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Python compatibility

PyPy implements the Python language version 2.7.18. It supports all of the core language, passing Python test suite (with minor modifications that were already accepted in the main python in newer versions). It supports most of the commonly used Python standard library modules; details below.

PyPy3 implements the Python language version 3.7.9. It has been released, but Python is a large language and it is quite possible that a few things are missing.

PyPy has support for the CPython C API, however there are constructs that are not compatible. We strongly advise use of CFFI instead. CFFI come builtin with PyPy. Many libraries will require a bit of effort to work, but there are known success stories. Check out PyPy blog for updates

C extensions need to be recompiled for PyPy in order to work. Depending on your build system, it might work out of the box or will be slightly harder.

Standard library modules supported by PyPy. Note that large parts of python library are implemented in pure python, so they don't have to be listed there. Please just check if it imports. If it imports, it should work:

__builtin__, __pypy__, _ast, _cffi_backend, _codecs, _collections, _continuation, _csv, _file, _hashlib, _io, _locale, _lsprof, _md5, _minimal_curses, _multibytecodec, _multiprocessing, _numpypy, _pickle_support, _pypyjson, _random, _rawffi, _sha, _socket, _sre, _ssl, _struct, _testing, _warnings, _weakref, array, binascii, bz2, cStringIO, cmath, cppyy, cpyext, crypt, errno, exceptions, fcntl, gc, imp, itertools, marshal, math, mmap, operator, parser, posix, pwd, pyexpat, pypyjit, select, signal, symbol, sys, termios, thread, time, token, unicodedata, zipimport, zlib

Supported, and written in pure Python:

cPickle, ctypes, datetime, dbm, _functools, grp, readline, resource, sqlite3, syslog

All modules that are pure python in CPython of course work.

The main difference that is not going to be fixed is that PyPy does not support refcounting semantics. The following code won't fill the file immediately, but only after a certain period of time, when the GC does a collection and flushes the output:

open("filename", "w").write("stuff")

The proper fix is

with open("filename", "w") as f:

The same problem---not closing your files---can also show up if your program opens a large number of files without closing them explicitly. In that case, you can easily hit the system limit on the number of file descriptors that are allowed to be opened at the same time.

PyPy can be run with the command-line option -X track-resources (as in, pypy -X track-resources This produces a ResourceWarning when the GC closes a non-closed file or socket. The traceback for the place where the file or socket was allocated is given as well, which aids finding places where close() is missing.

Similarly, remember that you must close() a non-exhausted generator in order to have its pending finally or with clauses executed immediately:

def mygen():
    with foo:
        yield 42

for x in mygen():
    if x == 42:
        break    # foo.__exit__ is not run immediately!

# fixed version:
gen = mygen()
    for x in gen:
        if x == 42:

More generally, __del__() methods are not executed as predictively as on CPython: they run "some time later" in PyPy (or not at all if the program finishes running in the meantime). See more details here.

Note that PyPy returns unused memory to the operating system if there is a madvise() system call (at least Linux, OS X, BSD) or on Windows. It is important to realize that you may not see this in top. The unused pages are marked with MADV_FREE, which tells the system "if you need more memory at some point, grab this page". As long as memory is plentiful, the RES column in top might remains high. (Exceptions to this rule are systems with no MADV_FREE, where we use MADV_DONTNEED, which forcefully lowers the RES. This includes Linux <= 4.4.)

A more complete list of known differences is available at our dev site.