Column and Data Types — SQLAlchemy 0.7.1 Documentation

# SQLAlchemy 0.7.1 Documentation

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Version: 0.7.1 Last Updated: 06/05/2011 13:13:59

# Column and Data Types

SQLAlchemy provides abstractions for most common database data types, and a mechanism for specifying your own custom data types.

The methods and attributes of type objects are rarely used directly. Type objects are supplied to `Table` definitions and can be supplied as type hints to functions for occasions where the database driver returns an incorrect type.

    >>> users = Table('users', metadata,
    ...               Column('id', Integer, primary_key=True)
    ...               Column('login', String(32))
    ...              )

SQLAlchemy will use the `Integer` and `String(32)` type information when issuing a `CREATE TABLE` statement and will use it again when reading back rows `SELECTed` from the database. Functions that accept a type (such as `Column()`) will typically accept a type class or instance; `Integer` is equivalent to `Integer()` with no construction arguments in this case.

## Generic Types

Generic types specify a column that can read, write and store a particular type of Python data. SQLAlchemy will choose the best database column type available on the target database when issuing a `CREATE TABLE` statement. For complete control over which column type is emitted in `CREATE TABLE`, such as `VARCHAR` see SQL Standard Types and the other sections of this chapter.

class `sqlalchemy.types.` `BigInteger`(*args, **kwargs)

  Bases: `sqlalchemy.types.Integer`

  A type for bigger `int` integers.

  Typically generates a `BIGINT` in DDL, and otherwise acts like a normal `Integer` on the Python side.

class `sqlalchemy.types.` `Boolean`(create_constraint=True, name=None)

  Bases: `sqlalchemy.types.TypeEngine`, `sqlalchemy.types.SchemaType`

  A bool datatype.

  Boolean typically uses BOOLEAN or SMALLINT on the DDL side, and on the Python side deals in `True` or `False`.

  `__init__`(create_constraint=True, name=None)

    Construct a Boolean.

    Parameters: | create_constraint – defaults to True. If the boolean is generated as an int/smallint, also create a CHECK constraint on the table that ensures 1 or 0 as a value. name – if a CHECK constraint is generated, specify the name of the constraint.

class `sqlalchemy.types.` `Date`(*args, **kwargs)

  Bases: `sqlalchemy.types._DateAffinity`, `sqlalchemy.types.TypeEngine`

  A type for `datetime.date()` objects.

class `sqlalchemy.types.` `DateTime`(timezone=False)

  Bases: `sqlalchemy.types._DateAffinity`, `sqlalchemy.types.TypeEngine`

  A type for `datetime.datetime()` objects.

  Date and time types return objects from the Python `datetime` module. Most DBAPIs have built in support for the datetime module, with the noted exception of SQLite. In the case of SQLite, date and time types are stored as strings which are then converted back to datetime objects when rows are returned.

class `sqlalchemy.types.` `Enum`(*enums, **kw)

  Bases: `sqlalchemy.types.String`, `sqlalchemy.types.SchemaType`

  Generic Enum Type.

  The Enum type provides a set of possible string values which the column is constrained towards.

  By default, uses the backend’s native ENUM type if available, else uses VARCHAR + a CHECK constraint.

  `__init__`(*enums, **kw)

    Construct an enum.

    Keyword arguments which don’t apply to a specific backend are ignored by that backend.

    Parameters: | *enums – string or unicode enumeration labels. If unicode labels are present, the convert_unicode flag is auto-enabled. convert_unicode – Enable unicode-aware bind parameter and result-set processing for this Enum’s data. This is set automatically based on the presence of unicode label strings. metadata – Associate this type directly with a `MetaData` object. For types that exist on the target database as an independent schema construct (Postgresql), this type will be created and dropped within `create_all()` and `drop_all()` operations. If the type is not associated with any `MetaData` object, it will associate itself with each `Table` in which it is used, and will be created when any of those individual tables are created, after a check is performed for it’s existence. The type is only dropped when `drop_all()` is called for that `Table` object’s metadata, however. name – The name of this type. This is required for Postgresql and any future supported database which requires an explicitly named type, or an explicitly named constraint in order to generate the type and/or a table that uses it. native_enum – Use the database’s native ENUM type when available. Defaults to True. When False, uses VARCHAR + check constraint for all backends. schema – Schemaname of this type. For types that exist on the target database as an independent schema construct (Postgresql), this parameter specifies the named schema in which the type is present. quote – Force quoting to be on or off on the type’s name. If left as the default of None, the usual schema-level “case sensitive”/”reserved name” rules are used to determine if this type’s name should be quoted.

class `sqlalchemy.types.` `Float`(precision=None, asdecimal=False, **kwargs)

  Bases: `sqlalchemy.types.Numeric`

  A type for `float` numbers.

  Returns Python `float` objects by default, applying conversion as needed.

  `__init__`(precision=None, asdecimal=False, **kwargs)

    Construct a Float.

    Parameters: | precision – the numeric precision for use in DDL `CREATE TABLE`. asdecimal – the same flag as that of `Numeric`, but defaults to `False`. Note that setting this flag to `True` results in floating point conversion.

class `sqlalchemy.types.` `Integer`(*args, **kwargs)

  Bases: `sqlalchemy.types._DateAffinity`, `sqlalchemy.types.TypeEngine`

  A type for `int` integers.

class `sqlalchemy.types.` `Interval`(native=True, second_precision=None, day_precision=None)

  Bases: `sqlalchemy.types._DateAffinity`, `sqlalchemy.types.TypeDecorator`

  A type for `datetime.timedelta()` objects.

