Frequently Asked Questions

See also: Frequently ask questions about RPython.

What is PyPy?

PyPy is a reimplementation of Python in Python, using the RPython translation toolchain.

PyPy tries to find new answers about ease of creation, flexibility, maintainability and speed trade-offs for language implementations. For further details see our goal and architecture document.

Is PyPy a drop in replacement for CPython?


The most likely stumbling block for any given project is support for extension modules. PyPy supports a continually growing number of extension modules, but so far mostly only those found in the standard library.

The language features (including builtin types and functions) are very refined and well tested, so if your project doesn’t use many extension modules there is a good chance that it will work with PyPy.

We list the known differences in cpython differences.

Module xyz does not work with PyPy: ImportError

A module installed for CPython is not automatically available for PyPy — just like a module installed for CPython 2.6 is not automatically available for CPython 2.7 if you installed both. In other words, you need to install the module xyz specifically for PyPy.

On Linux, this means that you cannot use apt-get or some similar package manager: these tools are only meant for the version of CPython provided by the same package manager. So forget about them for now and read on.

It is quite common nowadays that xyz is available on PyPI and installable with pip install xyz. The simplest solution is to use virtualenv (as documented here). Then enter (activate) the virtualenv and type: pip install xyz. If you don’t know or don’t want virtualenv, you can also install pip globally by saying pypy -m ensurepip.

If you get errors from the C compiler, the module is a CPython C Extension module using unsupported features. See below.

Alternatively, if either the module xyz is not available on PyPI or you don’t want to use virtualenv, then download the source code of xyz, decompress the zip/tarball, and run the standard command: pypy install. (Note: pypy here instead of python.) As usual you may need to run the command with sudo for a global installation. The other commands of are available too, like build.

Module xyz does not work in the sandboxed PyPy?

You cannot import any extension module in a sandboxed PyPy, sorry. Even the built-in modules available are very limited. Sandboxing in PyPy is a good proof of concept, and is without a doubt safe IMHO, however it is only a proof of concept. It currently requires some work from a motivated developer. However, until then it can only be used for “pure Python” example: programs that import mostly nothing (or only pure Python modules, recursively).

Do CPython Extension modules work with PyPy?

First note that some Linux distributions (e.g. Ubuntu, Debian) split PyPy into several packages. If you installed a package called “pypy”, then you may also need to install “pypy-dev” for the following to work.

We have experimental support for CPython extension modules, so they run with minor changes. This has been a part of PyPy since the 1.4 release, but support is still in beta phase. CPython extension modules in PyPy are often much slower than in CPython due to the need to emulate refcounting. It is often faster to take out your CPython extension and replace it with a pure python version that the JIT can see. If trying to install module xyz, and the module has both a C and a Python version of the same code, try first to disable the C version; this is usually easily done by changing some line in

We fully support ctypes-based extensions. But for best performance, we recommend that you use the cffi module to interface with C code.

For information on which third party extensions work (or do not work) with PyPy see the compatibility wiki.

For more information about how we manage refcounting semamtics see rawrefcount

On which platforms does PyPy run?

PyPy currently supports:

  • x86 machines on most common operating systems (Linux 32/64 bits, Mac OS X 64 bits, Windows 32 bits, OpenBSD, FreeBSD),
  • newer ARM hardware (ARMv6 or ARMv7, with VFPv3) running Linux,
  • big- and little-endian variants of PPC64 running Linux,
  • s390x running Linux

PyPy is regularly and extensively tested on Linux machines. It works on Mac and Windows: it is tested there, but most of us are running Linux so fixes may depend on 3rd-party contributions.

To bootstrap from sources, PyPy can use either CPython 2.7 or another (e.g. older) PyPy. Cross-translation is not really supported: e.g. to build a 32-bit PyPy, you need to have a 32-bit environment. Cross-translation is only explicitly supported between a 32-bit Intel Linux and ARM Linux (see here).

Which Python version (2.x?) does PyPy implement?

PyPy currently aims to be fully compatible with Python 2.7. That means that it contains the standard library of Python 2.7 and that it supports 2.7 features (such as set comprehensions).

Does PyPy have a GIL? Why?

Yes, PyPy has a GIL. Removing the GIL is very hard. On top of CPython, you have two problems: (1) GC, in this case reference counting; (2) the whole Python language.

