The goless library provides Go programming language semantics built on top of gevent, PyPy, or Stackless Python.

For an example of what goless can do, here is the Go program at reimplemented with goless:

c1 = goless.chan()
c2 = goless.chan()

def func1():

def func2():

for i in range(2):
    case, val =[goless.rcase(c1), goless.rcase(c2)])

It is surely a testament to Go’s style that it isn’t much less Python code than Go code, but I quite like this. Don’t you?


The goless.go() function mimics Go’s goroutines by, unsurprisingly, running the routine in a tasklet/greenlet. If an unhandled exception occurs in a goroutine, goless.on_panic() is called.

goless.go(func, *args, **kwargs)

Run a function in a new tasklet, like a goroutine. If the goroutine raises an unhandled exception (panics), the goless.on_panic() will be called, which by default logs the error and exits the process.

  • args – Positional arguments to func.
  • kwargs – Keyword arguments to func.
goless.on_panic(etype, value, tb)

Called when there is an unhandled error in a goroutine. By default, logs and exits the process.


There are three types of channels available in goless. Use the goless.chan() function to create a channel. The channel implementations contain more thorough documentation about how they actually work.


Returns a bidirectional channel.

A 0 or None size indicates a blocking channel (the send method will block until a receiver is available, and the recv method will block until a sender is available).

A positive integer value will return a channel with a buffer. Once the buffer is filled, the send method will block. When the buffer is empty, the recv method will block.

A negative integer will return a channel that will never block when the send method is called. The recv method will block if the buffer is empty.

Return type:goless.channels.GoChannel
class goless.channels.GoChannel

A Go-like channel that can be sent to, received from, and closed. Callers should never create this directly. Always use goless.chan() to create channels.


Closes the channel, not allowing further communication. Any blocking senders or receivers will be woken up and raise goless.ChannelClosed. Receiving or sending to a closed channel will raise goless.ChannelClosed.


Receive a value from the channel. Receiving will always block if no value is available. If the channel is already closed, goless.ChannelClosed will be raised. If the channel closes during a blocking recv, goless.ChannelClosed will be raised. (#TODO)


Sends the value. Blocking behavior depends on the channel type. Unbufferred channels block if no receiver is waiting. Buffered channels block if the buffer is full. Asynchronous channels never block on send.

If the channel is already closed, goless.ChannelClosed will be raised. If the channel closes during a blocking send, goless.ChannelClosed will be raised. (#TODO)

class goless.ChannelClosed

Exception raised to indicate a channel is closing or has closed.

The select function

Go’s select statement is implemented through the function. Because Python lacks anonymous blocks (multiline lambdas), works like Go’s reflect.Select function. Callers should create any number of classes that are passed into The function returns the chosen case, which the caller will usually switch off of. For example:

chan = goless.chan()
cases = [goless.rcase(chan), goless.scase(chan, 1), goless.dcase()]
chosen, value =
if chosen is cases[0]:
    print('Received %s' % value)
elif chosen is cases[1]:
    assert value is None
    assert chosen is cases[2]

Callers should never have to do anything with cases, other than create and switch off of them.*cases)

Select the first case that becomes ready. If a default case (goless.dcase) is present, return that if no other cases are ready. If there is no default case and no case is ready, block until one becomes ready.

See Go’s reflect.Select method for an analog (

Parameters:cases – List of case instances, such as goless.rcase, goless.scase, or goless.dcase.
Returns:(chosen case, received value). If the chosen case is not an goless.rcase, it will be None.
class goless.dcase

The default case.

class goless.rcase(chan)

A case that will chan.recv() when the channel is able to receive.

class goless.scase(chan, value)

A case that will chan.send(value) when the channel is able to send.

Exception Handling

Exception handling is a tricky topic and may change in the future. The default behavior right now is that an unhandled exception in a goroutine will log the exception and take down the entire process. This in theory emulates Go’s panic behavior: if a goroutine panics, the process will exit.

If you are not happy with this behavior, you should patch goless.on_panic to provide custom behavior.

If you find a better pattern, create an issue on GitHub.


The examples/ folder contains a number of examples.

In addtion, there are many examples from implemented via goless in the tests/ file.