  The Interval type deals with `datetime.timedelta` objects. In PostgreSQL, the native `INTERVAL` type is used; for others, the value is stored as a date which is relative to the “epoch” (Jan. 1, 1970).

  Note that the `Interval` type does not currently provide date arithmetic operations on platforms which do not support interval types natively. Such operations usually require transformation of both sides of the expression (such as, conversion of both sides into integer epoch values first) which currently is a manual procedure (such as via `func`).

  `__init__`(native=True, second_precision=None, day_precision=None)

    Construct an Interval object.

    Parameters: | native – when True, use the actual INTERVAL type provided by the database, if supported (currently Postgresql, Oracle). Otherwise, represent the interval data as an epoch value regardless. second_precision – For native interval types which support a “fractional seconds precision” parameter, i.e. Oracle and Postgresql day_precision – for native interval types which support a “day precision” parameter, i.e. Oracle.

  `impl`

    alias of `DateTime`

class `sqlalchemy.types.` `LargeBinary`(length=None)

  Bases: `sqlalchemy.types._Binary`

  A type for large binary byte data.

  The Binary type generates BLOB or BYTEA when tables are created, and also converts incoming values using the `Binary` callable provided by each DB-API.

  `__init__`(length=None)

    Construct a LargeBinary type.

    Parameters: | length – optional, a length for the column for use in DDL statements, for those BLOB types that accept a length (i.e. MySQL). It does not produce a small BINARY/VARBINARY type - use the BINARY/VARBINARY types specifically for those. May be safely omitted if no `CREATE TABLE` will be issued. Certain databases may require a length for use in DDL, and will raise an exception when the `CREATE TABLE` DDL is issued.

class `sqlalchemy.types.` `Numeric`(precision=None, scale=None, asdecimal=True)

  Bases: `sqlalchemy.types._DateAffinity`, `sqlalchemy.types.TypeEngine`

  A type for fixed precision numbers.

  Typically generates DECIMAL or NUMERIC. Returns `decimal.Decimal` objects by default, applying conversion as needed.

  Note

  The cdecimal library is a high performing alternative to Python’s built-in `decimal.Decimal` type, which performs very poorly in high volume situations. SQLAlchemy 0.7 is tested against `cdecimal` and supports it fully. The type is not necessarily supported by DBAPI implementations however, most of which contain an import for plain `decimal` in their source code, even though some such as psycopg2 provide hooks for alternate adapters. SQLAlchemy imports `decimal` globally as well. While the alternate `Decimal` class can be patched into SQLA’s `decimal` module, overall the most straightforward and foolproof way to use “cdecimal” given current DBAPI and Python support is to patch it directly into sys.modules before anything else is imported:

      import sys
      import cdecimal
      sys.modules["decimal"] = cdecimal

  While the global patch is a little ugly, it’s particularly important to use just one decimal library at a time since Python Decimal and cdecimal Decimal objects are not currently compatible with each other:

      >>> import cdecimal
      >>> import decimal
      >>> decimal.Decimal("10") == cdecimal.Decimal("10")
      False

  SQLAlchemy will provide more natural support of cdecimal if and when it becomes a standard part of Python installations and is supported by all DBAPIs.

  `__init__`(precision=None, scale=None, asdecimal=True)

    Construct a Numeric.

    Parameters: | precision – the numeric precision for use in DDL `CREATE TABLE`. scale – the numeric scale for use in DDL `CREATE TABLE`. asdecimal – default True. Return whether or not values should be sent as Python Decimal objects, or as floats. Different DBAPIs send one or the other based on datatypes - the Numeric type will ensure that return values are one or the other across DBAPIs consistently.

    When using the `Numeric` type, care should be taken to ensure that the asdecimal setting is apppropriate for the DBAPI in use - when Numeric applies a conversion from Decimal->float or float-> Decimal, this conversion incurs an additional performance overhead for all result columns received.

    DBAPIs that return Decimal natively (e.g. psycopg2) will have better accuracy and higher performance with a setting of `True`, as the native translation to Decimal reduces the amount of floating- point issues at play, and the Numeric type itself doesn’t need to apply any further conversions. However, another DBAPI which returns floats natively will incur an additional conversion overhead, and is still subject to floating point data loss - in which case `asdecimal=False` will at least remove the extra conversion overhead.

class `sqlalchemy.types.` `PickleType`(protocol=2, pickler=None, mutable=False, comparator=None)

  Bases: `sqlalchemy.types.MutableType`, `sqlalchemy.types.TypeDecorator`

  Holds Python objects, which are serialized using pickle.

  PickleType builds upon the Binary type to apply Python’s `pickle.dumps()` to incoming objects, and `pickle.loads()` on the way out, allowing any pickleable Python object to be stored as a serialized binary field.

  `__init__`(protocol=2, pickler=None, mutable=False, comparator=None)

    Construct a PickleType.

    Parameters: | protocol – defaults to `pickle.HIGHEST_PROTOCOL`. pickler – defaults to cPickle.pickle or pickle.pickle if cPickle is not available. May be any object with pickle-compatible `dumps` and ``loads` methods. mutable – defaults to False; implements `AbstractType.is_mutable()`. When `True`, incoming objects will be compared against copies of themselves using the Python “equals” operator, unless the `comparator` argument is present. See `MutableType` for details on “mutable” type behavior. (default changed from `True` in 0.7.0). Note This functionality is now superseded by the `sqlalchemy.ext.mutable` extension described in Mutation Tracking. comparator – a 2-arg callable predicate used to compare values of this type. If left as `None`, the Python “equals” operator is used to compare values.

