Posts from 2012-03

Binary operators in Python

A two-hour train journey provided the opportunity to watch the video recording of the Panel with Guido van Rossum at the recent PyData Workshop. The lengthy discussion about PEP 225 (which proposes to add additional operators to Python that would enable to have both elementwise and aggregate operations on the same objects, in particular for providing both matrix and elementwise multiplication on arrays with a nice syntax) motivated me to write up my own thoughts about what's wrong with operators in Python from my computational scientist's point of view.

The real problem I see is that operators map to methods. In Python, a*b is just syntactic sugar for a.__mul__(b). This means that it's the type of a that decides how to do the multiplication. The method implementing this operation can of course check the type of b, and it can even decide to give up and let b handle everything, in which case Python does b.__rmul__(a). But this is just a kludge to work around the real weakness of the operators-map-to-methods approach. Binary operators fundamentally require a dispatch on both types, the type of a and the type of b. What a*b should map to is __builtins__.__mul__(a, b), a global function that would then implement a binary dispatch operation. Implementing that dispatch would in fact be the real problem to solve, as Python currently has no multiple dispatch mechanisms at all.

But would multiple dispatch solve the issue addressed by PEP 225? Not at all, directly. But it would make some of the alternatives mentioned there feasible. A proper multiple dispatch system would allow NumPy (or any other library) to decide what multiplication of its own objects by a number means, no matter if the number is the first or the second factor.

More importantly, multiple dispatch would allow a major cleanup of many scientific packages, including NumPy, and even clean up the basic Python language by getting rid of __rmul__ and friends. NumPy's current aggressive handling of binary operations is actually more of a problem for me than the lack of a nice syntax for matrix multiplication.

There are many details that would need to be discussed before binary dispatch could be proposed as a PEP. Of course the old method-based approach would need to remain in place as a fallback, to ensure compatibility with existing code. But the real work is defining a good multiple dispatch system that integrates well with Python's dynamical type system and allows the right kind of extensibility. That same multiple dispatch method could then also be made available for use in plain functions.

Python becomes a platform

The recent announcement of clojure-py made some noise in the Clojure community, but not, as far as I can tell, in the Python community. For those who haven't heard of it before, clojure-py is an implementation of the Clojure language in Python, compiling Clojure code to bytecode for Python's virtual machine. It's still incomplete, but already usable if you can live with the subset of Clojure that has been implemented.

I think that this is an important event for the Python community, because it means that Python is no longer just a language, but is becoming a platform. One of the stated motivations of the clojure-py developers is to tap into the rich set of libraries that the Python ecosystem provides, in particular for scientific applications. Python is thus following the path that Java already went in the past: the Java virtual machine, initially designed only to support the Java language, became the target of many different language implementations which all provide interoperation with Java itself.

It will of course be interesting to see if more languages will follow once people realize it can be done. The prospect of speed through PyPy's JIT, another stated motivation for the clojure-py community, could also get more lanuage developers interested in Python as a platform.

Should Python programmers care about clojure-py? I'd say yes. Clojure is strong in two areas in which Python isn't. One of them is metaprogramming, a feature absent from Python which Clojure had from the start through its Lisp heritage. The other feature is persistent immutable data structures, for which clojure-py provides an implementation in Python. Immutable data structures make for more robust code, in particular but not exclusively for concurrent applications.


Tags: computational science, computer-aided research, emacs, mmtk, mobile computing, programming, proteins, python, rants, reproducible research, science, scientific computing, scientific software, social networks, software, source code repositories, sustainable software

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