IPython Documentation

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Note

This documentation is for a development version of IPython. There may be significant differences from the latest stable release.

Parallel Magic Commands

We provide a few IPython magic commands that make it a bit more pleasant to execute Python commands on the engines interactively. These are mainly shortcuts to DirectView.execute() and AsyncResult.display_outputs() methods respectively.

These magics will automatically become available when you create a Client:

In [2]: rc = parallel.Client()

The initially active View will have attributes targets='all', block=True, which is a blocking view of all engines, evaluated at request time (adding/removing engines will change where this view’s tasks will run).

The Magics

%px

The %px magic executes a single Python command on the engines specified by the targets attribute of the DirectView instance:

# import numpy here and everywhere
In [25]: with rc[:].sync_imports():
   ....:    import numpy
importing numpy on engine(s)

In [27]: %px a = numpy.random.rand(2,2)
Parallel execution on engines: [0, 1, 2, 3]

In [28]: %px numpy.linalg.eigvals(a)
Parallel execution on engines: [0, 1, 2, 3]
Out [0:68]: array([ 0.77120707, -0.19448286])
Out [1:68]: array([ 1.10815921,  0.05110369])
Out [2:68]: array([ 0.74625527, -0.37475081])
Out [3:68]: array([ 0.72931905,  0.07159743])

In [29]: %px print 'hi'
Parallel execution on engine(s): all
[stdout:0] hi
[stdout:1] hi
[stdout:2] hi
[stdout:3] hi

Since engines are IPython as well, you can even run magics remotely:

In [28]: %px %pylab inline
Parallel execution on engine(s): all
[stdout:0]
Populating the interactive namespace from numpy and matplotlib
[stdout:1]
Populating the interactive namespace from numpy and matplotlib
[stdout:2]
Populating the interactive namespace from numpy and matplotlib
[stdout:3]
Populating the interactive namespace from numpy and matplotlib

And once in pylab mode with the inline backend, you can make plots and they will be displayed in your frontend if it supports the inline figures (e.g. notebook or qtconsole):

In [40]: %px plot(rand(100))
Parallel execution on engine(s): all
<plot0>
<plot1>
<plot2>
<plot3>
Out[0:79]: [<matplotlib.lines.Line2D at 0x10a6286d0>]
Out[1:79]: [<matplotlib.lines.Line2D at 0x10b9476d0>]
Out[2:79]: [<matplotlib.lines.Line2D at 0x110652750>]
Out[3:79]: [<matplotlib.lines.Line2D at 0x10c6566d0>]

%%px Cell Magic

%%px can be used as a Cell Magic, which accepts some arguments for controlling the execution.

Targets and Blocking

%%px accepts --targets for controlling which engines on which to run, and --[no]block for specifying the blocking behavior of this cell, independent of the defaults for the View.

In [6]: %%px --targets ::2
   ...: print "I am even"
   ...:
Parallel execution on engine(s): [0, 2]
[stdout:0] I am even
[stdout:2] I am even

In [7]: %%px --targets 1
   ...: print "I am number 1"
   ...:
Parallel execution on engine(s): 1
I am number 1

In [8]: %%px
   ...: print "still 'all' by default"
   ...:
Parallel execution on engine(s): all
[stdout:0] still 'all' by default
[stdout:1] still 'all' by default
[stdout:2] still 'all' by default
[stdout:3] still 'all' by default

In [9]: %%px --noblock
   ...: import time
   ...: time.sleep(1)
   ...: time.time()
   ...:
Async parallel execution on engine(s): all
Out[9]: <AsyncResult: execute>

In [10]: %pxresult
Out[0:12]: 1339454561.069116
Out[1:10]: 1339454561.076752
Out[2:12]: 1339454561.072837
Out[3:10]: 1339454561.066665

See also

%pxconfig accepts these same arguments for changing the default values of targets/blocking for the active View.

Output Display

%%px also accepts a --group-outputs argument, which adjusts how the outputs of multiple engines are presented.

See also

AsyncResult.display_outputs() for the grouping options.

In [50]: %%px --block --group-outputs=engine
   ....: import numpy as np
   ....: A = np.random.random((2,2))
   ....: ev = numpy.linalg.eigvals(A)
   ....: print ev
   ....: ev.max()
   ....:
Parallel execution on engine(s): all
[stdout:0] [ 0.60640442  0.95919621]
Out [0:73]: 0.9591962130899806
[stdout:1] [ 0.38501813  1.29430871]
Out [1:73]: 1.2943087091452372
[stdout:2] [-0.85925141  0.9387692 ]
Out [2:73]: 0.93876920456230284
[stdout:3] [ 0.37998269  1.24218246]
Out [3:73]: 1.2421824618493817

%pxresult

If you are using %px in non-blocking mode, you won’t get output. You can use %pxresult to display the outputs of the latest command, just as is done when %px is blocking:

In [39]: dv.block = False

In [40]: %px print 'hi'
Async parallel execution on engine(s): all

In [41]: %pxresult
[stdout:0] hi
[stdout:1] hi
[stdout:2] hi
[stdout:3] hi

%pxresult simply calls AsyncResult.display_outputs() on the most recent request. It accepts the same output-grouping arguments as %%px, so you can use it to view a result in different ways.

