Databases

Below the Model level, peewee uses an abstraction for representing the database. The Database is responsible for establishing and closing connections, making queries, and gathering information from the database.

The Database in turn uses another abstraction called an Adapter, which is backend-specific and encapsulates functionality specific to a given db driver. Since there is some difference in column types across database engines, this information also resides in the adapter. The adapter is responsible for smoothing out the quirks of each database driver to provide a consistent interface, for example sqlite uses the question-mark ”?” character for parameter interpolation, while all the other backends use “%s”.

For a high-level overview of working with transactions, check out the transactions cookbook.

For notes on deferring instantiation of database, for example if loading configuration at run-time, see the notes on deferring initialization.

Note

The internals of the Database and BaseAdapter will be of interest to anyone interested in adding support for another database driver.

Database and its subclasses

class Database

A high-level api for working with the supported database engines. Database provides a wrapper around some of the functions performed by the Adapter, in addition providing support for:

  • execution of SQL queries
  • creating and dropping tables and indexes
__init__(adapter, database[, threadlocals=False[, autocommit=True[, **connect_kwargs]]])
Parameters:
  • adapter – an instance of a BaseAdapter subclass
  • database – the name of the database (or filename if using sqlite)
  • threadlocals – whether to store connections in a threadlocal
  • autocommit – automatically commit every query executed by calling execute()
  • connect_kwargs – any arbitrary parameters to pass to the database driver when connecting

Note

if your database name is not known when the class is declared, you can pass None in as the database name which will mark the database as “deferred” and any attempt to connect while in this state will raise an exception. To initialize your database, call the Database.init() method with the database name

init(database[, **connect_kwargs])

If the database was instantiated with database=None, the database is said to be in a ‘deferred’ state (see notes) – if this is the case, you can initialize it at any time by calling the init method.

Parameters:
  • database – the name of the database (or filename if using sqlite)
  • connect_kwargs – any arbitrary parameters to pass to the database driver when connecting
connect()

Establishes a connection to the database

Note

If you initialized with threadlocals=True, then this will store the connection inside a threadlocal, ensuring that connections are not shared across threads.

close()

Closes the connection to the database (if one is open)

Note

If you initialized with threadlocals=True, only a connection local to the calling thread will be closed.

get_conn()
Return type:a connection to the database, creates one if does not exist
get_cursor()
Return type:a cursor for executing queries
set_autocommit(autocommit)
Parameters:autocommit – a boolean value indicating whether to turn on/off autocommit for the current connection
get_autocommit()
Return type:a boolean value indicating whether autocommit is on for the current connection
execute(sql[, params=None])
Parameters:
  • sql – a string sql query
  • params – a list or tuple of parameters to interpolate

Note

You can configure whether queries will automatically commit by using the set_autocommit() and Database.get_autocommit() methods.

commit()

Call commit() on the active connection, committing the current transaction

rollback()

Call rollback() on the active connection, rolling back the current transaction

commit_on_success(func)

Decorator that wraps the given function in a single transaction, which, upon success will be committed. If an error is raised inside the function, the transaction will be rolled back and the error will be re-raised.

Parameters:func – function to decorate 
@database.commit_on_success
def transfer_money(from_acct, to_acct, amt):
    from_acct.charge(amt)
    to_acct.pay(amt)
    return amt
transaction()

Return a context manager that executes statements in a transaction. If an error is raised inside the context manager, the transaction will be rolled back, otherwise statements are committed when exiting.

# delete a blog instance and all its associated entries, but
# do so within a transaction
with database.transaction():
    blog.delete_instance(recursive=True)
last_insert_id(cursor, model)
Parameters:
  • cursor – the database cursor used to perform the insert query
  • model – the model class that was just created
Return type:

the primary key of the most recently inserted instance
rows_affected(cursor)
Return type:number of rows affected by the last query
create_table(model_class[, safe=False])
Parameters:
  • model_class – Model class to create table for
  • safe – if True, query will add a IF NOT EXISTS clause
create_index(model_class, field_name[, unique=False])
Parameters:
  • model_class – Model table on which to create index
  • field_name – name of field to create index on
  • unique – whether the index should enforce uniqueness
create_foreign_key(model_class, field)
Parameters:
  • model_class – Model table on which to create foreign key index / constraint
  • field – Field object
drop_table(model_class[, fail_silently=False])
Parameters:
  • model_class – Model table to drop
  • fail_silently – if True, query will add a IF EXISTS clause

Note

Cascading drop tables are not supported at this time, so if a constraint exists that prevents a table being dropped, you will need to handle that in application logic.

create_sequence(sequence_name)
Parameters:sequence_name – name of sequence to create

Note

only works with database engines that support sequences

drop_sequence(sequence_name)
Parameters:sequence_name – name of sequence to drop

Note

only works with database engines that support sequences

get_indexes_for_table(table)
Parameters:table – the name of table to introspect
Return type:a list of (index_name, is_unique) tuples

Warning

Not implemented – implementations exist in subclasses

get_tables()
Return type:a list of table names in the database

Warning

Not implemented – implementations exist in subclasses

sequence_exists(sequence_name)
Rtype boolean:
class SqliteDatabase(Database)

Database subclass that communicates to the “sqlite3” driver

class MySQLDatabase(Database)

Database subclass that communicates to the “MySQLdb” driver

class PostgresqlDatabase(Database)

Database subclass that communicates to the “psycopg2” driver

BaseAdapter and its subclasses

class BaseAdapter

The various subclasses of BaseAdapter provide a bridge between the high- level Database abstraction and the underlying python libraries like psycopg2. It also provides a way to unify the pythonic field types with the underlying column types used by the database engine.

The BaseAdapter provides two types of mappings: - mapping between filter operations and their database equivalents - mapping between basic field types and their database column types

The BaseAdapter also is the mechanism used by the Database class to: - handle connections with the database - extract information from the database cursor

operations = {'eq': '= %s'}

A mapping of query operation to SQL

interpolation = '%s'

The string used by the driver to interpolate query parameters

sequence_support = False

Whether the given backend supports sequences

reserved_tables = []

Table names that are reserved by the backend – if encountered in the application a warning will be issued.

get_field_types()
Return type:a dictionary mapping “user-friendly field type” to specific column type, e.g. {'string': 'VARCHAR', 'float': 'REAL', ... }
get_field_type_overrides()
Return type:a dictionary similar to that returned by get_field_types().

Provides a mechanism to override any number of field types without having to override all of them.

connect(database, **kwargs)
Parameters:
  • database – string representing database name (or filename if using sqlite)
  • kwargs – any keyword arguments to pass along to the database driver when connecting
Return type:

a database connection
close(conn)
Parameters:conn – a database connection

Close the given database connection

lookup_cast(lookup, value)
Parameters:
  • lookup – a string representing the lookup type
  • value – a python value that will be passed in to the lookup
Return type:

a converted value appropriate for the given lookup

Used as a hook when a specific lookup requires altering the given value, like for example when performing a LIKE query you may need to insert wildcards.

last_insert_id(cursor, model)
Return type:most recently inserted primary key
rows_affected(cursor)
Return type:number of rows affected by most recent query
class SqliteAdapter(BaseAdapter)

Subclass of BaseAdapter that works with the “sqlite3” driver

class MySQLAdapter(BaseAdapter)

Subclass of BaseAdapter that works with the “MySQLdb” driver

class PostgresqlAdapter(BaseAdapter)

Subclass of BaseAdapter that works with the “psycopg2” driver