Articles from November 2020

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The psycopg3 adaptation system

The adaptation system between Python objects and PostgreSQL types is at the core of psycopg2 and psycopg3. The flexibility of the psycopg2 adaptation system provides good out-of-the-box object mapping and allows users to customise it to suit any need. Do you want your decimal numbers returned as float because you need speed over pennies? Do you want to map PostgreSQL Infinity dates to the 25th of December 3099? That's certainly doable.

The psycopg3 adaptation system needs some modification compared to psycopg2, because psycopg3 uses the "extended query protocol" to send query parameters separately from the query. Together, with the differences to accommodate, there is also a chance to improve a system that has been in use for several years and has shown its shortcomings together with its strengths.

The new COPY support in psycopg3

psycopg2 allows interaction with PostgreSQL COPY commands. However what is possible to do with them is relatively limited: the only possible interaction is with file-like objects:

there is no adaptation from Python objects to PostgreSQL, as there is for normal queries: data must be formatted "manually" by the user;
psycopg2 "pulls" data from the file: writing a system that produces data and pushes it into PostgreSQL is a very contrived operation, requiring to write a blocking file-like object;
there is no support for asynchronous copy.

psycopg3 addresses these shortcomings and makes it easy to write Python programs producing data and pushing it efficiently to the database using the COPY protocol.