Little_PGer.py

What is it?

It's a thin layer just a tad above SQL, for use with Postgres and psycopg2, when you want to wrap queries in a convenient way, using plain data structures (but you don't feel like using an ORM, for some reason).

Why?

Of course psycopg2 already does a very fine job on its own, but in the context of webapp backend development, I often found myself wanting for an extra-frictionless way of shuffling around Ajax/JSON data. As composing raw SQL queries quickly induces string-manipulation fatigue, I gradually evolved little_pger for that simple purpose.

If you want to know more about it, I have also discussed its use in some particular contexts, on my blog:

• http://cjauvin.blogspot.com/2012/10/a-tribute-to-unsung-pattern.html

• http://cjauvin.blogspot.com/2013/04/impossibly-lean-access-control-with.html

To install

$ pip install little_pger

or

$ pip install -e git+git@github.com:cjauvin/little_pger.git#egg=little_pger

Note that psycopg2 will be automatically installed if it isn't already.

Testing it with this README document

Note that this README.md file can be executed as a test suite. To do so, simply create a dummy database (that you can destroy afterward):

$ createdb little_pger_test -U <your_pg_user>

and set this variable appropriately for your setup:

>>> pg_user = '' # an empty string works when the OS and PG users share the same name

Then simply execute the script with:

$ python -m doctest -f -v README.md

Let's go!

>>> from little_pger import LittlePGer

The first and mandatory parameter to a LittlePGer object is a connection, either as a string or as a psycopg2 object (resulting from psycopg2.connect). A LittlePGer object can be used in two ways. The first is as a context manager, which implies that the transaction is encapsulated under the with statement (with a rollback or commit performed automatically at exit):

>>> conn_str = 'dbname=little_pger_test user={}'.format(pg_user)
>>> with LittlePGer(conn=conn_str, commit=False) as pg:
...     _ = pg.pg_version # (9, 5, 0) for me, perhaps not for you

You can also use it without the context manager:

>>> pg = LittlePGer(conn=conn_str, commit=False)
>>> _ = pg.pg_version # (9, 5, 0) for me, perhaps not for you

in which case you are in charge of managing the transaction yourself. In this document we will not use the context manager because it makes things easier on the eyes.

Insert and update

Suppose we have two SQL tables:

>>> pg.sql("""
...     create table book (
...         book_id serial primary key,
...         author_id int,
...         title text,
...         n_pages int,
...         topics text[]
...     )
... """)

>>> pg.sql("""
...     create table author (
...         author_id serial primary key,
...         name text
...      )
... """)

you can insert a new book, along with its author:

>>> book = pg.insert('book', values={'title': 'PG is Fun!'})
>>> author = pg.insert('author', values={'name': 'Joe Foo', 'author_id': 100})

and update it:

>>> book = pg.update(
...     'book', set={'author_id': author['author_id'], 'n_pages': 200},
...     where={'book_id': book['book_id']}
... )
>>> sorted(book.items()) # just to clamp the field order
[('author_id', 100), ('book_id', 1), ('n_pages', 200), ('title', 'PG is Fun!'), ('topics', None)]

As shown above, insert and update by default return a dict record. However, insert has a convenient return_id keyword argument, which means that the primary key value of the newly created record should be returned directly:

>>> pg.insert(
...     'book', values={'title': 'Python and PG, a Love Story'},
...     return_id=True
... )
2

Upsert

Even though upsert only appeared recently (with PG 9.5), little_pger supports it for every version of PG, with a "fake implementation" (i.e. check existence, then insert or update accordingly) in the cases where it is not natively supported (and when it is, a "real" implementation is used). Both implementations are simplified versions where the primary key is implicitly used to determine uniqueness.

>>> # does not yet exist, will be created
>>> book_id = pg.upsert('book', set={'title': 'A Boring Story'}, return_id=True)
>>> book_id
3

>>> # already exists, will be updated
>>> book = pg.upsert('book', values={'n_pages': 123, 'book_id': book_id})
>>> book_id, book['book_id']
(3, 3)

insert, update and upsert all have a convenient filter_values parameter which, if used, will remove any item in the values dict that doesn't belong to the target table. Without it here, an exception would be thrown, as the book table does not have a publisher column:

>>> _ = pg.upsert(
...     'book', filter_values=True,
...      values={'book_id': book_id, 'publisher': 'Joe North'}
... )

Select

To select all books:

>>> books = pg.select('book')
>>> len(books)
3

or a particular book:

>>> books = pg.select('book', where={'book_id': book_id})
>>> len(books)
1

or:

>>> book = pg.select1('book', where={'book_id': book_id})
>>> type(book)
<class 'psycopg2.extras.RealDictRow'>

It's easy to (inner) join books and authors:

>>> book = pg.select1(
...     'book', join='author', where={'book_id': 1}
... )
>>> sorted(book.items()) # just to clamp the field order
[('author_id', 100), ('book_id', 1), ('n_pages', 200), ('name', 'Joe Foo'), ('title', 'PG is Fun!'), ('topics', None)]

or left join them:

>>> book_author = pg.select1(
...     'book', left_join='author', where={'book_id': 2}
... )
>>> sorted(book_author.items()) # just to clamp the field order
[('author_id', None), ('book_id', 2), ('n_pages', None), ('name', None), ('title', 'Python and PG, a Love Story'), ('topics', None)]

Using a tuple value in the where clause:

>>> books = pg.select('book', where={'book_id': (1, 2, 3)})
>>> len(books)
3

translates to a SQL query using the in operator:

select * from book where book_id in (1, 2, 3)

Make sure that you do not use tuples and lists interchangeably when working with psycopg2 and little_pger, as they are used for very different purposes. Python arrays translate into PG arrays (note that the book.topics column has type text[]):

>>> book = pg.update(
...     'book', set={'topics': ['database', 'programming']},
...     where={'book_id': 1}
... )
>>> book['topics']
['database', 'programming']

You can use operators other than =, like this:

>>> books = pg.select('book', where={('book_id', '<='): 2})
>>> len(books)
2

Using a set (instead of a tuple or a list) will result in a third type of semantics:

>>> pg.select1(
...     'book', where={('title', 'like'): {'%PG%', '%Fun%'}}
... )['title']
'PG is Fun!'

which translates to:

select * from book where title like '%PG%' and title like '%Fun%'

which can be a powerful way to implement an autocomplete mechanism, as I explain in more details elsewhere.

Until now we have assumed * selection, but the what keyword allows for more flexibility:

>>> res = pg.select(
...     'book', what={'*':1, 'title is not null': 'has_title'}
... )
>>> [book['has_title'] for book in res]
[True, True, True]

Similarly:

>>> res = pg.select(
...     'book', left_join='author',
...      what=['name', 'count(*)'],
...      group_by='name', order_by='count desc'
... )
>>> res[0]['name'], int(res[0]['count'])
(None, 2)
>>> res[1]['name'], int(res[1]['count'])
('Joe Foo', 1)

Delete

The delete function includes an option to "tighten" the primary key sequence, to make sure that if you delete a row with some ID that is the maximum one currently existing, it will be reused the next time you create a new row (in other words: it prevents "gaps" in the ID sequences).

Without tighten_sequence:

>>> pg.delete('book', where={'book_id': 3})
>>> pg.insert('book', return_id=True)
4

With it:

>>> pg.delete('book', where={'book_id': 4}, tighten_sequence=True)
>>> pg.insert('book', return_id=True)
3