For PyPy, the hard issue is (2): by that I mean issues like what occurs if a mutable object is changed from one thread and read from another concurrently. This is a problem for any mutable type: it needs careful review and fixes (fine-grained locks, mostly) through the whole Python interpreter. It is a major effort, although not completely impossible, as Jython/IronPython showed. This includes subtle decisions about whether some effects are ok or not for the user (i.e. the Python programmer).

CPython has additionally the problem (1) of reference counting. With PyPy, this sub-problem is simpler: we need to make our GC multithread-aware. This is easier to do efficiently in PyPy than in CPython. It doesn’t solve the issue (2), though.

Note that since 2012 there is work going on on a still very experimental Software Transactional Memory (STM) version of PyPy. This should give an alternative PyPy which works without a GIL, while at the same time continuing to give the Python programmer the complete illusion of having one. This work is currently a bit stalled because of its own technical difficulties.

Is PyPy more clever than CPython about Tail Calls?

No. PyPy follows the Python language design, including the built-in debugger features. This prevents tail calls, as summarized by Guido van Rossum in two blog posts. Moreover, neither the JIT nor Stackless change anything to that.

How fast is PyPy?

This really depends on your code. For pure Python algorithmic code, it is very fast. For more typical Python programs we generally are 3 times the speed of CPython 2.7. You might be interested in our benchmarking site and our jit documentation.

Your tests are not a benchmark: tests tend to be slow under PyPy because they run exactly once; if they are good tests, they exercise various corner cases in your code. This is a bad case for JIT compilers. Note also that our JIT has a very high warm-up cost, meaning that any program is slow at the beginning. If you want to compare the timings with CPython, even relatively simple programs need to run at least one second, preferrably at least a few seconds. Large, complicated programs need even more time to warm-up the JIT.

Couldn’t the JIT dump and reload already-compiled machine code?

No, we found no way of doing that. The JIT generates machine code containing a large number of constant addresses — constant at the time the machine code is generated. The vast majority is probably not at all constants that you find in the executable, with a nice link name. E.g. the addresses of Python classes are used all the time, but Python classes don’t come statically from the executable; they are created anew every time you restart your program. This makes saving and reloading machine code completely impossible without some very advanced way of mapping addresses in the old (now-dead) process to addresses in the new process, including checking that all the previous assumptions about the (now-dead) object are still true about the new object.

Would type annotations help PyPy’s performance?

Two examples of type annotations that are being proposed for improved performance are Cython types and PEP 484 - Type Hints.

Cython types are, by construction, similar to C declarations. For example, a local variable or an instance attribute can be declared "cdef int" to force a machine word to be used. This changes the usual Python semantics (e.g. no overflow checks, and errors when trying to write other types of objects there). It gives some extra performance, but the exact benefits are unclear: right now (January 2015) for example we are investigating a technique that would store machine-word integers directly on instances, giving part of the benefits without the user-supplied "cdef int".

PEP 484 - Type Hints, on the other hand, is almost entirely useless if you’re looking at performance. First, as the name implies, they are hints: they must still be checked at runtime, like PEP 484 says. Or maybe you’re fine with a mode in which you get very obscure crashes when the type annotations are wrong; but even in that case the speed benefits would be extremely minor.

There are several reasons for why. One of them is that annotations are at the wrong level (e.g. a PEP 484 “int” corresponds to Python 3’s int type, which does not necessarily fits inside one machine word; even worse, an “int” annotation allows arbitrary int subclasses). Another is that a lot more information is needed to produce good code (e.g. “this f() called here really means this function there, and will never be monkey-patched” – same with len() or list(), btw). The third reason is that some “guards” in PyPy’s JIT traces don’t really have an obvious corresponding type (e.g. “this dict is so far using keys which don’t override __hash__ so a more efficient implementation was used”). Many guards don’t even have any correspondence with types at all (“this class attribute was not modified”; “the loop counter did not reach zero so we don’t need to release the GIL”; and so on).

As PyPy works right now, it is able to derive far more useful information than can ever be given by PEP 484, and it works automatically. As far as we know, this is true even if we would add other techniques to PyPy, like a fast first-pass JIT.

Can I use PyPy’s translation toolchain for other languages besides Python?

Yes. The toolsuite that translates the PyPy interpreter is quite general and can be used to create optimized versions of interpreters for any language, not just Python. Of course, these interpreters can make use of the same features that PyPy brings to Python: translation to various languages, stackless features, garbage collection, implementation of various things like arbitrarily long integers, etc.

