Version Compatibility

Reference examples tested with: BioPython 1.83+, NCBI Datasets CLI 16.0+, Entrez Direct 21.0+

Before using code patterns, verify installed versions match. If versions differ:

• Python: pip show biopython then help(Bio.Entrez.efetch) to check signatures
• CLI: datasets --version and efetch -version

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Batch Downloads

"Download N thousand records from NCBI without getting blocked" -> The right answer is rarely "parallelize requests". For >5000 records the answer is the history server: search once, fetch in chunks server-side. For >100,000 records or whole genomes, the modern answer is NCBI Datasets v2 CLI -- the E-utilities are not optimized for bulk genome/gene data anymore.

This skill encodes (a) when to use each retrieval strategy, (b) the precise rate-limit math, (c) WebEnv lifecycle for long-running jobs, (d) how to design retry/resume, and (e) when to defect to Datasets CLI instead.

• Python: Entrez.esearch(usehistory='y') + chunked Entrez.efetch() (BioPython)
• CLI: datasets download genome accession ... (NCBI Datasets v2 -- preferred for genome/gene bulk)
• CLI: epost | efetch -mode webenv (Entrez Direct)

Required Setup

from Bio import Entrez
import time
Entrez.email = 'researcher@institution.edu'
Entrez.api_key = 'YOUR_KEY'  # 3 -> 10 req/sec; mandatory for bulk
Entrez.tool = 'project-name'

Decision matrix: which retrieval strategy?

Record count	Source	Strategy	Why
< 200 known IDs	Any db	EFetch with comma-joined id=	Single round-trip; trivial
200-5,000 known IDs	Any db	EPost (chunked at 200) -> history -> chunked EFetch	URL length limit + chunked retrieval
5,000-100,000 from a query	Any db	ESearch with usehistory='y' -> chunked EFetch	Push to server once; pull in batches
> 100,000 sequences	nucleotide/protein	Consider FTP mirror or Datasets CLI; chunk if E-utils still	NCBI throttles bulk; offline mirror is faster
Whole genome assemblies	Assembly/Datasets	datasets download genome accession ...	Datasets v2 is the modern bulk endpoint
All RefSeq for a species	Datasets	datasets download genome taxon ...	Replaces assembly_summary.txt scraping
All gene records for a list	Datasets	datasets download gene gene-id ...	Cleaner output than EFetch gene XML
Raw sequencing reads	SRA	prefetch + fasterq-dump (or ENA mirror)	See sra-data skill

The Datasets CLI is the right answer for any genome- or gene-centric bulk workflow as of 2023+. The E-utilities remain right for PubMed, ESummary metadata, custom queries, and anything not in the Datasets API. See ncbi-datasets-cli skill.

Rate-limit math (precise)

Auth	req/sec	Sleep between calls	Bulk-friendly notes
Email only	3	0.34 s	Single-threaded only; parallelism violates ToS
Email + API key	10	0.10 s	Modest parallelism (max ~4 workers) safe
Institutional bulk	Negotiated	Email eutilities@ncbi.nlm.nih.gov	For >100K queries; courtesy expected

NCBI's terms ask that heavy automated downloads run outside US weekday business hours (9 AM-5 PM ET). Cron the job for nights/weekends; pipelines that ignore this get IP-throttled.

Critical: parallelizing API calls is the WRONG bulk strategy. One stream with history server + larger batches is faster AND more polite than N parallel streams. The bottleneck is rarely NCBI's throughput at small N -- it's the round-trip count.

History server lifecycle (the long-running-job trap)

Property	Value	Failure mode
TTL	8 hours absolute (per NCBI E-utils help)	Job started Friday evening dies Saturday morning
Idle eviction	~15 min empirically under load	A worker that stalls loses its WebEnv
Per-session isolation	One WebEnv string per session	Don't share across processes if isolation matters
Expired session behavior	HTTP 200 with <ERROR>WebEnv not found</ERROR>	Won't surface as HTTP error -- must parse body
Recovery	Re-run ESearch; resume at retstart	Need to checkpoint progress to disk

Production pattern: checkpoint the retstart cursor after each successful chunk to disk; on restart, re-run ESearch (cheap), pick up retstart from checkpoint, continue.

EPost specifics

EPost pushes a list of UIDs to the history server so downstream EFetch can pull by WebEnv/QueryKey instead of by ID. Two constraints:

• 200 IDs per EPost call is the hard limit.
• Chained posts share a WebEnv: pass the WebEnv from the first call into subsequent calls to accumulate IDs under one session; a new QueryKey is issued per call.

To intersect: term=#{key1} AND #{key2} against the WebEnv produces a new key.

