What’s new in gevent 1.1

Detailed information an what has changed is available in the Changelog. This document summarizes the most important changes since gevent 1.0.2.

Broader Platform Support

gevent 1.1 supports Python 2.6, 2.7, 3.3, and 3.4 on the CPython (python.org) interpreter. It also supports PyPy 2.6.1 and above (PyPy 4.0.1 or higher is recommended); PyPy3 is not supported.

Support for Python 2.5 was removed when support for Python 3 was added. Any further releases in the 1.0.x line will maintain support for Python 2.5.


Version 1.1.x will be the last series of gevent releases to support Python 2.6. The next major release will only support Python 2.7 and above.

Python 3.5 has preliminary support, which means that gevent is expected to generally run and function with the same level of support as on Python 3.4, but new features and APIs introduced in 3.5 may not be properly supported (e.g., DevpollSelector) and due to the recent arrival of Python 3.5, the level of testing it has received is lower.

For ease of installation on Windows and OS X, gevent 1.1 is distributed as pre-compiled binary wheels, in addition to source code.

PyPy Notes

PyPy has been tested on OS X and 64-bit Linux from version 2.6.1 through 4.0.0 and 4.0.1, and on 32-bit ARM on Raspbian with version 4.0.1.


PyPy is not supported on Windows. (gevent’s CFFI backend is not available on Windows.)

  • Version 4.0.1 or above is highly recommended due to its extensive bug fixes relative to earlier versions.
  • Version 2.6.1 or above is required for proper signal handling. Prior to 2.6.1 and its inclusion of cffi 1.3.0, signals could be delivered incorrectly or fail to be delivered during a blocking operation. (PyPy 2.5.0 includes CFFI 0.8.6 while 2.6.0 has 1.1.0; the necessary feature was added in 1.2.0 which is not itself directly present in any PyPy release.) CFFI 1.3.0 also allows using the CFFI backend on CPython.
  • Overall performance seems to be quite acceptable with newer versions of PyPy. The benchmarks distributed with gevent typically perform as well or better on PyPy than on CPython at least on some platforms. Things that are known or expected to be (relatively) slower under PyPy include the c-ares resolver and Semaphore. Whether or not these matter will depend on the workload of each application (pull request #708 mentions some specific benchmarks for Semaphore).


The c-ares resolver is considered highly experimental under PyPy and is not recommended for production use. Released versions of PyPy through at least 4.0.1 have a bug that can cause a memory leak when subclassing objects that are implemented in Cython, as is the c-ares resolver. In addition, thanks to reports like issue #704, we know that the PyPy garbage collector can interact badly with Cython-compiled code, leading to crashes. While the intended use of the ares resolver has been loosely audited for these issues, no guarantees are made.


PyPy 4.0.x on Linux is known to rarely (once per 24 hours) encounter crashes when running heavily loaded, heavily networked gevent programs (even without c-ares). The exact cause is unknown and is being tracked in issue #677.

Operating Systems

gevent is regularly built and tested on Mac OS X, Ubuntu Linux, and Windows, in both 32- and 64-bit configurations. All three platforms are primarily tested on the x86/amd64 architecture, while Linux is also occasionally tested on Raspian on ARM.

In general, gevent should work on any platform that both Python and libev support. However, some less commonly used platforms may require tweaks to the gevent source code or user environment to compile (e.g., SmartOS). Also, due to differences in things such as timing, some platforms may not be able to fully pass gevent’s extensive test suite (e.g., OpenBSD).

Bug Fixes

Since 1.0.2, gevent 1.1 contains over 600 commits from nearly two dozen contributors. Over 200 issues were closed, and over 50 pull requests were merged.

Improved subprocess support

In gevent 1.0, support and monkey patching for the subprocess module was added. Monkey patching this module was off by default.

In 1.1, monkey patching subprocess is on by default due to improvements in handling child processes and requirements by downstream libraries, notably gunicorn.

  • gevent.os.fork(), which is monkey patched by default (and should be used to fork a gevent-aware process that expects to use gevent in the child process) has been improved and cooperates with gevent.os.waitpid() (again monkey patched by default) and gevent.signal.signal() (which is monkey patched only for the signal.SIGCHLD case). The latter two patches are new in 1.1.
  • In gevent 1.0, use of libev child watchers (which are used internally by gevent.subprocess) had race conditions with user-provided SIGCHLD handlers, causing many types of unpredictable breakage. The two new APIs described above are intended to rectify this.
  • Fork-watchers will be called, even in multi-threaded programs (except on Windows).
  • The default threadpool and threaded resolver work in child processes.
  • File descriptors are no longer leaked if gevent.subprocess.Popen fails to start the child.

In addition, simple use of multiprocessing.Process is now possible in a monkey patched system, at least on POSIX platforms.


Use of multiprocessing.Queue when thread has been monkey-patched will lead to a hang due to Queue‘s internal use of a blocking pipe and threads. For the same reason, concurrent.futures.ProcessPoolExecutor, which internally uses a Queue, will hang.


It is not possible to use gevent.subprocess from native threads. See gevent.subprocess for details.


If the SIGCHLD signal is to be handled, it is important to monkey patch (or directly use) both os and signal; this is the default for patch_all(). Failure to do so can result in the SIGCHLD signal being lost.


All of the above entail forking a child process. Forking a child process that uses gevent, greenlets, and libev can have some unexpected consequences if the child doesn’t immediately exec a new binary. Be sure you understand these consequences before using this functionality, especially late in a program’s lifecycle. For a more robust solution to certain uses of child process, consider gipc.

Monkey patching

Monkey patching is more robust, especially if the standard library threading or logging modules had been imported before applying the patch. In addition, there are now supported ways to determine if something has been monkey patched.

