Relational Database Access with Python

Relational Database Access
with Python
Why you should be using SQLAlchemy
Mark Rees
CTO
Century Software (M) Sdn. Bhd.

Is This Your Current Relational Database Access
Style?
# Django ORM
>>> from ip2country.models import Ip2Country
>>> Ip2Country.objects.all()
[<Ip2Country: Ip2Country object>, <Ip2Country: Ip2Country
object>, '...(remaining elements truncated)...']
>>> myp = Ip2Country.objects.filter(assigned__year=2015)
... .filter(countrycode2=’MY')
>>> myp[0].ipfrom
736425984.0

Is This Your Current Relational Database Access
Style?
# SQLAlchemy ORM
>>> from sqlalchemy import create_engine, extract
>>> from sqlalchemy.orm import sessionmaker
>>> from models import Ip2Country
>>> engine =
create_engine('postgresql://ip2country_rw:secret@localhost/i
p2country')
>>> Session = sessionmaker(bind=engine)
>>> session = Session()
>>> all_data = session.query(Ip2Country).all()
>>> myp = session.query(Ip2Country).
... filter(extract('year',Ip2Country.assigned) == 2015).
... filter(Ip2Country.countrycode2 == ’MY')
print(myp[0].ipfrom)
736425984.0

SQL Relational Database Access
SELECT * FROM ip2country;
“id”,"ipfrom";"ipto";"registry";"assigned";"countrycode2";"countrycode3";"countryname"
1,1729522688;1729523711;"apnic";"2011-08-05";"CN";"CHN";"China"
2,1729523712;1729524735;"apnic";"2011-08-05";"CN";"CHN";"China”
. . .
SELECT * FROM ip2country
WHERE date_part('year', assigned) = 2015
AND countrycode2 = ’MY';
“id”,"ipfrom";"ipto";"registry";"assigned";"countrycode2";"countrycode3";"countryname"
5217;736425984;736427007;"apnic";"2015-01-13 00:00:00";"MY";"MYS";"Malaysia”
5218;736427008;736428031;"apnic";"2015-01-13 00:00:00";"MY";"MYS";"Malaysia”
. . .
SELECT ipfrom FROM ip2country
WHERE date_part('year', assigned) = 2015
AND countrycode2 = ’MY';
"ipfrom"
736425984
736427008
. . .

Python + SQL == Python DB-API 2.0
• The Python standard for a consistent
interface to relational databases is the
Python DB-API (PEP 249)
• The majority of Python database interfaces
adhere to this standard

Python DB-API Connection Object
Access the database via the connection object
• Use connect constructor to create a
connection with database
conn = psycopg2.connect(parameters…)
• Create cursor via the connection
cur = conn.cursor()
• Transaction management (implicit begin)
conn.commit()
conn.rollback()
• Close connection (will rollback current
transaction)
conn.close()
• Check module capabilities by globals
psycopg2.apilevel psycopg2.threadsafety
psycopg2.paramstyle

Python DB-API Cursor Object
A cursor object is used to represent a database
cursor, which is used to manage the context of
fetch operations.
• Cursors created from the same connection
are not isolated
cur = conn.cursor()
cur2 = conn.cursor()
• Cursor methods
cur.execute(operation, parameters)
cur.executemany(op,seq_of_parameters)
cur.fetchone()
cur.fetchmany([size=cursor.arraysize])
cur.fetchall()
cur.close()

Python DB-API Cursor Object
• Optional cursor methods
cur.scroll(value[,mode='relative'])
cur.next()
cur.callproc(procname[,parameters])
cur.__iter__()
• Results of an operation
cur.description
cur.rowcount
cur.lastrowid
• DB adaptor specific “proprietary” cursor
methods

Python DB-API Parameter Styles
Allows you to keep SQL separate from parameters
Improves performance & security
Warning Never, never, NEVER use Python string
concatenation (+) or string parameters
interpolation (%) to pass variables to a SQL query
string. Not even at gunpoint.
From http://initd.org/psycopg/docs/usage.html#query-parameters