Batch size guidelines per rettype

Database	rettype	Optimal batch	Per-record payload
nucleotide	fasta	500-1000	~1 KB
nucleotide	gb	100-200	~10-50 KB
protein	fasta	500-1000	~0.5 KB
protein	gp	100-200	~5-30 KB
pubmed	medline	1000-2000	~2 KB
pubmed	xml	200-500	~10-30 KB
any	esummary (docsum)	500 per call	~1 KB

Smaller batches for GenBank/XML because per-record payload is larger; larger batches for FASTA because the per-call HTTP overhead dominates.

Code patterns

Production batch fetch (history server + retry + checkpoint)

Goal: Download all records matching a query, robust to mid-job failures and session expiry.

Approach: ESearch with history; checkpoint cursor to disk; on error, retry the chunk; on session expiry, re-run ESearch and resume from checkpoint.

Reference (BioPython 1.83+):

import json
import time
from pathlib import Path
from urllib.error import HTTPError
from Bio import Entrez


def checkpointed_batch_download(db, term, out_path, ckpt_path, rettype='fasta',
                                 retmode='text', batch_size=500, max_retries=3):
    '''Download all matching records with disk checkpoint for resumability.'''
    delay = 0.1 if Entrez.api_key else 0.34
    ckpt = Path(ckpt_path)
    start = json.loads(ckpt.read_text())['start'] if ckpt.exists() else 0

    h = Entrez.esearch(db=db, term=term, usehistory='y', retmax=0)
    s = Entrez.read(h); h.close()
    webenv, query_key, total = s['WebEnv'], s['QueryKey'], int(s['Count'])
    print(f'{total:,} records matched; resuming at {start:,}')

    mode = 'a' if start else 'w'
    with open(out_path, mode) as out:
        while start < total:
            for attempt in range(max_retries):
                try:
                    h = Entrez.efetch(db=db, rettype=rettype, retmode=retmode,
                                      retstart=start, retmax=batch_size,
                                      webenv=webenv, query_key=query_key)
                    body = h.read(); h.close()
                    if isinstance(body, bytes):
                        body = body.decode('utf-8', errors='replace')
                    if '<ERROR>' in body[:500]:
                        raise RuntimeError(f'Server error in body: {body[:200]}')
                    out.write(body)
                    break
                except HTTPError as e:
                    if e.code == 429:
                        wait = 10 * (attempt + 1)
                        print(f'  Rate-limited; sleeping {wait}s')
                        time.sleep(wait)
                    elif attempt == max_retries - 1:
                        raise
                    else:
                        time.sleep(5 * (attempt + 1))
                except RuntimeError as e:
                    # Likely WebEnv expired; re-run ESearch
                    print(f'  {e}; refreshing WebEnv')
                    h = Entrez.esearch(db=db, term=term, usehistory='y', retmax=0)
                    s = Entrez.read(h); h.close()
                    webenv, query_key = s['WebEnv'], s['QueryKey']

            start += batch_size
            ckpt.write_text(json.dumps({'start': start, 'total': total}))
            time.sleep(delay)
            print(f'  {min(start, total):,}/{total:,}')
    ckpt.unlink(missing_ok=True)

EPost large ID list, then EFetch

Goal: Download by a known list of 5,000 accessions without 414 URI errors.

Approach: EPost in 200-ID chunks; reuse WebEnv across chunks; final fetch reads from history.

Reference (BioPython 1.83+):

def epost_and_fetch(db, ids, out_path, rettype='fasta', retmode='text', batch_size=500):
    delay = 0.1 if Entrez.api_key else 0.34
    webenv = None
    posted_keys = []  # (query_key, n_ids) so we iterate each key's actual size
    for i in range(0, len(ids), 200):
        chunk = ids[i:i+200]
        kwargs = {'db': db, 'id': ','.join(chunk)}
        if webenv:
            kwargs['WebEnv'] = webenv
        h = Entrez.epost(**kwargs)
        r = Entrez.read(h); h.close()
        webenv = r['WebEnv']
        posted_keys.append((r['QueryKey'], len(chunk)))
        time.sleep(delay)

    with open(out_path, 'w') as out:
        for qk, n in posted_keys:
            for start in range(0, n, batch_size):
                h = Entrez.efetch(db=db, rettype=rettype, retmode=retmode,
                                  retstart=start, retmax=min(batch_size, n - start),
                                  webenv=webenv, query_key=qk)
                out.write(h.read()); h.close()
                time.sleep(delay)

Integrity check after download

Goal: Confirm downloaded FASTA has the expected record count and no truncation.

Approach: Count expected (from ESearch Count) vs observed (from SeqIO.parse).

from Bio import SeqIO

def verify_fasta_count(path, expected):
    observed = sum(1 for _ in SeqIO.parse(path, 'fasta'))
    assert observed == expected, f'Expected {expected:,} records, found {observed:,}'
    return True

For genome assemblies and known-checksum files, NCBI provides MD5 manifests (e.g. md5checksums.txt in FTP genome directories). NCBI Datasets CLI verifies checksums automatically; the FTP-direct route needs explicit md5sum -c.

