cs-sqlalchemy-utils 20190812

Assorted utility functions to support working with SQLAlchemy.

Release 20190812: Make ORM a MultiOpenMixin. get_json_field: use forgotten default parameter. Other minor changes.

Assorted utility functions to support working with SQLAlchemy.

Function auto_session(func)

Decorator to run a function in a session if one is not presupplied. The function func runs within a transaction, nested if the session already exists.

Function find_json_field(column_value, field_name, *, infill=False)

Descend a JSONable Python object column_value to field_name. Return column_value (possibly infilled), final_field, final_field_name.

This supports database row columns which are JSON columns.

Parameters:

• column_value: the original value of the column
• field_name: the field within the column to locate
• infill: optional keyword parameter, default False. If true, column_value and its innards will be filled in as dicts to allow deferencing the field_name.

The field_name is a str consisting of a period ('.') separated sequence of field parts. Each field part becomes a key to index the column mapping. These keys are split into the leading field parts and the final field part, which is returned as final_field_name above.

The final_field return value above is the mapping within which final_field_value may lie and where final_field_value may be set. Note: it may not be present.

If a leading key is missing and infill is true the corresponding part of the column_value is set to an empty dictionary in order to allow deferencing the leading key. This includes the case when column_value itself is None, which is why the column_value is part of the return.

If a leading key is missing and infill is false this function will raise a KeyError for the portion of the field_name which failed.

Examples:

>>> find_json_field({'a':{'b':{}}}, 'a.b')
({'a': {'b': {}}}, {'b': {}}, 'b')
>>> find_json_field({'a':{}}, 'a.b')
({'a': {}}, {}, 'b')
>>> find_json_field({'a':{'b':{}}}, 'a.b.c.d')
Traceback (most recent call last):
    ...
KeyError: 'a.b.c'
>>> find_json_field({'a':{'b':{}}}, 'a.b.c.d', infill=True)
({'a': {'b': {'c': {}}}}, {}, 'd')
>>> find_json_field(None, 'a.b.c.d')
Traceback (most recent call last):
    ...
KeyError: 'a'
>>> find_json_field(None, 'a.b.c.d', infill=True)
({'a': {'b': {'c': {}}}}, {}, 'd')

Function get_json_field(column_value, field_name, *, default=None)

Return the value of field_name from column_value or a defaault if the field is not present.

Parameters:

• column_value: the original value of the column
• field_name: the field within the column to locate
• default: default value to return if the field is not present, default: None

Examples:

>>> get_json_field({'a': 1}, 'a')
1
>>> get_json_field({'b': 1}, 'a')
>>> get_json_field({'a': {}}, 'a.b')
>>> get_json_field({'a': {'b': 2}}, 'a.b')
2

Class ORM

MRO: cs.resources.MultiOpenMixin
A convenience base class for an ORM class.

This defines a .Base attribute which is a new DeclarativeBase and provides various Session related convenience methods. It is also a MultiOpenMixin subclass supporting nested open/close sequences and use as a context manager.

Subclasses must define the following:

• .Session: a factory in their own __init__, typically self.Session=sessionmaker(bind=engine)
• .startup and .shutdown methods to support the MultiOpenMixin, even if these just pass

Function orm_auto_session(method)

Decorator to run a method in a session derived from self.orm if a session is not presupplied. Intended to assist classes with a .orm attribute.

Function set_json_field(column_value, field_name, value, *, infill=False)

Set a new value for field_name of column_value. Return the new column_value.

Parameters:

• column_value: the original value of the column
• field_name: the field within the column to locate
• value: the value to store as field_name
• infill: optional keyword parameter, default False. If true, column_value and its innards will be filled in as dicts to allow deferencing the field_name.

As with find_json_field, a true infill may modify column_value to provide field_name which is why this function returns the new column_value.

Examples:

>>> set_json_field({'a': 2}, 'a', 3)
{'a': 3}
>>> set_json_field({'a': 2, 'b': {'c': 5}}, 'b.c', 4)
{'a': 2, 'b': {'c': 4}}
>>> set_json_field({'a': 2}, 'b.c', 4)
Traceback (most recent call last):
    ...
KeyError: 'b'
>>> set_json_field({'a': 2}, 'b.c', 4, infill=True)
{'a': 2, 'b': {'c': 4}}
>>> set_json_field(None, 'b.c', 4, infill=True)
{'b': {'c': 4}}

Function with_session(func, *a, orm=None, session=None, **kw)

Call func(*a,session=session,**kw), creating a session if required. The function func runs within a transaction, nested if the session already exists.

This is the inner mechanism of @auto_session and ORM.auto_session_method.

Parameters:

• func: the function to call
• a: the positional parameters
• orm: optional ORM class with a .session() context manager method such as the ORM base class supplied by this module.
• session: optional existing ORM session

One of orm or session must be not None; if session is None then one is made from orm.session() and used as a context manager.

The session is also passed to func as the keyword parameter session to support nested calls.

Release Log

Release 20190812: Make ORM a MultiOpenMixin. get_json_field: use forgotten default parameter. Other minor changes.

Release 20190526: Support for virtual columns mapped to a JSON column interior value: New functions find_json_field, get_json_field, set_json_field. New decorator @json_column for declaritive_base tables.

Release 20190517: Make ORM._Session private session factory the public ORM.Session factory for external use. with_session: preexisting sessions still trigger a session.begin_nested, removes flush/commit tension elsewhere.

Release 20190403: Rename @ORM.orm_auto_session to @ORM.auto_session. New @orm_auto_session decorator for methods of objects with a .orm attribute.

Release 20190319.1: Initial release. ORM base class, @auto_session decorator.