%autopx

The %autopx magic switches to a mode where everything you type is executed on the engines until you do %autopx again.

In [30]: dv.block=True

In [31]: %autopx
%autopx enabled

In [32]: max_evals = []

In [33]: for i in range(100):
   ....:     a = numpy.random.rand(10,10)
   ....:     a = a+a.transpose()
   ....:     evals = numpy.linalg.eigvals(a)
   ....:     max_evals.append(evals[0].real)
   ....:

In [34]: print "Average max eigenvalue is: %f" % (sum(max_evals)/len(max_evals))
[stdout:0] Average max eigenvalue is: 10.193101
[stdout:1] Average max eigenvalue is: 10.064508
[stdout:2] Average max eigenvalue is: 10.055724
[stdout:3] Average max eigenvalue is: 10.086876

In [35]: %autopx
Auto Parallel Disabled

%pxconfig

The default targets and blocking behavior for the magics are governed by the block and targets attribute of the active View. If you have a handle for the view, you can set these attributes directly, but if you don’t, you can change them with the %pxconfig magic:

In [3]: %pxconfig --block

In [5]: %px print 'hi'
Parallel execution on engine(s): all
[stdout:0] hi
[stdout:1] hi
[stdout:2] hi
[stdout:3] hi

In [6]: %pxconfig --targets ::2

In [7]: %px print 'hi'
Parallel execution on engine(s): [0, 2]
[stdout:0] hi
[stdout:2] hi

In [8]: %pxconfig --noblock

In [9]: %px print 'are you there?'
Async parallel execution on engine(s): [0, 2]
Out[9]: <AsyncResult: execute>

In [10]: %pxresult
[stdout:0] are you there?
[stdout:2] are you there?

Multiple Active Views

The parallel magics are associated with a particular DirectView object. You can change the active view by calling the activate() method on any view.

In [11]: even = rc[::2]

In [12]: even.activate()

In [13]: %px print 'hi'
Async parallel execution on engine(s): [0, 2]
Out[13]: <AsyncResult: execute>

In [14]: even.block = True

In [15]: %px print 'hi'
Parallel execution on engine(s): [0, 2]
[stdout:0] hi
[stdout:2] hi

When activating a View, you can also specify a suffix, so that a whole different set of magics are associated with that view, without replacing the existing ones.

# restore the original DirecView to the base %px magics
In [16]: rc.activate()
Out[16]: <DirectView all>

In [17]: even.activate('_even')

In [18]: %px print 'hi all'
Parallel execution on engine(s): all
[stdout:0] hi all
[stdout:1] hi all
[stdout:2] hi all
[stdout:3] hi all

In [19]: %px_even print "We aren't odd!"
Parallel execution on engine(s): [0, 2]
[stdout:0] We aren't odd!
[stdout:2] We aren't odd!

This suffix is applied to the end of all magics, e.g. %autopx_even, %pxresult_even, etc.

For convenience, the Client has a activate() method as well, which creates a DirectView with block=True, activates it, and returns the new View.

The initial magics registered when you create a client are the result of a call to rc.activate() with default args.

Engines as Kernels

Engines are really the same object as the Kernels used elsewhere in IPython, with the minor exception that engines connect to a controller, while regular kernels bind their sockets, listening for connections from a QtConsole or other frontends.

Sometimes for debugging or inspection purposes, you would like a QtConsole connected to an engine for more direct interaction. You can do this by first instructing the Engine to also bind its kernel, to listen for connections:

In [50]: %px from IPython.parallel import bind_kernel; bind_kernel()

Then, if your engines are local, you can start a qtconsole right on the engine(s):

In [51]: %px %qtconsole

Careful with this one, because if your view is of 16 engines it will start 16 QtConsoles!

Or you can view just the connection info, and work out the right way to connect to the engines, depending on where they live and where you are:

In [51]: %px %connect_info
Parallel execution on engine(s): all
[stdout:0]
{
  "stdin_port": 60387,
  "ip": "127.0.0.1",
  "hb_port": 50835,
  "key": "eee2dd69-7dd3-4340-bf3e-7e2e22a62542",
  "shell_port": 55328,
  "iopub_port": 58264
}

Paste the above JSON into a file, and connect with:
    $> ipython <app> --existing <file>
or, if you are local, you can connect with just:
    $> ipython <app> --existing kernel-60125.json
or even just:
    $> ipython <app> --existing
if this is the most recent IPython session you have started.
[stdout:1]
{
  "stdin_port": 61869,
...

Note

%qtconsole will call bind_kernel() on an engine if it hasn’t been done already, so you can often skip that first step.