Getting Started with RPython


Please read this FAQ entry first!

RPython is a subset of Python2 that can be statically compiled. The PyPy interpreter is written mostly in RPython (with pieces in Python), while the RPython compiler is written in Python. The hard to understand part is that Python is a meta-programming language for RPython, that is, “being valid RPython” is a question that only makes sense on the live objects after the imports are done. This might require more explanation. You start writing RPython from entry_point, a good starting point is rpython/translator/goal/ This does not do all that much, but is a start. Now if code analyzed (in this case entry_point) calls some functions, those calls will be followed. Those followed calls have to be RPython themselves (and everything they call etc.), however not entire module files. To show how you can use metaprogramming, we can do a silly example (note that closures are not RPython):

def generator(operation):
    if operation == 'add':
       def f(a, b):
           return a + b
       def f(a, b):
           return a - b
    return f

add = generator('add')
sub = generator('sub')

def entry_point(argv):
    print add(sub(int(argv[1]), 3) 4)
    return 0

In this example entry_point is RPython, add and sub are RPython, however, generator is not.

The following introductory level articles are available:

Trying out the translator

The translator is a tool based on the PyPy interpreter which can translate sufficiently static RPython programs into low-level code (in particular it can be used to translate the full Python interpreter). To be able to experiment with it you need to download and install the usual (CPython2) version of:

To start the interactive translator shell do:

cd rpython
python2 bin/

Test snippets of translatable code are provided in the file rpython/translator/test/, which is imported under the name snippet. For example:

>>> t = Translation(snippet.is_perfect_number, [int])
>>> t.view()

After that, the graph viewer pops up, that lets you interactively inspect the flow graph. To move around, click on something that you want to inspect. To get help about how to use it, press ‘H’. To close it again, press ‘Q’.

Trying out the type annotator

We have a type annotator that can completely infer types for functions like is_perfect_number (as well as for much larger examples):

>>> t.annotate()
>>> t.view()

Move the mouse over variable names (in red) to see their inferred types.

Translating the flow graph to C code

The graph can be turned into C code:

>>> t.rtype()
>>> lib = t.compile_c()

The first command replaces the operations with other low level versions that only use low level types that are available in C (e.g. int). The compiled version is now in a .so library. You can run it say using ctypes:

>>> f = get_c_function(lib, snippet.is_perfect_number)
>>> f(5)
>>> f(6)

A slightly larger example

There is a small-to-medium demo showing the translator and the annotator:

python2 bin/rpython --view --annotate translator/goal/

This causes to display itself as a call graph and class hierarchy. Clicking on functions shows the flow graph of the particular function. Clicking on a class shows the attributes of its instances. All this information (call graph, local variables’ types, attributes of instances) is computed by the annotator.

To turn this example to C code (compiled to the executable bpnn-c), type simply:

python2 bin/rpython translator/goal/

Translating Full Programs

To translate full RPython programs, there is the script rpython/bin/rpython. Examples for this are a slightly changed version of Pystone:

python2 bin/rpython translator/goal/targetrpystonedalone

This will produce the executable “targetrpystonedalone-c”.

The largest example of this process is to translate the full Python interpreter. There is also an FAQ about how to set up this process for your own interpreters.

There are several environment variables you can find useful while playing with the RPython:

RPython uses temporary session directories to store files that are generated during the translation process(e.g., translated C files). PYPY_USESSION_DIR serves as a base directory for these session dirs. The default value for this variable is the system’s temporary dir.
By default RPython keeps only the last PYPY_USESSION_KEEP (defaults to 3) session dirs inside PYPY_USESSION_DIR. Increase this value if you want to preserve C files longer (useful when producing lots of lldebug builds).


  • rpython/translator contains the code analysis and generation stuff. Start reading from, from which it should be easy to follow the pieces of code involved in the various translation phases.
  • rpython/annotator contains the data model for the type annotation that can be inferred about a graph. The graph “walker” that uses this is in rpython/annotator/
  • rpython/rtyper contains the code of the RPython typer. The typer transforms annotated flow graphs in a way that makes them very similar to C code so that they can be easy translated. The graph transformations are controlled by the code in rpython/rtyper/ The object model that is used can be found in rpython/rtyper/lltypesystem/ For each RPython type there is a file that contains the low level functions needed for this type.
  • rpython/rlib contains the RPython standard library, things that you can use from rpython.