If there is an example you’d like to see, or an idiomatic Go example you’d like converted, please see Contributing below.


You can run benchmarks using the current Python interpreter and configured backend by running the following from the goless project directory:

$ python -m benchmark

Developers may run benchmarks locally and report them into the following table. The Go versions of the benchmarks are also run. The numbers are useful for relative comparisons only:

Current goless Benchmarks
Platform Backend Benchmark Time
go gc chan_async 0.00236
PyPy2 stackless chan_async 0.03200
CPython2 stackless chan_async 0.09000
PyPy2 gevent chan_async 0.39600
CPython3 gevent chan_async 0.91000
CPython2 gevent chan_async 1.05000
go gc chan_buff 0.00235
PyPy2 stackless chan_buff 0.03200
CPython2 stackless chan_buff 0.10000
PyPy2 gevent chan_buff 0.39600
CPython3 gevent chan_buff 0.97000
CPython2 gevent chan_buff 1.11000
go gc chan_sync 0.00507
PyPy2 stackless chan_sync 0.05200
CPython2 stackless chan_sync 0.10000
PyPy2 gevent chan_sync 0.80000
CPython3 gevent chan_sync 0.89000
CPython2 gevent chan_sync 1.07000
go gc select 0.03031
PyPy2 stackless select 0.06400
CPython2 stackless select 0.28000
PyPy2 gevent select 0.49200
CPython3 gevent select 1.38000
CPython2 gevent select 1.49000
PyPy2 gevent select_default 0.00800
PyPy2 stackless select_default 0.00800
go gc select_default 0.02645
CPython2 stackless select_default 0.14000
CPython3 gevent select_default 0.15000
CPython2 gevent select_default 0.20000

To regenerate this table, run:

$ python

To print the table to stdout, run (notice the trailing - char):

$ python -

Assuming you have Go installed, you can run the benchmarks with:

$ go run benchmark.go


There are two backends for concurrently available in goless.backends. Backends should only be used by goless, and not by any client code. You can choose between backends by setting the environment variable GOLESS_BACKEND to "gevent" or "stackless". Otherwise, an appropriate backend will be chosen. If neither gevent or stackless are available, goless will raise an error when used (but will still be importable).

Compatibility Details

The good news is that you probably don’t need to worry about any of this, and goless works almost everywhere.

The bad news is, almost all abstractions are leaky, and there can be some nuances to compatibility. If you run into an issue where goless cannot create a backend, you may need to read the following sections.


goless works under PyPy out of the box with the stackless backend, because PyPy includes a file in its standard library. This appears to work properly, but fails the goless test suite. We are not sure why yet, as does not have a real maintainer and the bug is difficult to track down. However, the examples and common usages seem to all work fine.

Using PyPy 2.2+ and the tip of gevent’s GitHub repo ( ), the gevent backend works and is fully tested.

Python 2 (CPython)

Using Python 2 and the CPython interpreter, you can use the gevent backend for goless with no problems. Under Python 2, you can just do:

$ pip install gevent
$ pip install goless

Python 3 (CPython)

Newer versions of gevent include Python 3 compatibility. To install gevent on Python3, you also must install Cython. So you can use thew following commands to install goless under Python3 with its gevent backend:

$ pip install cython
$ pip install git+
$ pip install goless

This works and is tested.

Stackless Python

All versions of Stackless Python (2 and 3) should work with goless. However, we cannot test with Stackless Python on Travis, so we only test with it locally. If you find any problems, please report an issue.

goless and the GIL

goless does not address CPython’s Global Interpreter Lock (GIL) at all. goless does not magically provide any parallelization. It provides Go-like semantics, but not its performance. Perhaps this will change in the future if the GIL is removed. Another option is PyPy’s STM branch, which goless will (probably) benefit heartily.



I am definitely not a Go expert, so improvements to make things more idiomatic are very welcome. Please create an issue or pull request on GitHub:

goless was created by a number of people at the PyCon 2014 sprints. Even a small library like goless is the product of lots of collaboration.


Special thanks:


Coverage is wrong. It should be higher. The coverage module does not work properly with gevent and stackless.

Indices and tables