  `impl`

    alias of `LargeBinary`

  `is_mutable`()

    Return True if the target Python type is ‘mutable’.

    When this method is overridden, `copy_value()` should also be supplied. The `MutableType` mixin is recommended as a helper.

class `sqlalchemy.types.` `SchemaType`(**kw)

  Bases: `sqlalchemy.events.SchemaEventTarget`

  Mark a type as possibly requiring schema-level DDL for usage.

  Supports types that must be explicitly created/dropped (i.e. PG ENUM type) as well as types that are complimented by table or schema level constraints, triggers, and other rules.

  `SchemaType` classes can also be targets for the `DDLEvents.before_parent_attach()` and `DDLEvents.after_parent_attach()` events, where the events fire off surrounding the association of the type object with a parent `Column`.

  `bind`
  `create`(bind=None, checkfirst=False)

    Issue CREATE ddl for this type, if applicable.

  `drop`(bind=None, checkfirst=False)

    Issue DROP ddl for this type, if applicable.

class `sqlalchemy.types.` `SmallInteger`(*args, **kwargs)

  Bases: `sqlalchemy.types.Integer`

  A type for smaller `int` integers.

  Typically generates a `SMALLINT` in DDL, and otherwise acts like a normal `Integer` on the Python side.

class `sqlalchemy.types.` `String`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

  Bases: `sqlalchemy.types.Concatenable`, `sqlalchemy.types.TypeEngine`

  The base for all string and character types.

  In SQL, corresponds to VARCHAR. Can also take Python unicode objects and encode to the database’s encoding in bind params (and the reverse for result sets.)

  The length field is usually required when the String type is used within a CREATE TABLE statement, as VARCHAR requires a length on most databases.

  `__init__`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

    Create a string-holding type.

    Parameters: | length – optional, a length for the column for use in DDL statements. May be safely omitted if no `CREATE TABLE` will be issued. Certain databases may require a length for use in DDL, and will raise an exception when the `CREATE TABLE` DDL is issued. Whether the value is interpreted as bytes or characters is database specific. convert_unicode – defaults to False. If True, the type will do what is necessary in order to accept Python Unicode objects as bind parameters, and to return Python Unicode objects in result rows. This may require SQLAlchemy to explicitly coerce incoming Python unicodes into an encoding, and from an encoding back to Unicode, or it may not require any interaction from SQLAlchemy at all, depending on the DBAPI in use. When SQLAlchemy performs the encoding/decoding, the encoding used is configured via `encoding`, which defaults to utf-8. The “convert_unicode” behavior can also be turned on for all String types by setting `sqlalchemy.engine.base.Dialect.convert_unicode` on create_engine(). To instruct SQLAlchemy to perform Unicode encoding/decoding even on a platform that already handles Unicode natively, set convert_unicode=’force’. This will incur significant performance overhead when fetching unicode result columns. assert_unicode – Deprecated. A warning is raised in all cases when a non-Unicode object is passed when SQLAlchemy would coerce into an encoding (note: but not when the DBAPI handles unicode objects natively). To suppress or raise this warning to an error, use the Python warnings filter documented at: http://docs.python.org/library/warnings.html unicode_error – Optional, a method to use to handle Unicode conversion errors. Behaves like the ‘errors’ keyword argument to the standard library’s string.decode() functions. This flag requires that convert_unicode is set to “force” - otherwise, SQLAlchemy is not guaranteed to handle the task of unicode conversion. Note that this flag adds significant performance overhead to row-fetching operations for backends that already return unicode objects natively (which most DBAPIs do). This flag should only be used as an absolute last resort for reading strings from a column with varied or corrupted encodings, which only applies to databases that accept invalid encodings in the first place (i.e. MySQL. not PG, Sqlite, etc.)

class `sqlalchemy.types.` `Text`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

  Bases: `sqlalchemy.types.String`

  A variably sized string type.

  In SQL, usually corresponds to CLOB or TEXT. Can also take Python unicode objects and encode to the database’s encoding in bind params (and the reverse for result sets.)

class `sqlalchemy.types.` `Time`(timezone=False)

  Bases: `sqlalchemy.types._DateAffinity`, `sqlalchemy.types.TypeEngine`

  A type for `datetime.time()` objects.

class `sqlalchemy.types.` `Unicode`(length=None, **kwargs)

  Bases: `sqlalchemy.types.String`

  A variable length Unicode string.

  The `Unicode` type is a `String` which converts Python `unicode` objects (i.e., strings that are defined as `u'somevalue'`) into encoded bytestrings when passing the value to the database driver, and similarly decodes values from the database back into Python `unicode` objects.

  It’s roughly equivalent to using a `String` object with `convert_unicode=True`, however the type has other significances in that it implies the usage of a unicode-capable type being used on the backend, such as NVARCHAR. This may affect what type is emitted when issuing CREATE TABLE and also may effect some DBAPI-specific details, such as type information passed along to `setinputsizes()`.

