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Entrez Search

Search NCBI databases using Biopython's Entrez module (ESearch, EInfo, EGQuery utilities).

Required Setup

python
from Bio import Entrez

Entrez.email = 'your.email@example.com'  # Required by NCBI
Entrez.api_key = 'your_api_key'          # Optional, raises rate limit 3->10 req/sec

Core Functions

Entrez.esearch() - Search a Database

Search any NCBI database and get matching record IDs.

python
handle = Entrez.esearch(db='nucleotide', term='human[orgn] AND BRCA1[gene]')
record = Entrez.read(handle)
handle.close()

print(f"Found {record['Count']} records")
print(f"IDs: {record['IdList']}")  # First 20 IDs by default

Key Parameters: | Parameter | Description | Default | |-----------|-------------|---------| | db | Database to search | Required | | term | Search query | Required | | retmax | Max IDs to return | 20 | | retstart | Starting index (pagination) | 0 | | usehistory | Store results on server | 'n' | | sort | Sort order | database-specific | | datetype | Date field to search | 'pdat' | | reldate | Records from last N days | None | | mindate | Start date (YYYY/MM/DD) | None | | maxdate | End date (YYYY/MM/DD) | None |

ESearch Result Fields:

python
record['Count']        # Total matching records (string)
record['IdList']       # List of record IDs
record['RetMax']       # Number of IDs returned
record['RetStart']     # Starting index
record['QueryKey']     # For history server (if usehistory='y')
record['WebEnv']       # For history server (if usehistory='y')
record['TranslationSet']  # Query translations applied
record['QueryTranslation']  # Final translated query

Entrez.einfo() - Database Information

Get information about available databases or specific database fields.

python
# List all available databases
handle = Entrez.einfo()
record = Entrez.read(handle)
handle.close()
print(record['DbList'])  # ['pubmed', 'protein', 'nucleotide', ...]

# Get info about specific database
handle = Entrez.einfo(db='nucleotide')
record = Entrez.read(handle)
handle.close()

print(f"Description: {record['DbInfo']['Description']}")
print(f"Record count: {record['DbInfo']['Count']}")

# List searchable fields
for field in record['DbInfo']['FieldList']:
    print(f"{field['Name']}: {field['Description']}")

Database Info Fields:

python
record['DbInfo']['DbName']       # Database name
record['DbInfo']['Description']  # Database description
record['DbInfo']['Count']        # Total records in database
record['DbInfo']['LastUpdate']   # Last update date
record['DbInfo']['FieldList']    # Searchable fields
record['DbInfo']['LinkList']     # Available links to other databases

Entrez.egquery() - Global Query

Search across all NCBI databases simultaneously.

python
handle = Entrez.egquery(term='CRISPR')
record = Entrez.read(handle)
handle.close()

for result in record['eGQueryResult']:
    if int(result['Count']) > 0:
        print(f"{result['DbName']}: {result['Count']} records")

Search Query Syntax

NCBI uses a specific query syntax:

Field Tags

python
# Search specific fields using [field_name]
term = 'BRCA1[gene]'                    # Gene name field
term = 'human[orgn]'                    # Organism field
term = 'Homo sapiens[ORGN]'             # Full organism name
term = 'NM_007294[accn]'                # Accession number
term = 'Smith J[auth]'                  # Author (PubMed)
term = 'Nature[jour]'                   # Journal (PubMed)
term = '1000:5000[slen]'                # Sequence length range
term = 'mRNA[fkey]'                     # Feature key

Boolean Operators

python
term = 'BRCA1 AND human'                # Both terms
term = 'cancer OR tumor'                # Either term
term = 'human NOT mouse'                # Exclude term
term = '(BRCA1 OR BRCA2) AND human'     # Grouping

Date Ranges

python
# Using date parameters
handle = Entrez.esearch(
    db='pubmed',
    term='CRISPR',
    datetype='pdat',     # Publication date
    mindate='2023/01/01',
    maxdate='2024/12/31'
)