Python DB-API Parameter Styles
Global paramstyle gives supported style for the
adaptor
qmark Question mark style
WHERE countrycode2 = ?
numeric Numeric positional style
WHERE countrycode2 = :1
named Named style
WHERE countrycode2 = :code
format ANSI C printf format style
WHERE countrycode2 = %s
pyformat Python format style
WHERE countrycode2 = %(name)s

Python + SQL: INSERT
import csv, datetime, psycopg2
conn = psycopg2.connect("dbname=ip2countrydb user=ip2country_rw
password=secret")
cur = conn.cursor()
with open("IpToCountry.csv", "rt") as f:
reader = csv.reader(f)
try:
for row in reader:
if row[0][0] != "#":
row[3] =
datetime.datetime.utcfromtimestamp(float(row[3]))
cur.execute("""INSERT INTO ip2country(
ipfrom, ipto, registry, assigned,
countrycode2, countrycode3, countryname)
VALUES (%s, %s, %s, %s, %s, %s, %s)""", row)
except (Exception) as error:
print(error)
conn.rollback()
else:
conn.commit()
finally:
cur.close()
conn.close()

Python + SQL: SELECT
# Find ipv4 address ranges assigned to Malaysia
import psycopg2, socket, struct
def num_to_dotted_quad(n):
"""convert long int to dotted quad string
http://code.activestate.com/recipes/66517/"""
return socket.inet_ntoa(struct.pack('!L', n))
conn = psycopg2.connect("dbname=ip2countrydb user=ip2country_rw
password=secret")
cur = conn.cursor()
cur.execute("""SELECT * FROM ip2country
WHERE countrycode2 = 'MY'
ORDER BY ipfrom""")
for row in cur:
print("%s - %s" % (num_to_dotted_quad(int(row[0])),
num_to_dotted_quad(int(row[1]))))

SQLite
• sqlite3
• CPython 2.5 & 3
• DB-API 2.0
• Part of CPython distribution since 2.5

PostgreSQL
• psycopg
• CPython 2 & 3
• DB-API 2.0, level 2 thread safe
• Appears to be most popular
• http://initd.org/psycopg/
• py-postgresql
• CPython 3
• DB-API 2.0
• Written in Python with optional C
optimizations
• pg_python - console
• http://python.projects.postgresql.org/

PostgreSQL
• PyGreSQL
• CPython 2.5+
• Classic & DB-API 2.0 interfaces
• http://www.pygresql.org/
• pyPgSQL
• CPython 2
• Classic & DB-API 2.0 interfaces
• http://pypgsql.sourceforge.net/
• Last release 2006

PostgreSQL
• pypq
• CPython 2.7 & pypy 1.7+
• Uses ctypes
• DB-API 2.0 interface
• psycopg2-like extension API
• https://bitbucket.org/descent/pypq
• psycopg2cffi
• CPython 2.6+ & pypy 2.0+
• Uses cffi
• psycopg2 compat layer
• https://github.com/chtd/psycopg2cffi

MySQL
• MySQL-python
• CPython 2.3+
• http://sourceforge.net/projects/mysql-
python/
• PyMySQL
• CPython 2.4+ & 3
• Pure Python DB-API 2.0 interface
• http://www.pymysql.org/
• MySQL-Connector
• CPython 2.4+ & 3
• Pure Python DB-API 2.0 interface
• https://launchpad.net/myconnpy

Other “Enterprise” Databases
• cx_Oracle
• CPython 2 & 3
• http://cx-oracle.sourceforge.net/
• informixda
• CPython 2
• http://informixdb.sourceforge.net/
• Last release 2007
• Ibm-db
• CPython 2
• DB-API 2.0 for DB2 & Informix
• http://code.google.com/p/ibm-db/