  When using the `Unicode` type, it is only appropriate to pass Python `unicode` objects, and not plain `str`. If a bytestring (`str`) is passed, a runtime warning is issued. If you notice your application raising these warnings but you’re not sure where, the Python `warnings` filter can be used to turn these warnings into exceptions which will illustrate a stack trace:

      import warnings
      warnings.simplefilter('error')

  Bytestrings sent to and received from the database are encoded using the dialect’s `encoding`, which defaults to utf-8.

  `__init__`(length=None, **kwargs)

    Create a Unicode-converting String type.

    Parameters: | length – optional, a length for the column for use in DDL statements. May be safely omitted if no `CREATE TABLE` will be issued. Certain databases may require a length for use in DDL, and will raise an exception when the `CREATE TABLE` DDL is issued. Whether the value is interpreted as bytes or characters is database specific. **kwargs – passed through to the underlying `String` type.

class `sqlalchemy.types.` `UnicodeText`(length=None, **kwargs)

  Bases: `sqlalchemy.types.Text`

  An unbounded-length Unicode string.

  See `Unicode` for details on the unicode behavior of this object.

  Like `Unicode`, usage the `UnicodeText` type implies a unicode-capable type being used on the backend, such as NCLOB.

  `__init__`(length=None, **kwargs)

    Create a Unicode-converting Text type.

    Parameters: | length – optional, a length for the column for use in DDL statements. May be safely omitted if no `CREATE TABLE` will be issued. Certain databases may require a length for use in DDL, and will raise an exception when the `CREATE TABLE` DDL is issued. Whether the value is interpreted as bytes or characters is database specific.

## SQL Standard Types

The SQL standard types always create database column types of the same name when `CREATE TABLE` is issued. Some types may not be supported on all databases.

class `sqlalchemy.types.` `BIGINT`(*args, **kwargs)

  Bases: `sqlalchemy.types.BigInteger`

  The SQL BIGINT type.

class `sqlalchemy.types.` `BINARY`(length=None)

  Bases: `sqlalchemy.types._Binary`

  The SQL BINARY type.

class `sqlalchemy.types.` `BLOB`(length=None)

  Bases: `sqlalchemy.types.LargeBinary`

  The SQL BLOB type.

class `sqlalchemy.types.` `BOOLEAN`(create_constraint=True, name=None)

  Bases: `sqlalchemy.types.Boolean`

  The SQL BOOLEAN type.

class `sqlalchemy.types.` `CHAR`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

  Bases: `sqlalchemy.types.String`

  The SQL CHAR type.

class `sqlalchemy.types.` `CLOB`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

  Bases: `sqlalchemy.types.Text`

  The CLOB type.

  This type is found in Oracle and Informix.

class `sqlalchemy.types.` `DATE`(*args, **kwargs)

  Bases: `sqlalchemy.types.Date`

  The SQL DATE type.

class `sqlalchemy.types.` `DATETIME`(timezone=False)

  Bases: `sqlalchemy.types.DateTime`

  The SQL DATETIME type.

class `sqlalchemy.types.` `DECIMAL`(precision=None, scale=None, asdecimal=True)

  Bases: `sqlalchemy.types.Numeric`

  The SQL DECIMAL type.

class `sqlalchemy.types.` `FLOAT`(precision=None, asdecimal=False, **kwargs)

  Bases: `sqlalchemy.types.Float`

  The SQL FLOAT type.

`sqlalchemy.types.` `INT`

  alias of `INTEGER`

class `sqlalchemy.types.` `INTEGER`(*args, **kwargs)

  Bases: `sqlalchemy.types.Integer`

  The SQL INT or INTEGER type.

class `sqlalchemy.types.` `NCHAR`(length=None, **kwargs)

  Bases: `sqlalchemy.types.Unicode`

  The SQL NCHAR type.

class `sqlalchemy.types.` `NVARCHAR`(length=None, **kwargs)

  Bases: `sqlalchemy.types.Unicode`

  The SQL NVARCHAR type.

class `sqlalchemy.types.` `NUMERIC`(precision=None, scale=None, asdecimal=True)

  Bases: `sqlalchemy.types.Numeric`

  The SQL NUMERIC type.

class `sqlalchemy.types.` `REAL`(precision=None, asdecimal=False, **kwargs)

  Bases: `sqlalchemy.types.Float`

  The SQL REAL type.

class `sqlalchemy.types.` `SMALLINT`(*args, **kwargs)

  Bases: `sqlalchemy.types.SmallInteger`

  The SQL SMALLINT type.

class `sqlalchemy.types.` `TEXT`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

  Bases: `sqlalchemy.types.Text`

  The SQL TEXT type.

class `sqlalchemy.types.` `TIME`(timezone=False)

  Bases: `sqlalchemy.types.Time`

  The SQL TIME type.

class `sqlalchemy.types.` `TIMESTAMP`(timezone=False)

  Bases: `sqlalchemy.types.DateTime`

  The SQL TIMESTAMP type.

class `sqlalchemy.types.` `VARBINARY`(length=None)

  Bases: `sqlalchemy.types._Binary`

  The SQL VARBINARY type.

class `sqlalchemy.types.` `VARCHAR`(length=None, convert_unicode=False, assert_unicode=None, unicode_error=None, _warn_on_bytestring=False)

  Bases: `sqlalchemy.types.String`

  The SQL VARCHAR type.

## Vendor-Specific Types

Database-specific types are also available for import from each database’s dialect module. See the sqlalchemy.dialects_toplevel reference for the database you’re interested in.

