# Green mode

PyTango supports cooperative green Tango objects. Since version 8.1 two green modes have been added: `Futures` and `Gevent`.

The `Futures` uses the standard python module `concurrent.futures`. The `Gevent` mode uses the well known gevent library.

Currently, in version 8.1, only `DeviceProxy` has been modified to work in a green cooperative way. If the work is found to be useful, the same can be implemented in the future for `AttributeProxy`, `Database`, `Group` or even in the server side.

You can set the PyTango green mode at a global level. Set the environment variable `PYTANGO_GREEN_MODE` to either futures or gevent (case incensitive). If this environment variable is not defined the PyTango global green mode defaults to Synchronous.

You can also change the active global green mode at any time in your program:

    >>> from PyTango import DeviceProxy, GreenMode
    >>> from PyTango import set_green_mode, get_green_mode

    >>> get_green_mode()
    PyTango.GreenMode.Synchronous

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> dev.get_green_mode()
    PyTango.GreenMode.Synchronous

    >>> set_green_mode(GreenMode.Futures)
    >>> get_green_mode()
    PyTango.GreenMode.Futures

    >>> dev.get_green_mode()
    PyTango.GreenMode.Futures

As you can see by the example, the global green mode will affect any previously created `DeviceProxy` using the default DeviceProxy constructor parameters.

You can specificy green mode on a `DeviceProxy` at creation time. You can also change the green mode at any time:

    >>> from PyTango.futures import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> dev.get_green_mode()
    PyTango.GreenMode.Futures

    >>> dev.set_green_mode(GreenMode.Synchronous)
    >>> dev.get_green_mode()
    PyTango.GreenMode.Synchronous

## futures mode

Using `concurrent.futures` cooperative mode in PyTango is relatively easy:

    >>> from PyTango.futures import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> dev.get_green_mode()
    PyTango.GreenMode.Futures

    >>> print(dev.state())
    RUNNING

The `PyTango.futures.DeviceProxy()` API is exactly the same as the standard `DeviceProxy`. The difference is in the semantics of the methods that involve synchronous network calls (constructor included) which may block the execution for a relatively big amount of time. The list of methods that have been modified to accept futures semantics are, on the `PyTango.futures.DeviceProxy()`:

- Constructor
- `state()`
- `status()`
- `read_attribute()`
- `write_attribute()`
- `write_read_attribute()`
- `read_attributes()`
- `write_attributes()`
- `ping()`

So how does this work in fact? I see no difference from using the standard `DeviceProxy`. Well, this is, in fact, one of the goals: be able to use a futures cooperation without changing the API. Behind the scenes the methods mentioned before have been modified to be able to work cooperatively.

All of the above methods have been boosted with two extra keyword arguments wait and timeout which allow to fine tune the behaviour. The wait parameter is by default set to True meaning wait for the request to finish (the default semantics when not using green mode). If wait is set to True, the timeout determines the maximum time to wait for the method to execute. The default is None which means wait forever. If wait is set to False, the timeout is ignored.

If wait is set to True, the result is the same as executing the standard method on a `DeviceProxy`. If, wait is set to False, the result will be a `concurrent.futures.Future`. In this case, to get the actual value you will need to do something like:

    >>> from PyTango.futures import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> result = dev.state(wait=False)
    >>> result
    <Future at 0x16cb310 state=pending>

    >>> # this will be the blocking code
    >>> state = result.result()
    >>> print(state)
    RUNNING

Here is another example using `read_attribute()`:

    >>> from PyTango.futures import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> result = dev.read_attribute('wave', wait=False)
    >>> result
    <Future at 0x16cbe50 state=pending>

    >>> dev_attr = result.result()
    >>> print(dev_attr)
    DeviceAttribute[
    data_format = PyTango.AttrDataFormat.SPECTRUM
          dim_x = 256
          dim_y = 0
     has_failed = False
       is_empty = False
           name = 'wave'
        nb_read = 256
     nb_written = 0
        quality = PyTango.AttrQuality.ATTR_VALID
    r_dimension = AttributeDimension(dim_x = 256, dim_y = 0)
           time = TimeVal(tv_nsec = 0, tv_sec = 1383923329, tv_usec = 451821)
           type = PyTango.CmdArgType.DevDouble
          value = array([ -9.61260664e-01,  -9.65924853e-01,  -9.70294813e-01,
            -9.74369212e-01,  -9.78146810e-01,  -9.81626455e-01,
            -9.84807087e-01,  -9.87687739e-01,  -9.90267531e-01,
            ...
            5.15044507e-1])
        w_dim_x = 0
        w_dim_y = 0
    w_dimension = AttributeDimension(dim_x = 0, dim_y = 0)
        w_value = None]