ODBC
• mxODBC
• CPython 2.3+
• DB-API 2.0 interfaces
• http://www.egenix.com/products/pytho
n/mxODBC/doc
• Commercial product
• PyODBC
• CPython 2 & 3
• DB-API 2.0 interfaces with extensions
• https://github.com/mkleehammer/pyod
bc
• ODBC interfaces not limited to Windows
thanks to iODBC and unixODBC

Jython + SQL
• zxJDBC
• DB-API 2.0 Written in Java using JDBC API
so can utilize JDBC drivers
• Support for connection pools and JNDI
lookup
• Included with standard Jython
installation http://www.jython.org/
• jyjdbc
• DB-API 2.0 compliant
• Written in Python/Jython so can utilize
JDBC drivers
• Decimal data type support
• https://bitbucket.org/clach04/jyjdbc/

IronPython + SQL
• adodbapi
• IronPython 2+
• Also works with CPython 2.3+ with
pywin32
• http://adodbapi.sourceforge.net/

Gerald, the half a schema
import gerald
s1 = gerald.PostgresSchema(’public',
'postgres://ip2country_rw:secret@localhost/ip2country')
s2 = gerald.PostgresSchema(’public',
'postgres://ip2country_rw:secret@localhost/ip2countryv4')
print s1.schema['ip2country'].compare(s2.schema['ip2country'])
DIFF: Definition of assigned is different
DIFF: Column countryname not in ip2country
DIFF: Definition of registry is different
DIFF: Column countrycode3 not in ip2country
DIFF: Definition of countrycode2 is different
• Database schema toolkit
• via DB-API currently supports
• PostgreSQL
• MySQL
• Oracle
• http://halfcooked.com/code/gerald/

SQLPython
$ sqlpython --postgresql ip2country ip2country_rw
Password:
0:ip2country_rw@ip2country> select * from ip2country where countrycode2='SG';
...
1728830464.0 1728830719.0 apnic 2011-11-02 SG SGP Singapore
551 rows selected.
0:ip2country_rw@ip2country> select * from ip2country where countrycode2='SG'j
[...
{"ipfrom": 1728830464.0, "ipto": 1728830719.0, "registry": "apnic”,"assigned":
"2011-11-02", "countrycode2": "SG", "countrycode3": "SGP", "countryname":
"Singapore"}]
• A command-line interface to relational
databases
• via DB-API currently supports
• PostgreSQL
• MySQL
• Oracle
• http://packages.python.org/sqlpython/

SQLPython, batteries included
0:ip2country_rw@ip2country> select * from ip2country where countrycode2 =’MY’;
...
1728830464.0 1728830719.0 apnic 2011-11-02 MY MYS Malaysia
551 rows selected.
0:ip2country_rw@ip2country> py
Python 2.6.6 (r266:84292, May 20 2011, 16:42:25)
[GCC 4.4.5 20110214 (Red Hat 4.4.5-6)] on linux2
py <command>: Executes a Python command.
py: Enters interactive Python mode.
End with `Ctrl-D` (Unix) / `Ctrl-Z` (Windows), `quit()`, 'exit()`.
Past SELECT results are exposed as list `r`;
most recent resultset is `r[-1]`.
SQL bind, substitution variables are exposed as `binds`, `substs`.
Run python code from external files with ``run("filename.py")``
>>> r[-1][-1]
(1728830464.0, 1728830719.0, 'apnic', datetime.date(2011, 11, 2), ’MY', ’MYS',
’Malaysia')
>>> import socket, struct
>>> def num_to_dotted_quad(n):
... return socket.inet_ntoa(struct.pack('!L',n))
...
>>> num_to_dotted_quad(int(r[-1][-1].ipfrom))
'103.11.220.0'

SpringPython – Database Templates
# Find ipv4 address ranges assigned to Malaysia
# using SpringPython DatabaseTemplate & DictionaryRowMapper
from springpython.database.core import *
from springpython.database.factory import *
conn_factory = PgdbConnectionFactory(
user="ip2country_rw", password="secret",
host="localhost", database="ip2countrydb")
dt = DatabaseTemplate(conn_factory)
results = dt.query(
"SELECT * FROM ip2country WHERE countrycode2=%s",
(”MY",), DictionaryRowMapper())
for row in results:
print("%s - %s" % (num_to_dotted_quad(int(row['ipfrom'])),
num_to_dotted_quad(int(row['ipto']))))