For example, MySQL has a `BIGINTEGER` type and PostgreSQL has an `INET` type. To use these, import them from the module explicitly:

    from sqlalchemy.dialects import mysql

    table = Table('foo', meta,
        Column('id', mysql.BIGINTEGER),
        Column('enumerates', mysql.ENUM('a', 'b', 'c'))
    )

Or some PostgreSQL types:

    from sqlalchemy.dialects import postgresql

    table = Table('foo', meta,
        Column('ipaddress', postgresql.INET),
        Column('elements', postgresql.ARRAY(str))
        )

Each dialect provides the full set of typenames supported by that backend within its __all__ collection, so that a simple import * or similar will import all supported types as implemented for that backend:

    from sqlalchemy.dialects.postgresql import *

    t = Table('mytable', metadata,
               Column('id', INTEGER, primary_key=True),
               Column('name', VARCHAR(300)),
               Column('inetaddr', INET)
    )

Where above, the INTEGER and VARCHAR types are ultimately from sqlalchemy.types, and INET is specific to the Postgresql dialect.

Some dialect level types have the same name as the SQL standard type, but also provide additional arguments. For example, MySQL implements the full range of character and string types including additional arguments such as collation and charset:

    from sqlalchemy.dialects.mysql import VARCHAR, TEXT

    table = Table('foo', meta,
        Column('col1', VARCHAR(200, collation='binary')),
        Column('col2', TEXT(charset='latin1'))
    )

## Custom Types

A variety of methods exist to redefine the behavior of existing types as well as to provide new ones.

### Overriding Type Compilation

A frequent need is to force the “string” version of a type, that is the one rendered in a CREATE TABLE statement or other SQL function like CAST, to be changed. For example, an application may want to force the rendering of `BINARY` for all platforms except for one, in which is wants `BLOB` to be rendered. Usage of an existing generic type, in this case `LargeBinary`, is preferred for most use cases. But to control types more accurately, a compilation directive that is per-dialect can be associated with any type:

    from sqlalchemy.ext.compiler import compiles
    from sqlalchemy.types import BINARY

    @compiles(BINARY, "sqlite")
    def compile_binary_sqlite(type_, compiler, **kw):
        return "BLOB"

The above code allows the usage of `types.BINARY`, which will produce the string `BINARY` against all backends except SQLite, in which case it will produce `BLOB`.

See the section Changing Compilation of Types, a subsection of Custom SQL Constructs and Compilation Extension, for additional examples.

### Augmenting Existing Types

The `TypeDecorator` allows the creation of custom types which add bind-parameter and result-processing behavior to an existing type object. It is used when additional in-Python marshalling of data to and from the database is required.

class `sqlalchemy.types.` `TypeDecorator`(*args, **kwargs)

  Bases: `sqlalchemy.types.TypeEngine`

  Allows the creation of types which add additional functionality to an existing type.

  This method is preferred to direct subclassing of SQLAlchemy’s built-in types as it ensures that all required functionality of the underlying type is kept in place.

  Typical usage:

      import sqlalchemy.types as types

      class MyType(types.TypeDecorator):
          '''Prefixes Unicode values with "PREFIX:" on the way in and
          strips it off on the way out.
          '''

          impl = types.Unicode

          def process_bind_param(self, value, dialect):
              return "PREFIX:" + value

          def process_result_value(self, value, dialect):
              return value[7:]

          def copy(self):
              return MyType(self.impl.length)

  The class-level “impl” variable is required, and can reference any TypeEngine class. Alternatively, the load_dialect_impl() method can be used to provide different type classes based on the dialect given; in this case, the “impl” variable can reference `TypeEngine` as a placeholder.

  Types that receive a Python type that isn’t similar to the ultimate type used may want to define the `TypeDecorator.coerce_compared_value()` method. This is used to give the expression system a hint when coercing Python objects into bind parameters within expressions. Consider this expression:

      mytable.c.somecol + datetime.date(2009, 5, 15)

  Above, if “somecol” is an `Integer` variant, it makes sense that we’re doing date arithmetic, where above is usually interpreted by databases as adding a number of days to the given date. The expression system does the right thing by not attempting to coerce the “date()” value into an integer-oriented bind parameter.

  However, in the case of `TypeDecorator`, we are usually changing an incoming Python type to something new - `TypeDecorator` by default will “coerce” the non-typed side to be the same type as itself. Such as below, we define an “epoch” type that stores a date value as an integer:

      class MyEpochType(types.TypeDecorator):
          impl = types.Integer

          epoch = datetime.date(1970, 1, 1)

          def process_bind_param(self, value, dialect):
              return (value - self.epoch).days

          def process_result_value(self, value, dialect):
              return self.epoch + timedelta(days=value)

  Our expression of `somecol + date` with the above type will coerce the “date” on the right side to also be treated as `MyEpochType`.

  This behavior can be overridden via the `coerce_compared_value()` method, which returns a type that should be used for the value of the expression. Below we set it such that an integer value will be treated as an `Integer`, and any other value is assumed to be a date and will be treated as a `MyEpochType`:

      def coerce_compared_value(self, op, value):
          if isinstance(value, int):
              return Integer()
          else:
              return self

  `__init__`(*args, **kwargs)

    Construct a `TypeDecorator`.

    Arguments sent here are passed to the constructor of the class assigned to the `impl` class level attribute, where the `self.impl` attribute is assigned an instance of the implementation type. If `impl` at the class level is already an instance, then it’s assigned to `self.impl` as is.

