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You write Processing code. In Python.

Processing is a programming language, development environment, and online community.
Since 2001, Processing has promoted software literacy within the visual arts and visual literacy within technology.
Today, there are tens of thousands of students, artists, designers, researchers, and hobbyists who use Processing for learning, prototyping, and production.

Processing was initially released with a Java-based syntax, and with a lexicon of graphical primitives that took inspiration from OpenGL, Postscript, Design by Numbers, and other sources.
With the gradual addition of alternative progamming interfaces — including JavaScript, Python, and Ruby — it has become increasingly clear that Processing is not a single language, but rather, an arts-oriented approach to learning, teaching, and making things with code.