## gevent mode

Warning

Before using gevent mode please note that at the time of writing this documentation, PyTango.gevent requires the latest version 1.0 of gevent (which has been released the day before :-P). Also take into account that gevent 1.0 is not available on python 3. Please consider using the Futures mode instead.

Using gevent cooperative mode in PyTango is relatively easy:

    >>> from PyTango.gevent import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> dev.get_green_mode()
    PyTango.GreenMode.Gevent

    >>> print(dev.state())
    RUNNING

The `PyTango.gevent.DeviceProxy()` API is exactly the same as the standard `DeviceProxy`. The difference is in the semantics of the methods that involve synchronous network calls (constructor included) which may block the execution for a relatively big amount of time. The list of methods that have been modified to accept gevent semantics are, on the `PyTango.gevent.DeviceProxy()`:

- Constructor
- `state()`
- `status()`
- `read_attribute()`
- `write_attribute()`
- `write_read_attribute()`
- `read_attributes()`
- `write_attributes()`
- `ping()`

So how does this work in fact? I see no difference from using the standard `DeviceProxy`. Well, this is, in fact, one of the goals: be able to use a gevent cooperation without changing the API. Behind the scenes the methods mentioned before have been modified to be able to work cooperatively with other greenlets.

All of the above methods have been boosted with two extra keyword arguments wait and timeout which allow to fine tune the behaviour. The wait parameter is by default set to True meaning wait for the request to finish (the default semantics when not using green mode). If wait is set to True, the timeout determines the maximum time to wait for the method to execute. The default timeout is None which means wait forever. If wait is set to False, the timeout is ignored.

If wait is set to True, the result is the same as executing the standard method on a `DeviceProxy`. If, wait is set to False, the result will be a `gevent.event.AsyncResult`. In this case, to get the actual value you will need to do something like:

    >>> from PyTango.gevent import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> result = dev.state(wait=False)
    >>> result
    <gevent.event.AsyncResult at 0x1a74050>

    >>> # this will be the blocking code
    >>> state = result.get()
    >>> print(state)
    RUNNING

Here is another example using `read_attribute()`:

    >>> from PyTango.gevent import DeviceProxy

    >>> dev = DeviceProxy("sys/tg_test/1")
    >>> result = dev.read_attribute('wave', wait=False)
    >>> result
    <gevent.event.AsyncResult at 0x1aff54e>

    >>> dev_attr = result.get()
    >>> print(dev_attr)
    DeviceAttribute[
    data_format = PyTango.AttrDataFormat.SPECTRUM
          dim_x = 256
          dim_y = 0
     has_failed = False
       is_empty = False
           name = 'wave'
        nb_read = 256
     nb_written = 0
        quality = PyTango.AttrQuality.ATTR_VALID
    r_dimension = AttributeDimension(dim_x = 256, dim_y = 0)
           time = TimeVal(tv_nsec = 0, tv_sec = 1383923292, tv_usec = 886720)
           type = PyTango.CmdArgType.DevDouble
          value = array([ -9.61260664e-01,  -9.65924853e-01,  -9.70294813e-01,
            -9.74369212e-01,  -9.78146810e-01,  -9.81626455e-01,
            -9.84807087e-01,  -9.87687739e-01,  -9.90267531e-01,
            ...
            5.15044507e-1])
        w_dim_x = 0
        w_dim_y = 0
    w_dimension = AttributeDimension(dim_x = 0, dim_y = 0)
        w_value = None]

Note

due to the internal workings of gevent, setting the wait flag to True (default) doesn’t prevent other greenlets from running in parallel. This is, in fact, one of the major bonus of working with `gevent` when compared with `concurrent.futures`