    Subclasses can override this to customize the generation of `self.impl`.

  `adapt`(cls, **kw)

    Produce an “adapted” form of this type, given an “impl” class to work with.

    This method is used internally to associate generic types with “implementation” types that are specific to a particular dialect.

  `bind_processor`(dialect)

    Provide a bound value processing function for the given `Dialect`.

    This is the method that fulfills the `TypeEngine` contract for bound value conversion. `TypeDecorator` will wrap a user-defined implementation of `process_bind_param()` here.

    User-defined code can override this method directly, though its likely best to use `process_bind_param()` so that the processing provided by `self.impl` is maintained.

  `coerce_compared_value`(op, value)

    Suggest a type for a ‘coerced’ Python value in an expression.

    By default, returns self. This method is called by the expression system when an object using this type is on the left or right side of an expression against a plain Python object which does not yet have a SQLAlchemy type assigned:

        expr = table.c.somecolumn + 35

    Where above, if `somecolumn` uses this type, this method will be called with the value `operator.add` and `35`. The return value is whatever SQLAlchemy type should be used for `35` for this particular operation.

  `compare_values`(x, y)

    Given two values, compare them for equality.

    By default this calls upon `TypeEngine.compare_values()` of the underlying “impl”, which in turn usually uses the Python equals operator `==`.

    This function is used by the ORM to compare an original-loaded value with an intercepted “changed” value, to determine if a net change has occurred.

  `compile`(dialect=None)

    Produce a string-compiled form of this `TypeEngine`.

    When called with no arguments, uses a “default” dialect to produce a string result.

    Parameters: | dialect – a `Dialect` instance.

  `copy`()

    Produce a copy of this `TypeDecorator` instance.

    This is a shallow copy and is provided to fulfill part of the `TypeEngine` contract. It usually does not need to be overridden unless the user-defined `TypeDecorator` has local state that should be deep-copied.

  `copy_value`(value)

    Given a value, produce a copy of it.

    By default this calls upon `TypeEngine.copy_value()` of the underlying “impl”.

    `copy_value()` will return the object itself, assuming “mutability” is not enabled. Only the `MutableType` mixin provides a copy function that actually produces a new object. The copying function is used by the ORM when “mutable” types are used, to memoize the original version of an object as loaded from the database, which is then compared to the possibly mutated version to check for changes.

    Modern implementations should use the `sqlalchemy.ext.mutable` extension described in Mutation Tracking for intercepting in-place changes to values.

  `dialect_impl`(dialect)

    Return a dialect-specific implementation for this `TypeEngine`.

  `get_dbapi_type`(dbapi)

    Return the DBAPI type object represented by this `TypeDecorator`.

    By default this calls upon `TypeEngine.get_dbapi_type()` of the underlying “impl”.

  `is_mutable`()

    Return True if the target Python type is ‘mutable’.

    This allows systems like the ORM to know if a column value can be considered ‘not changed’ by comparing the identity of objects alone. Values such as dicts, lists which are serialized into strings are examples of “mutable” column structures.

    Note

    This functionality is now superseded by the `sqlalchemy.ext.mutable` extension described in Mutation Tracking.

  `load_dialect_impl`(dialect)

    Return a `TypeEngine` object corresponding to a dialect.

    This is an end-user override hook that can be used to provide differing types depending on the given dialect. It is used by the `TypeDecorator` implementation of `type_engine()` to help determine what type should ultimately be returned for a given `TypeDecorator`.

    By default returns `self.impl`.

  `process_bind_param`(value, dialect)

    Receive a bound parameter value to be converted.

    Subclasses override this method to return the value that should be passed along to the underlying `TypeEngine` object, and from there to the DBAPI `execute()` method.

    Parameters: | value – the value. Can be None. dialect – the `Dialect` in use.

  `process_result_value`(value, dialect)

    Receive a result-row column value to be converted.

    Subclasses override this method to return the value that should be passed back to the application, given a value that is already processed by the underlying `TypeEngine` object, originally from the DBAPI cursor method `fetchone()` or similar.

    Parameters: | value – the value. Can be None. dialect – the `Dialect` in use.

  `result_processor`(dialect, coltype)

    Provide a result value processing function for the given `Dialect`.

    This is the method that fulfills the `TypeEngine` contract for result value conversion. `TypeDecorator` will wrap a user-defined implementation of `process_result_value()` here.

    User-defined code can override this method directly, though its likely best to use `process_result_value()` so that the processing provided by `self.impl` is maintained.

  `type_engine`(dialect)

    Return a dialect-specific `TypeEngine` instance for this `TypeDecorator`.

    In most cases this returns a dialect-adapted form of the `TypeEngine` type represented by `self.impl`. Makes usage of `dialect_impl()` but also traverses into wrapped `TypeDecorator` instances. Behavior can be customized here by overriding `load_dialect_impl()`.

### TypeDecorator Recipes

A few key `TypeDecorator` recipes follow.

