Python

Charming Python: Decorators make magic easy

Python made metaprogramming possible, but each Python version has added slightly different -- and not quite compatible -- wrinkles to the way you accomplish metaprogramming tricks. Playing with first-class function objects has long been around, as have techniques for peaking and poking at magic attributes. With version 2.2, Python grew a custom metaclass mechanism that went a long way, but at the cost of melting users' brains. More recently, with version 2.4, Python has grown "decorators," which are the newest -- and by far the most user-friendly way, so far -- to perform most metaprogramming.

While metaprogramming is inherently a bit confusing, I think this article could have a been a little clearer. Still, it's a nice highlevel introduction to decorators.

Type inference for Python

The subject of type inference for dynamically-checked languages came up in the Buried Treasure thread. A question was raised in that thread having to do with why static type inference in these languages is difficult. Since there's a nascent body of literature which addresses that question, here are a few links to articles and papers about type inference for Python.

A nice overview can be found in Localized Type Inference of Atomic Types in Python, a Master's thesis by Brett Cannon. The whole thesis is relevant, but for an overview of the issues, see Chapter 3, "Challenges of Inferring Types in Python". Chapter 4 summarizes previous attempts involving static inference in Python, including Psyco (previously on LtU) and Starkiller. The limitations of these attempts are briefly addressed.

Type inference solutions for Python invariably involve restrictions to make the problem tractable. The above paper focuses on "inferring atomic types in the local namespace". Another approach is described in Aggressive Type Inference, by John Aycock. Aycock makes an important observation:

Giving people a dynamically-typed language does not mean that they write dynamically-typed programs.

The article offers a type inference approach which exploits this observation. (If the meaning of the above quote isn't clear, I recommend reviewing our mammoth three-part thread on the subject, "Why type systems are interesting", part I, part II, and part III.)

The PyPy implementation of Python in Python (previously on LtU) uses a restricted subset of Python, called RPython, to implement parts of the language. RPython is sufficiently static to be able to support full-program type inference. It is not a "soft" inference approach, and is not designed to be used with ordinary Python programs. The paper Compiling dynamic language implementations covers the approach used for static analysis of RPython. The PyPy Coding Guide, starting at section 1.4 may also be useful.

(It may be interesting to note that the PyPy approach is very similar to that used previously for Scheme 48. The core of Scheme 48 is implemented in PreScheme, a subset of Scheme that supports full-program type inference.)

Finally, Guido van Rossum has a number of blog entries on the subject of adding optional static typing to Python:

If anyone knows of any other good treatments of type inference in Python or similar languages, please post links here.

EasyExtend - Python MetaProgramming

Just saw this announcement on Google groups / comp.lang.python.

EasyExtend is a constructive approach to extend the Python language using pure Python. EasyExtend is developed as a Python framework depending only on tools provided by the CPython interpreter suite ( compiler ) and standard library as well as some pieces of code borrowed from the PyPy project. Opposite to toolkits for writing extension modules in C ( or RPython in future ) the EasyExtend framework is dedicated to extend the language itself by adding new grammar rules and transformation of parse trees. Acting directly on the nodes of syntax trees makes EasyExtend safe and extensible. Moreover the parser and the transformations are considerably fast. While EasyExtend can obviously be used to define "little languages" embedded in Python it can also be used to create Python oriented tools like a code coverage tool based on code generation. For both use-cases examples will be provided.

You'll want to probably want to check out the examples.

Python 2.5a1 released

Python 2.5 seems to be feature complete now and is released as a first alpha. See here for a complete list of new features.

From a language perspective enhanced generators and the new with-statement are probably the most interesting features. For many developers the incorporation of the small relational database sqlite, the new XML package elementree and the foreign function interface ctypes might be the highlights.

Class decorators in Python

Guido resisted the few calling for class decorators, because there wasn't a clear use case that wasn't more readable done another way... [but] Guido has conceded, class decorators will make it into some future version of Python.

More + links: here.

Tail call elimination decorator in Python

Features of a programming language, whether syntactic or semantic, are all part of the language's user interface. And a user interface can handle only so much complexity or it becomes unusable. This is also the reason why Python will never have continuations, and even why I'm uninterested in optimizing tail recursion.

Thus spoke Guido - as LtU readers already know.

Now, not even four weeks later, it has become clear that turning tail recursions into iterations can be achieved by an innocent little decorator in pure Python. No Rube Goldberg machine(s) in sight.

Guido: Language Design Is Not Just Solving Puzzles

And there's the rub: there's no way to make a Rube Goldberg language feature appear simple. Features of a programming language, whether syntactic or semantic, are all part of the language's user interface. And a user interface can handle only so much complexity or it becomes unusable.

The discussion is about multi-statement lambdas, but I don't want to discuss this specific issue. What's more interesting is the discussion of language as a user interface (an interface to what, you might ask), the underlying assumption that languages have character (e.g., Pythonicity), and the integrated view of semantics and syntax of language constructs when thinking about language usability.

PyPy

PyPy, the Python implementation written in Python, was mentioned here a couple of times in the past. After it was mentioned in a recent LtU discussion, I took another look, and boy did they make a lot of progress when I wasn't looking. PyPy can even compile itself now... You should check it out again if you are interested in this sort of thing.

There's even an introduction to the techniques used by PyPy, including a nice (but very high level) overview of abstract interpretation.

MyHDL

(via Daily Python-URL)

The key idea behind MyHDL is the use of Python generators to model hardware concurrency. Generators are best described as resumable functions. In MyHDL, generators are used in a specific way so that they become similar to always blocks in Verilog or processes in VHDL.

A hardware module is modeled as a function that returns any number of generators. This approach makes it straightforward to support features such as arbitrary hierarchy, named port association, arrays of instances, and conditional instantiation.

MyHDL is an open-source package for using Python as a hardware description and verification language. A Verilog converter is also included.

EE Times provides some background on MyHDL in this article.

NetKernel - XML processing pipeline

It rapidly became clear that a single language runtime is too limited for general applications ... as a minimum we needed both a linear-flow language and a recursive tree composition language ... while declarative languages are excellent for rapid assembly of XML operations, they are terrible for expressing business logic and logical flow-control ... Our other declarative language is XML Recursion Language (XRL). XRL is like XInclude with services, in which inclusion references fire service invocations into the URI address space in order to recursively compose an XML document. XRL is an elegant and powerful way of building XHTML applications ... The active URI, in combination with the local NetKernel environment, is a functional program - Introducing NetKernel.

Main site; Tour.

It's another XML pipeline (there's a Freshmeat project that lets Coccoon apps run in NetKernel), apparently from HP, which might interest people here.

XML feed