# Or in query string
term = 'CRISPR AND 2024[pdat]'
term = 'CRISPR AND 2023:2024[pdat]'

Wildcards and Phrases

python
term = 'immun*'                         # Wildcard
term = '"breast cancer"[title]'         # Exact phrase

Common Databases

| Database | db value | Common Fields | |----------|------------|---------------| | PubMed | pubmed | [auth], [title], [jour], [pdat] | | Nucleotide | nucleotide | [orgn], [gene], [accn], [slen] | | Protein | protein | [orgn], [gene], [accn], [molwt] | | Gene | gene | [orgn], [sym], [chr] | | SRA | sra | [orgn], [platform], [strategy] | | Taxonomy | taxonomy | [scin], [comn], [rank] | | Assembly | assembly | [orgn], [level], [refseq] |

Code Patterns

Basic Search with Pagination

python
from Bio import Entrez

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

def search_ncbi(db, term, max_results=100):
    handle = Entrez.esearch(db=db, term=term, retmax=max_results)
    record = Entrez.read(handle)
    handle.close()
    return record['IdList'], int(record['Count'])

ids, total = search_ncbi('nucleotide', 'human[orgn] AND insulin[gene]')
print(f'Retrieved {len(ids)} of {total} total records')

Paginated Search for Large Results

python
def search_all_ids(db, term, batch_size=10000):
    all_ids = []
    handle = Entrez.esearch(db=db, term=term, retmax=0)
    record = Entrez.read(handle)
    handle.close()
    total = int(record['Count'])

    for start in range(0, total, batch_size):
        handle = Entrez.esearch(db=db, term=term, retstart=start, retmax=batch_size)
        record = Entrez.read(handle)
        handle.close()
        all_ids.extend(record['IdList'])

    return all_ids

Search with History Server (for Large Results)

python
# Store results on NCBI server for subsequent fetching
handle = Entrez.esearch(db='nucleotide', term='human[orgn] AND mRNA[fkey]', usehistory='y')
record = Entrez.read(handle)
handle.close()

webenv = record['WebEnv']
query_key = record['QueryKey']
total = int(record['Count'])

# Use webenv and query_key with efetch for batch downloads
# See batch-downloads skill for details

Recent Records Only

python
# Records from last 30 days
handle = Entrez.esearch(db='pubmed', term='CRISPR', reldate=30, datetype='pdat')
record = Entrez.read(handle)
handle.close()

Get Available Fields for a Database

python
def get_search_fields(db):
    handle = Entrez.einfo(db=db)
    record = Entrez.read(handle)
    handle.close()
    return [(f['Name'], f['Description']) for f in record['DbInfo']['FieldList']]

fields = get_search_fields('nucleotide')
for name, desc in fields[:10]:
    print(f'{name}: {desc}')

Check Query Translation

python
handle = Entrez.esearch(db='nucleotide', term='human BRCA1')
record = Entrez.read(handle)
handle.close()

# See how NCBI interpreted your query
print(f"Your query was translated to: {record['QueryTranslation']}")
# e.g., '"homo sapiens"[Organism] AND BRCA1[All Fields]'

Common Errors

| Error | Cause | Solution | |-------|-------|----------| | HTTPError 429 | Rate limit exceeded | Add delays or use API key | | HTTPError 400 | Invalid query syntax | Check field names and operators | | Empty IdList | No matches or typo | Check QueryTranslation field | | RuntimeError | Missing email | Set Entrez.email |

Decision Tree

Need to search NCBI?
├── Finding records in one database?
│   └── Use Entrez.esearch()
├── Search across all databases?
│   └── Use Entrez.egquery()
├── Need database field names?
│   └── Use Entrez.einfo(db='database')
├── List all available databases?
│   └── Use Entrez.einfo() (no db argument)
├── Results > 10,000 records?
│   └── Use usehistory='y', then batch fetch
└── Need to fetch actual records?
    └── See entrez-fetch skill

Related Skills

• entrez-fetch - Retrieve full records after searching
• entrez-link - Find related records in other databases
• batch-downloads - Download large result sets efficiently
• geo-data - Search GEO expression datasets (specialized search)
• blast-searches - Search by sequence similarity instead of keywords

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