#### Rounding Numerics

Some database connectors like those of SQL Server choke if a Decimal is passed with too many decimal places. Here’s a recipe that rounds them down:

    from sqlalchemy.types import TypeDecorator, Numeric
    from decimal import Decimal

    class SafeNumeric(TypeDecorator):
        """Adds quantization to Numeric."""

        impl = Numeric

        def __init__(self, *arg, **kw):
            TypeDecorator.__init__(self, *arg, **kw)
            self.quantize_int = -(self.impl.precision - self.impl.scale)
            self.quantize = Decimal(10) ** self.quantize_int

        def process_bind_param(self, value, dialect):
            if isinstance(value, Decimal) and \
                value.as_tuple()[2] < self.quantize_int:
                value = value.quantize(self.quantize)
            return value

#### Backend-agnostic GUID Type

Receives and returns Python uuid() objects. Uses the PG UUID type when using Postgresql, CHAR(32) on other backends, storing them in stringified hex format. Can be modified to store binary in CHAR(16) if desired:

    from sqlalchemy.types import TypeDecorator, CHAR
    from sqlalchemy.dialects.postgresql import UUID
    import uuid

    class GUID(TypeDecorator):
        """Platform-independent GUID type.

        Uses Postgresql's UUID type, otherwise uses
        CHAR(32), storing as stringified hex values.

        """
        impl = CHAR

        def load_dialect_impl(self, dialect):
            if dialect.name == 'postgresql':
                return dialect.type_descriptor(UUID())
            else:
                return dialect.type_descriptor(CHAR(32))

        def process_bind_param(self, value, dialect):
            if value is None:
                return value
            elif dialect.name == 'postgresql':
                return str(value)
            else:
                if not isinstance(value, uuid.UUID):
                    return "%.32x" % uuid.UUID(value)
                else:
                    # hexstring
                    return "%.32x" % value

        def process_result_value(self, value, dialect):
            if value is None:
                return value
            else:
                return uuid.UUID(value)

#### Marshal JSON Strings

This type uses `simplejson` to marshal Python data structures to/from JSON. Can be modified to use Python’s builtin json encoder:

    from sqlalchemy.types import TypeDecorator, VARCHAR
    import json

    class JSONEncodedDict(TypeDecorator):
        """Represents an immutable structure as a json-encoded string.

        Usage::

            JSONEncodedDict(255)

        """

        impl = VARCHAR

        def process_bind_param(self, value, dialect):
            if value is not None:
                value = json.dumps(value)

            return value

        def process_result_value(self, value, dialect):
            if value is not None:
                value = json.loads(value)
            return value

Note that the ORM by default will not detect “mutability” on such a type - meaning, in-place changes to values will not be detected and will not be flushed. Without further steps, you instead would need to replace the existing value with a new one on each parent object to detect changes. Note that there’s nothing wrong with this, as many applications may not require that the values are ever mutated once created. For those which do have this requirment, support for mutability is best applied using the `sqlalchemy.ext.mutable` extension - see the example in Mutation Tracking.

### Creating New Types

The `UserDefinedType` class is provided as a simple base class for defining entirely new database types:

class `sqlalchemy.types.` `UserDefinedType`(*args, **kwargs)

  Bases: `sqlalchemy.types.TypeEngine`

  Base for user defined types.

  This should be the base of new types. Note that for most cases, `TypeDecorator` is probably more appropriate:

      import sqlalchemy.types as types

      class MyType(types.UserDefinedType):
          def __init__(self, precision = 8):
              self.precision = precision

          def get_col_spec(self):
              return "MYTYPE(%s)" % self.precision

          def bind_processor(self, dialect):
              def process(value):
                  return value
              return process

          def result_processor(self, dialect, coltype):
              def process(value):
                  return value
              return process

  Once the type is made, it’s immediately usable:

      table = Table('foo', meta,
          Column('id', Integer, primary_key=True),
          Column('data', MyType(16))
          )

  `__init__`(*args, **kwargs)

    Support implementations that were passing arguments

  `adapt`(cls, **kw)

    Produce an “adapted” form of this type, given an “impl” class to work with.

    This method is used internally to associate generic types with “implementation” types that are specific to a particular dialect.

  `adapt_operator`(op)

    A hook which allows the given operator to be adapted to something new.

    See also UserDefinedType._adapt_expression(), an as-yet- semi-public method with greater capability in this regard.

  `bind_processor`(dialect)

    Return a conversion function for processing bind values.

    Returns a callable which will receive a bind parameter value as the sole positional argument and will return a value to send to the DB-API.

    If processing is not necessary, the method should return `None`.

    Parameters: | dialect – Dialect instance in use.

  `compare_values`(x, y)

    Compare two values for equality.

  `compile`(dialect=None)

    Produce a string-compiled form of this `TypeEngine`.

    When called with no arguments, uses a “default” dialect to produce a string result.

    Parameters: | dialect – a `Dialect` instance.

  `copy_value`(value)
  `dialect_impl`(dialect)

    Return a dialect-specific implementation for this `TypeEngine`.

  `get_dbapi_type`(dbapi)

    Return the corresponding type object from the underlying DB-API, if any.

    > This can be useful for calling `setinputsizes()`, for example.

  `is_mutable`()

    Return True if the target Python type is ‘mutable’.

    This allows systems like the ORM to know if a column value can be considered ‘not changed’ by comparing the identity of objects alone. Values such as dicts, lists which are serialized into strings are examples of “mutable” column structures.

    Note

    This functionality is now superseded by the `sqlalchemy.ext.mutable` extension described in Mutation Tracking.

    When this method is overridden, `copy_value()` should also be supplied. The `MutableType` mixin is recommended as a helper.

  `result_processor`(dialect, coltype)

    Return a conversion function for processing result row values.

    Returns a callable which will receive a result row column value as the sole positional argument and will return a value to return to the user.

    If processing is not necessary, the method should return `None`.