API Additions

Numerous APIs offer slightly expanded functionality in this version. Look for “changed in version 1.1” or “added in version 1.1” throughout the documentation for specifics. Highlights include:

  • A gevent-friendly version of select.poll (on platforms that implement it).
  • FileObjectPosix uses the io package on both Python 2 and Python 3, increasing its functionality, correctness, and performance. (Previously, the Python 2 implementation used the undocumented class socket._fileobject.)
  • Locks raise the same error as standard library locks if they are over-released. Likewise, SSL sockets raise the same errors as their bundled counterparts if they are read or written after being closed.
  • ThreadPool.apply can now be used recursively.
  • The various pool objects (Group, Pool, ThreadPool) support the same improved APIs: imap and imap_unordered accept multiple iterables, apply raises any exception raised by the target callable, etc.
  • Killing a greenlet (with gevent.kill() or Greenlet.kill) before it is actually started and switched to now prevents the greenlet from ever running, instead of raising an exception when it is later switched to. Attempting to spawn a greenlet with an invalid target now immediately produces a useful TypeError, instead of spawning a greenlet that would (usually) immediately die the first time it was switched to.
  • Almost anywhere that gevent raises an exception from one greenlet to another (e.g., Greenlet.get), the original traceback is preserved and raised.
  • Various logging/debugging outputs have been cleaned up.
  • The WSGI server found in gevent.pywsgi is more robust against errors in either the client or the WSGI application, fixing several hangs or HTTP protocol violations. It also supports new functionality such as configurable error handling and logging.
  • Documentation has been expanded and clarified.

Library Updates

The two C libraries that are bundled with gevent have been updated. libev has been updated from 4.19 to 4.20 (libev release notes) and c-ares has been updated from 1.9.1 to 1.10.0 (c-ares release notes).


The c-ares configure script is now much stricter about the contents of compilation environment variables such as $CFLAGS and $LDFLAGS. For example, $CFLAGS is no longer allowed to contain -I directives; instead, these must be placed in $CPPFLAGS. That’s one common cause of an error like the following when compiling from scratch on a POSIX platform:

Running '(cd  "/tmp/easy_install-NT921u/gevent-1.1b2/c-ares"  && if [ -e ares_build.h ]; then cp ares_build.h ares_build.h.orig; fi   && /bin/sh ./configure CONFIG_COMMANDS= CONFIG_FILES=   && cp ares_config.h ares_build.h "$OLDPWD"   && mv ares_build.h.orig ares_build.h) > configure-output.txt' in /tmp/easy_install-NT921u/gevent-1.1b2/build/temp.linux-x86_64-2.7/c-ares
configure: error: Can not continue. Fix errors mentioned immediately above this line.


This release is intended to be compatible with 1.0.x with minimal or no changes to client source code. However, there are a few changes to be aware of that might affect some applications. Most of these changes are due to the increased platform support of Python 3 and PyPy and reduce the cases of undocumented or non-standard behaviour.

  • gevent.baseserver.BaseServer deterministically closes its sockets.

    As soon as a request completes (the request handler returns), the BaseServer and its subclasses including gevent.server.StreamServer and gevent.pywsgi.WSGIServer close the client socket.

    In gevent 1.0, the client socket was left to the mercies of the garbage collector (this was undocumented). In the typical case, the socket would still be closed as soon as the request handler returned due to CPython’s reference-counting garbage collector. But this meant that a reference cycle could leave a socket dangling open for an indeterminate amount of time, and a reference leak would result in it never being closed. It also meant that Python 3 would produce ResourceWarnings, and PyPy (which, unlike CPython, does not use a reference-counted GC) would only close (and flush!) the socket at an arbitrary time in the future.

    If your application relied on the socket not being closed when the request handler returned (e.g., you spawned a greenlet that continued to use the socket) you will need to keep the request handler from returning (e.g., join the greenlet). If for some reason that isn’t possible, you may subclass the server to prevent it from closing the socket, at which point the responsibility for closing and flushing the socket is now yours; but the former approach is strongly preferred, and subclassing the server for this reason may not be supported in the future.

  • gevent.pywsgi.WSGIServer ensures that headers (names and values) and the status line set by the application can be encoded in the ISO-8859-1 (Latin-1) charset and are of the native string type.

    Under gevent 1.0, non-bytes headers (that is, unicode, since gevent 1.0 only ran on Python 2, although objects like int were also allowed) were encoded according to the current default Python encoding. In some cases, this could allow non-Latin-1 characters to be sent in the headers, but this violated the HTTP specification, and their interpretation by the recipient is unknown. In other cases, gevent could send malformed partial HTTP responses. Now, a UnicodeError will be raised proactively.

    Most applications that adhered to the WSGI PEP, PEP 3333, will not need to make any changes. See issue #614 for more discussion.

  • Under Python 2, the previously undocumented timeout parameter to Popen.wait (a gevent extension ) now throws an exception, just like the documented parameter to the same stdlib method in Python 3.

  • Under Python 3, several standard library methods added timeout parameters. These often default to -1 to mean “no timeout”, whereas gevent uses a default of None to mean the same thing, potentially leading to great confusion and bugs in portable code. In gevent, using a negative value has always been ill-defined and hard to reason about. Because of those two things, as of this release, negative timeout values should be considered deprecated (unless otherwise documented). The current ill-defined behaviour is maintained, but future releases may choose to treat it the same as None or raise an error. No runtime warnings are issued for this change for performance reasons.

  • The previously undocumented class gevent.fileobject.SocketAdapter has been removed, as have the internal gevent._util module and some internal implementation modules found in early pre-releases of 1.1.

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