Compare E-utils to Datasets CLI cost

def estimate_efetch_calls(total, batch_size):
    return -(-total // batch_size)  # ceiling division

For 100,000 nucleotide records at 500/batch with API key: 200 calls * 0.1s = 20s minimum. For the same workflow via datasets download gene gene-id 100000: one CLI invocation, parallel download, automatic checksum. For genome-scale bulk, Datasets wins by an order of magnitude.

Parallelization design (modest)

Goal: Pull from two independent queries concurrently without violating rate limits.

Approach: Async with a global semaphore that enforces the API-key-permitted rate. Max 4 concurrent workers is the polite cap.

import asyncio
from asyncio import Semaphore

# Pseudo-pattern; real impl needs aiohttp + Bio.Entrez async wrappers
async def fetch_with_semaphore(sem, db, id_, rettype):
    async with sem:
        # call EFetch
        await asyncio.sleep(0.1)  # rate gate
        # ... actual call

sem = Semaphore(4)

Never exceed 4 concurrent workers with an API key, or 1 without. Above that NCBI throttles by IP and the whole pipeline grinds.

Failure modes

Session expires mid-pipeline

• Trigger: Job runs >8h or worker idles >15 min.
• Mechanism: WebEnv evicted; EFetch returns HTTP 200 with <ERROR>WebEnv not found</ERROR> body.
• Symptom: Silently truncated output mid-file; downstream parsing fails on empty chunks.
• Fix: Parse body for <ERROR>; re-run ESearch and resume at checkpointed retstart.

URL too long on >200 IDs

• Trigger: Comma-joined id= to EFetch with 250+ IDs.
• Mechanism: GET URL exceeds NCBI's ~2000 char limit.
• Symptom: HTTP 414 URI Too Long, or silent truncation.
• Fix: EPost in chunks of 200 first, then EFetch by WebEnv/QueryKey.

Rate-limit cascade

• Trigger: Parallelizing without API key; or >10 req/s with key.
• Mechanism: NCBI returns 429; aggressive retry triggers IP-level throttle.
• Symptom: Pipeline gets slower and eventually stops.
• Fix: Add jittered exponential backoff; reduce concurrency; reach out for institutional access if bulk is the norm.

Datasets / E-utils confusion

• Trigger: Building a custom assembly_summary.txt scraper instead of using Datasets.
• Mechanism: Datasets API is the official, supported bulk endpoint for genome/gene data; E-utils is not optimized for it.
• Symptom: Slow downloads, stale snapshots, missing fields.
• Fix: Use datasets download genome ... for genomes; datasets download gene ... for gene records. See ncbi-datasets-cli.

Silent retmax cap

• Trigger: ESearch without usehistory='y'; Count > 9999.
• Mechanism: Legacy esearch enforces 9999 cap; the rest of the result set is silently dropped.
• Symptom: Batch loop terminates early; missing thousands of records.
• Fix: Always set usehistory='y' for any query expected to return >5000.

Checkpoint corruption / partial chunk

• Trigger: Job crashes mid-chunk; checkpoint hasn't been written.
• Mechanism: Output file has half a record at the end.
• Symptom: SeqIO.parse fails on the partial record.
• Fix: Write checkpoint AFTER successful chunk write + file flush; on resume, truncate the output file at the last newline before continuing.

Common errors

Error / symptom	Cause	Solution
HTTPError 429	Rate limit	Sleep with backoff; get API key
HTTPError 414	URL too long	EPost first
<ERROR>WebEnv not found</ERROR> (HTTP 200)	Session expired	Re-run ESearch; resume at checkpoint
Output file ends mid-record	Crash mid-chunk	Truncate-to-newline on resume
Slow despite API key	Too few records per call	Increase batch_size to 500+ for FASTA
Datasets CLI faster than EFetch	Workflow is genome/gene bulk	Switch to ncbi-datasets-cli

References

• Sayers EW et al. (2024) Database resources of the National Center for Biotechnology Information in 2024. Nucleic Acids Res 52:D33-D43.
• Kans J. (2024) Entrez Direct: E-utilities on the Unix Command Line. NCBI Bookshelf NBK179288.
• NCBI. EPost help and Usage Guidelines. NBK25499.
• NCBI Datasets documentation: https://www.ncbi.nlm.nih.gov/datasets/docs/v2/

Related Skills

• entrez-search - Build the query that batch-downloads will fetch
• entrez-fetch - Single-record EFetch and ESummary
• entrez-link - Chain ELink with neighbor_history for cross-db bulk
• ncbi-datasets-cli - Modern bulk endpoint for genome/gene data; preferred over E-utils for that scope
• sra-data - Raw read downloads via SRA toolkit (not via E-utilities)
• geo-data - GEO supplementary file downloads