SQLAlchemy
http://www.sqlalchemy.org/
First release in 2005
Now at version 1.0.8
What is it
• Provides helpers, tools & components to
assist with database access
• Provides a consisdent and full featured
façade over the Python DBAPI
• Provides an optional object relational
mapper(ORM)
• Foundation for many Python third party
libraries & tools
• It doesn’t hide the database, you need
understand SQL

SQLAlchemy Core – The Engine
from sqlalchemy import create_engine
engine =
create_engine('postgresql://ip2country_rw:secret@localhost/i
p2countrydb')
engine.execute("""
create table registry (
id serial primary key,
name text
)
""")
engine.execute("""
insert into registry(name) values('apnic')
""")
engine.execute("""
insert into registry(name) values('aprn')
""”)
engine.execute("""
insert into registry(name) values('lacnic')
""")

SQLAlchemy Core – SQL Expression Language
from sqlalchemy import create_engine, Table, Column, Integer, String,
MetaData
engine =
create_engine('postgresql://ip2country_rw:secret@localhost/ip2countrydb',
echo=True)
metadata = MetaData()
registry = Table('registry', metadata,
Column('id', Integer,
autoincrement=True,
primary_key=True),
Column('name', String(10)))
metadata.create_all(engine) # create table if it doesn't exist
# auto construct insert statement with binding parameters
ins = registry.insert().values(name='dummy’)
conn = engine.connect() # get database connection
# insert multiple rows with explicit commit
conn.execute(ins, [{'name': 'apnic'},
{'name': 'aprn'}, {'name': 'lacnic'}])

SQLAlchemy Core – SQL Expression Language
from sqlalchemy import create_engine, Table, Column, Integer, String,
MetaData
from sqlalchemy.sql import select
engine =
echo=True)
metadata = MetaData()
registry = Table('registry', metadata,
Column('id', Integer, autoincrement=True,
primary_key=True,
Column('name', String(10)))
# auto create select statement
s = select([registry])
conn = engine.connect()
result = conn.execute(s)
for row in result:
print(row)

SQLAlchemy ORM
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import create_engine, Table, Column, Integer, String
Base = declarative_base()
class Registry(Base):
__tablename__ = 'registry'
id = Column(Integer, autoincrement=True, primary_key=True)
name = Column(String(10))
def __repr__(self):
return "<Registry(%r, %r)>" % (
self.id, self.name
)
engine =
echo=True)
Base.metadata.create_all(engine)
from sqlalchemy.orm import Session
session = Session(bind=engine)
apnic = session.query(Registry).filter_by(name='apnic').first()
print(apnic)

SQLAlchemy ORM
. . .
Base = declarative_base()
class Registry(Base):
__tablename__ = 'registry'
id = Column(Integer, autoincrement=True, primary_key=True)
name = Column(String(10))
def __repr__(self):
return "<Registry(%r, %r)>" % (
self.id, self.name
)
engine =
echo=True)
Base.metadata.create_all(engine)
from sqlalchemy.orm import Session
session = Session(bind=engine)
mynic = Registry(name='mynic')
session.add(mynic)

DB-API 2.0 PEP
http://www.python.org/dev/peps/pep-0249/
Travis Spencer’s DB-API UML Diagram
http://travisspencer.com/
Andrew Kuchling's introduction to the DB-API
http://www.amk.ca/python/writing/DB-API.html
Attributions

Andy Todd’s OSDC paper
http://halfcooked.com/presentations/osdc2006/p
ython_databases.html
Source of csv data used in examples from
WebNet77 licensed under GPLv3
http://software77.net/geo-ip/
Attributions

Mark Rees
mark at censof dot com
+Mark Rees
@hexdump42
hex-dump.blogspot.com
Contact Details

Relational Database Access with Python

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