    Parameters: | dialect – Dialect instance in use. coltype – DBAPI coltype argument received in cursor.description.

## Base Type API

class `sqlalchemy.types.` `AbstractType`

  Bases: `sqlalchemy.sql.visitors.Visitable`

  Base for all types - not needed except for backwards compatibility.

  `__init__`

    x.__init__(...) initializes x; see help(type(x)) for signature

class `sqlalchemy.types.` `TypeEngine`(*args, **kwargs)

  Bases: `sqlalchemy.types.AbstractType`

  Base for built-in types.

  `__init__`(*args, **kwargs)

    Support implementations that were passing arguments

  `adapt`(cls, **kw)

    Produce an “adapted” form of this type, given an “impl” class to work with.

    This method is used internally to associate generic types with “implementation” types that are specific to a particular dialect.

  `bind_processor`(dialect)

    Return a conversion function for processing bind values.

    Returns a callable which will receive a bind parameter value as the sole positional argument and will return a value to send to the DB-API.

    If processing is not necessary, the method should return `None`.

    Parameters: | dialect – Dialect instance in use.

  `compare_values`(x, y)

    Compare two values for equality.

  `compile`(dialect=None)

    Produce a string-compiled form of this `TypeEngine`.

    When called with no arguments, uses a “default” dialect to produce a string result.

    Parameters: | dialect – a `Dialect` instance.

  `copy_value`(value)
  `dialect_impl`(dialect)

    Return a dialect-specific implementation for this `TypeEngine`.

  `get_dbapi_type`(dbapi)

    Return the corresponding type object from the underlying DB-API, if any.

    > This can be useful for calling `setinputsizes()`, for example.

  `is_mutable`()

    Return True if the target Python type is ‘mutable’.

    This allows systems like the ORM to know if a column value can be considered ‘not changed’ by comparing the identity of objects alone. Values such as dicts, lists which are serialized into strings are examples of “mutable” column structures.

    Note

    This functionality is now superseded by the `sqlalchemy.ext.mutable` extension described in Mutation Tracking.

    When this method is overridden, `copy_value()` should also be supplied. The `MutableType` mixin is recommended as a helper.

  `result_processor`(dialect, coltype)

    Return a conversion function for processing result row values.

    Returns a callable which will receive a result row column value as the sole positional argument and will return a value to return to the user.

    If processing is not necessary, the method should return `None`.

    Parameters: | dialect – Dialect instance in use. coltype – DBAPI coltype argument received in cursor.description.

class `sqlalchemy.types.` `MutableType`

  Bases: `object`

  A mixin that marks a `TypeEngine` as representing a mutable Python object type. This functionality is used only by the ORM.

  Note

  `MutableType` is superseded as of SQLAlchemy 0.7 by the `sqlalchemy.ext.mutable` extension described in Mutation Tracking. This extension provides an event driven approach to in-place mutation detection that does not incur the severe performance penalty of the `MutableType` approach.

  “mutable” means that changes can occur in place to a value of this type. Examples includes Python lists, dictionaries, and sets, as well as user-defined objects. The primary need for identification of “mutable” types is by the ORM, which applies special rules to such values in order to guarantee that changes are detected. These rules may have a significant performance impact, described below.

  A `MutableType` usually allows a flag called `mutable=False` to enable/disable the “mutability” flag, represented on this class by `is_mutable()`. Examples include `PickleType` and `ARRAY`. Setting this flag to `True` enables mutability-specific behavior by the ORM.

  The `copy_value()` and `compare_values()` functions represent a copy and compare function for values of this type - implementing subclasses should override these appropriately.

  Warning

  The usage of mutable types has significant performance implications when using the ORM. In order to detect changes, the ORM must create a copy of the value when it is first accessed, so that changes to the current value can be compared against the “clean” database-loaded value. Additionally, when the ORM checks to see if any data requires flushing, it must scan through all instances in the session which are known to have “mutable” attributes and compare the current value of each one to its “clean” value. So for example, if the Session contains 6000 objects (a fairly large amount) and autoflush is enabled, every individual execution of `Query` will require a full scan of that subset of the 6000 objects that have mutable attributes, possibly resulting in tens of thousands of additional method calls for every query.

  As of SQLAlchemy 0.7, the `sqlalchemy.ext.mutable` is provided which allows an event driven approach to in-place mutation detection. This approach should now be favored over the usage of `MutableType` with `mutable=True`. `sqlalchemy.ext.mutable` is described in Mutation Tracking.

  `__init__`

    x.__init__(...) initializes x; see help(type(x)) for signature

  `compare_values`(x, y)

    Compare x == y.

  `copy_value`(value)

    Unimplemented.

  `is_mutable`()

    Return True if the target Python type is ‘mutable’.

    For `MutableType`, this method is set to return `True`.

class `sqlalchemy.types.` `Concatenable`

  Bases: `object`

  A mixin that marks a type as supporting ‘concatenation’, typically strings.

  `__init__`

    x.__init__(...) initializes x; see help(type(x)) for signature

class `sqlalchemy.types.` `NullType`(*args, **kwargs)

  Bases: `sqlalchemy.types.TypeEngine`

  An unknown type.

  NullTypes will stand in if `Table` reflection encounters a column data type unknown to SQLAlchemy. The resulting columns are nearly fully usable: the DB-API adapter will handle all translation to and from the database data type.

  NullType does not have sufficient information to particpate in a `CREATE TABLE` statement and will raise an exception if encountered during a `create()` operation.