pyDAL
last modified January 29, 2024
The pyDAL tutorial shows how to use the pyDAL database abstraction layer to program databases in Python. We use SQLite in our code examples.
pyDAL
pyDAL is a pure Python Database Abstraction Layer. The pyDAL module dynamically generates the SQL in the specified dialect for the database back end. The resulting code will be portable among different types of databases.
pyDAL installation
$ sudo pip3 install pyDAL
We use the pip3 tool to install pyDAL.
pyDAL create database table
In the following example, we create a database table.
#!/usr/bin/python
from pydal import DAL, Field
db = DAL('sqlite://test.db', folder='dbs')
try:
db.define_table('cars', Field('name'), Field('price', type='integer'))
db.cars.insert(name='Audi', price=52642)
db.cars.insert(name='Skoda', price=9000)
db.cars.insert(name='Volvo', price=29000)
db.cars.insert(name='Bentley', price=350000)
db.cars.insert(name='Citroen', price=21000)
db.cars.insert(name='Hummer', price=41400)
db.cars.insert(name='Volkswagen', price=21600)
finally:
if db:
db.close()
The example creates a cars table with seven rows.
db = DAL('sqlite://test.db', folder='dbs')
DAL represents a database connection. It takes a database
connection string as the first parameter. We connect to an SQLite database.
db.define_table('cars', Field('name'), Field('price', type='integer'))
A database table is defined with define_table. It is
created if it does not exist. It has two fields: name and price. An id
field is automatically generated.
db.cars.insert(name='Audi', price=52642)
We insert a new row into the table with insert. The method
is called on the cars table of a db connection.
$ ls dbs c95cf9bab36fcb04c2424cdf9be0f6e3_cars.table sql.log test.db
In addition to the test.db database, we have a migration
file with the .table extension and a log file.
pyDAL drop table
A database table is removed with drop.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
cars = db.define_table('cars', Field('name'), Field('price', 'integer'))
cars.drop()
finally:
if db:
db.close()
In the example, we delete the cars table using the drop
method.
pyDAL select rows
Table rows are selected with select.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
db.define_table('cars', Field('name'), Field('price'))
rows = db().select(db.cars.ALL)
for row in rows:
print("{} {} {}".format(row['id'], row['name'], row['price']))
finally:
if db:
db.close()
In the example, we retrieve all rows from the cars table.
rows = db().select(db.cars.ALL)
We fetch all rows with the select method.
The db.cars.ALL tells to select all columns from the table.
for row in rows:
print("{} {} {}".format(row['id'], row['name'], row['price']))
We go throught each of the rows and print its fields.
$ ./select_all_cars.py 1 Audi 52642 2 Skoda 9000 3 Volvo 29000 4 Bentley 350000 5 Citroen 21000 6 Hummer 41400 7 Volkswagen 21600
pyDAL ordering
The following example shows how to order data with pyDAL.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db')
db.define_table('cars', Field('name'), Field('price', 'integer'))
rows = db(db.cars).select(orderby=db.cars.price)
for row in rows:
print("{} {} {}".format(row['id'], row['name'], row['price']))
print("**************************************")
rows = db(db.cars).select(orderby=~db.cars.price)
for row in rows:
print("{} {} {}".format(row['id'], row['name'], row['price']))
finally:
if db:
db.close()
The example prints all rows from the table and orders them by price in ascending and descending order.
rows = db(db.cars).select(orderby=db.cars.price)
Ordering is done with the orderby parameter of the
select method.
rows = db(db.cars).select(orderby=~db.cars.price)
To order by descending order, we use the tilda character.
$ ./order_by.py 5 Citroen 21000 7 Volkswagen 21600 3 Volvo 29000 4 Bentley 350000 6 Hummer 41400 1 Audi 52642 2 Skoda 9000 ************************************** 2 Skoda 9000 1 Audi 52642 6 Hummer 41400 4 Bentley 350000 3 Volvo 29000 7 Volkswagen 21600 5 Citroen 21000
pyDAL limit data output
The data output can be limited with limitby parameter
of the select method.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
db.define_table('cars', Field('name'), Field('price', 'integer'))
rows = db(db.cars).select(limitby=(2, 5))
for row in rows:
print("{} {} {}".format(row['id'], row['name'], row['price']))
finally:
if db:
db.close()
In the code example, we limit the output to three rows with offset 2.
$ ./limit_by.py 3 Volvo 29000 4 Bentley 350000 5 Citroen 21000
pyDAL count rows
With count, we can get the number of rows in the table.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
db.define_table('cars', Field('name'), Field('price', 'integer'))
n = db(db.cars.id).count()
print("There are {} rows in the table".format(n))
finally:
if db:
db.close()
In the example, we print the number of rows in the cars
table.
$ ./count_rows.py There are 7 rows in the table
We have seven rows in the table.
pyDAL JSON output
We can get the data in JSON format with as_json.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
db.define_table('cars', Field('name'), Field('price', 'integer'))
rows = db(db.cars).select()
print(rows.as_json())
finally:
if db:
db.close()
The example shows all rows in JSON format.
$ ./json_output.py
[{"id": 1, "price": 52642, "name": "Audi"},
{"id": 2, "price": 9000, "name": "Skoda"},
{"id": 3, "price": 29000, "name": "Volvo"},
{"id": 4, "price": 350000, "name": "Bentley"},
{"id": 5, "price": 21000, "name": "Citroen"},
{"id": 6, "price": 41400, "name": "Hummer"},
{"id": 7, "price": 21600, "name": "Volkswagen"}]
pyDAL last SQL
The SQL that was last executed by pyDAL can be found with
_lastsql.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
db.define_table('cars', Field('name'), Field('price', 'integer'))
# we ignore the result
db(db.cars.id).select(db.cars.name, db.cars.price)
print(db._lastsql)
finally:
if db:
db.close()
In the example, we print the SQL executed by pyDAL when doing a select statement.
$ ./lastsql.py
('SELECT "cars"."name", "cars"."price" FROM "cars" WHERE ("cars"."id" IS NOT NULL);', 0.0005686283111572266)
This SQL was generated by pyDAL.
pyDAL execute raw SQL
We can execute raw SQL with the executesql method.
#!/usr/bin/python
from pydal import DAL, Field
try:
db = DAL('sqlite://test.db', folder='dbs')
db.define_table('cars', Field('name'), Field('price', 'integer'))
data = db.executesql('SELECT * FROM cars WHERE id=6')[0]
print(data)
finally:
if db:
db.close()
In the example, we execute an SQL SELECT statement with executesql.
$ ./raw_sql.py (6, 'Hummer', '41400')
Source
In this article we have used pyDAL to work with an SQLite database.
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