': 0, 'r2_orphan': 0}
    with open(r1_out, 'w') as p1, open(r2_out, 'w') as p2, \
         open(r1_orphan, 'w') as o1, open(r2_orphan, 'w') as o2:
        for r1, r2 in zip(r1_records, r2_records):
            r1_ok = sum(r1.letter_annotations['phred_quality']) / len(r1.seq) >= min_qual
            r2_ok = sum(r2.letter_annotations['phred_quality']) / len(r2.seq) >= min_qual
            if r1_ok and r2_ok:
                SeqIO.write(r1, p1, 'fastq')
                SeqIO.write(r2, p2, 'fastq')
                counts['paired'] += 1
            elif r1_ok:
                SeqIO.write(r1, o1, 'fastq')
                counts['r1_orphan'] += 1
            elif r2_ok:
                SeqIO.write(r2, o2, 'fastq')
                counts['r2_orphan'] += 1
    return counts

The paired outputs stay synchronized because every pass-both write goes to BOTH files in the same iteration. Mean quality is one criterion; swap the test for a length threshold, adapter check, or any predicate, but always apply it to both mates and route orphans the same way. min_qual=25 is a common Q-score cutoff (Phred 25 ~= 0.3% error); tune per experiment.

Interleave Pairs

Interleaved FASTQ holds both mates in one file alternating R1, R2, R1, R2 (record 2k = forward, 2k+1 = reverse). bwa mem -p reads this format. The strict alternation IS the pairing, so a single mate-less read shifts every downstream record by one and desyncs everything. Only interleave files that are known to be synchronized, and pull orphans out first.

Create Interleaved File

Goal: Merge synchronized R1/R2 files into one interleaved file.

Approach: Zip both iterators and yield alternating records through a generator so nothing is materialized in memory.

Reference (BioPython 1.83+):

def interleave_pairs(r1_file, r2_file, output_file, format='fastq'):
    r1_records = SeqIO.parse(r1_file, format)
    r2_records = SeqIO.parse(r2_file, format)

    def interleaved():
        for r1, r2 in zip(r1_records, r2_records):
            yield r1
            yield r2

    count = SeqIO.write(interleaved(), output_file, format)
    return count // 2  # Number of pairs

pairs = interleave_pairs('reads_R1.fastq', 'reads_R2.fastq', 'reads_interleaved.fastq')

Deinterleave

Split Interleaved to Paired Files

Goal: Recover separate R1/R2 files from an interleaved file, streaming to avoid loading everything.

Approach: Parse once; route even-indexed records to R1, odd-indexed to R2.

Reference (BioPython 1.83+):

def deinterleave_streaming(interleaved_file, r1_file, r2_file, format='fastq'):
    records = SeqIO.parse(interleaved_file, format)

    pairs = 0
    with open(r1_file, 'w') as r1_h, open(r2_file, 'w') as r2_h:
        for i, record in enumerate(records):
            if i % 2 == 0:
                SeqIO.write(record, r1_h, format)
            else:
                SeqIO.write(record, r2_h, format)
                pairs += 1
    return pairs

Even/odd splitting only stays correct if the interleaved file has perfect alternation. If an upstream per-read filter left an orphan in the file, every record after it lands in the wrong output. Guard by checking mate_key equality between each even/odd pair after splitting, or deinterleave with a name check.

Paired Statistics

Count and Verify Pairs

def paired_stats(r1_file, r2_file):
    r1_count = sum(1 for _ in SeqIO.parse(r1_file, 'fastq'))
    r2_count = sum(1 for _ in SeqIO.parse(r2_file, 'fastq'))

    if r1_count != r2_count:
        print(f'WARNING: Unequal counts! R1={r1_count}, R2={r2_count} -> files are desynced')
    else:
        print(f'Pairs: {r1_count}, total reads: {r1_count * 2}')
    return r1_count, r2_count

Paired Quality Summary

def paired_quality_summary(r1_file, r2_file):
    r1_quals, r2_quals = [], []
    for r1, r2 in zip(SeqIO.parse(r1_file, 'fastq'), SeqIO.parse(r2_file, 'fastq')):
        r1_quals.append(sum(r1.letter_annotations['phred_quality']) / len(r1.seq))
        r2_quals.append(sum(r2.letter_annotations['phred_quality']) / len(r2.seq))
    print(f'R1 mean quality: {sum(r1_quals)/len(r1_quals):.1f}')
    print(f'R2 mean quality: {sum(r2_quals)/len(r2_quals):.1f}')

R2 commonly shows lower mean quality than R1 (the reverse read is sequenced later in the run); a modest R1/R2 gap is expected, not a defect.

Find Paired Files

Auto-Detect R2 from R1

from pathlib import Path

def find_r2(r1_path):
    r1_path = Path(r1_path)
    name = r1_path.name
    patterns = [('_R1', '_R2'), ('_R1_', '_R2_'), ('.R1.', '.R2.'), ('_1', '_2')]

    for p1, p2 in patterns:
        if p1 in name:
            r2_path = r1_path.parent / name.replace(p1, p2, 1)
            if r2_path.exists():
                return r2_path
    return None

Order matters: test the specific _R1 patterns before the bare _1, otherwise sample_R1.fastq would match _1 and produce sample_R2.fastq only by luck. replace(..., 1) replaces the first occurrence only, so a sample named sample_R1_lane_R1.fastq swaps just the first token.

Compressed Paired Files

import gzip

def iterate_gzipped_pairs(r1_gz, r2_gz):
    with gzip.open(r1_gz, 'rt') as r1_h, gzip.open(r2_gz, 'rt') as r2_h:
        for r1, r2 in zip(SeqIO.parse(r1_h, 'fastq'), SeqIO.parse(r2_h, 'fastq')):
            yield r1, r2

Use text mode 'rt', not 'rb', when handing a gzip handle to SeqIO.parse; the parser expects decoded text.

Common Errors

Related Skills

• read-sequences - Parse individual FASTQ files and choose parse vs index
• fastq-quality - Phred encoding and quality interpretation before paired filtering
• filter-sequences - Single-file filtering criteria (apply to both mates here)
• compressed-files - gzip vs BGZF handling for paired files
• read-qc/quality-reports - FastQC/MultiQC per-mate quality assessment
• alignment-files/sam-bam-basics - After filtering, align paired reads with bwa mem; proper pairs in BAM