Tuesday, January 25, 2011

A Helper Module for PostgreSQL and Psycopg2

I have created little_pger, a Python "modulet" meant to help with common PostgreSQL/Psycopg2 tasks, by wrapping up a few things handily, and offering a coherent and pythonic interface to it.

Say we have a PG table like this:

create table document (
    document_id serial primary key,  
    title text,  
    type text check (type in ('book', 'article', 'essay'),  
    topics text[]

and a pair of connection/cursor objects:

>>> conn = psycopg2.connect("dbname=...")
>>> cur = conn.cursor()

You can then insert a new document record like this:

>>> insert(cur, 'document', values={'title':'PG is Easy'})

and update it like this:

>>> update(cur, 'document', set={'type':'article'}, where={'title':'PG is Easy'})

Note that you are still responsible for managing any transaction externally:

>>> conn.commit()

With the 'return_id' option (which restricts the default 'returning *' clause to the primary key's value, which is assumed to be named '<table>_id'), the insert/update above could also be done this way:

>>> doc_id = insert(cur, 'document', values={'title':'PG is Easy'}, return_id=True)
>>> update(cur, 'document', values={'type':'article'}, where={'document_id':doc_id})

Note that the 'set' or 'values' keywords can both be used with 'update'. Using a tuple (but not a list!) as a value in the 'where' dict param is translated to the proper SQL 'in' operator:

>>> select(cur, 'document', where={'type':('article', 'book')})

will return all article or book documents, whereas:

>>> select(cur, 'document', what='title', where={'type':('article', 'book')})

will only get their titles. Using a list (but not a tuple!) as a value in either the 'values' or 'where' dict params is translated to the proper SQL array syntax:

>>> update(cur, 'document', set={'topics':['database', 'programming']}, where={'document_id':doc_id})
>>> select(cur, 'document', where={'topics':['database', 'programming']})

The 'filter_values' option is useful if you do not wish to care about the exact values sent to the function. This for instance would fail:

>>> insert(cur, 'document', values={'title':'PG is Easy', 'author':'John Doe'})

because there is no 'author' column in our document table. This however would work:

>>> insert(cur, 'document', values={'title':'PG is Easy', 'author':'John Doe'}, filter_values=True)

because it trims any extra items in 'values' (i.e. corresponding to columns not belonging to the table). Note that since this option requires an extra SQL query, it makes a single call a little less efficient.

You can always append additional projection elements to a select query with the 'what' argument (which can be a string, a list or a dict, depending on your needs):

>>> select(cur, 'document', what={'*':1, 'title is not null':'has_title'})

will be translated as:

select *, (title is not null) as has_title from document

Similarly, by using the 'group_by' argument:

>>> select(cur, 'document', what=['type', 'count(*)'], group_by='type')

will yield:

select type, count(*) from document group by type

A select query can also be called with 'order_by', 'limit' and 'offset' optional arguments. You can also restrict the results to only one row by using the 'rows' argument (default is rows='all'):

>>> select(cur, 'document', where={'type':'article'], rows='one')

would return directly a document row (and not a list of rows), and would actually throw an assertion exception if there was more than one article in the document table.

This module is available for download and as a repository on GitHub.