Currently, we have Topaz, a Ruby interpreter; Hippy, a PHP interpreter; preliminary versions of a JavaScript interpreter (Leonardo Santagada as his Summer of PyPy project); a Prolog interpreter (Carl Friedrich Bolz as his Bachelor thesis); and a SmallTalk interpreter (produced during a sprint). On the PyPy bitbucket page there is also a Scheme and an Io implementation; both of these are unfinished at the moment.

How do I get into PyPy development? Can I come to sprints?

Certainly you can come to sprints! We always welcome newcomers and try to help them as much as possible to get started with the project. We provide tutorials and pair them with experienced PyPy developers. Newcomers should have some Python experience and read some of the PyPy documentation before coming to a sprint.

Coming to a sprint is usually the best way to get into PyPy development. If you get stuck or need advice, contact us. IRC is the most immediate way to get feedback (at least during some parts of the day; most PyPy developers are in Europe) and the mailing list is better for long discussions.

OSError: ... cannot restore segment prot after reloc... Help?

On Linux, if SELinux is enabled, you may get errors along the lines of “OSError: cannot restore segment prot after reloc: Permission denied.” This is caused by a slight abuse of the C compiler during configuration, and can be disabled by running the following command with root privileges:

# setenforce 0

This will disable SELinux’s protection and allow PyPy to configure correctly. Be sure to enable it again if you need it!

How should I report a bug?

Our bug tracker is here:

Missing features or incompatibilities with CPython are considered bugs, and they are welcome. (See also our list of known incompatibilities.)

For bugs of the kind “I’m getting a PyPy crash or a strange exception”, please note that: We can’t do anything without reproducing the bug ourselves. We cannot do anything with tracebacks from gdb, or core dumps. This is not only because the standard PyPy is compiled without debug symbols. The real reason is that a C-level traceback is usually of no help at all in PyPy. Debugging PyPy can be annoying.

This is a clear and useful bug report. (Admittedly, sometimes the problem is really hard to reproduce, but please try to.)

In more details:

  • First, please give the exact PyPy version, and the OS.
  • It might help focus our search if we know if the bug can be reproduced on a “pypy --jit off” or not. If “pypy --jit off” always works, then the problem might be in the JIT. Otherwise, we know we can ignore that part.
  • If you got the bug using only Open Source components, please give a step-by-step guide that we can follow to reproduce the problem ourselves. Don’t assume we know anything about any program other than PyPy. We would like a guide that we can follow point by point (without guessing or having to figure things out) on a machine similar to yours, starting from a bare PyPy, until we see the same problem. (If you can, you can try to reduce the number of steps and the time it needs to run, but that is not mandatory.)
  • If the bug involves Closed Source components, or just too many Open Source components to install them all ourselves, then maybe you can give us some temporary ssh access to a machine where the bug can be reproduced. Or, maybe we can download a VirtualBox or VMWare virtual machine where the problem occurs.
  • If giving us access would require us to use tools other than ssh, make appointments, or sign a NDA, then we can consider a commerical support contract for a small sum of money.
  • If even that is not possible for you, then sorry, we can’t help.

Of course, you can try to debug the problem yourself, and we can help you get started if you ask on the #pypy IRC channel, but be prepared: debugging an annoying PyPy problem usually involves quite a lot of gdb in auto-generated C code, and at least some knowledge about the various components involved, from PyPy’s own RPython source code to the GC and possibly the JIT.

Why doesn’t PyPy move to GitHub, Gitlab, ...?

We’ve been quite happy with Moving version control systems and hosting is a lot of hard work: On the one hand, PyPy’s mercurial history is long and gnarly. On the other hand, all our infrastructure (buildbots, benchmarking, etc) would have to be adapted. So unless somebody steps up and volunteers to do all that work, it will likely not happen.

What is needed for Windows 64 support of PyPy?

First, please note that the Windows 32 PyPy binary works just fine on Windows 64. The only problem is that it only supports up to 4GB of heap per process.

As to real Windows 64 support: Currently we don’t have an active PyPy developer whose main development platform is Windows. So if you are interested in getting Windows 64 support, we encourage you to volunteer to make it happen! Another option would be to pay some PyPy developers to implement Windows 64 support, but so far there doesn’t seem to be an overwhelming commercial interest in it.

How long will PyPy support Python2?

Since RPython is built on top of Python2 and that is extremely unlikely to change, the Python2 version of PyPy will be around “forever”, i.e. as long as PyPy itself is around.