Question

I’ve been reading and re-reading the IPython documentation/tutorial, and I can’t figure out the issue with this particular piece of code. It seems to be that the function dimensionless_run is not visible to the namespace delivered to each of the engines, but I’m confused because the function is defined in __main__, and clearly visible as part of the global namespace.

wrapper.py:

import math, os

def dimensionless_run(inputs):
    output_file = open(inputs['fn'],'w')
    ...
    return output_stats

def parallel_run(inputs):
    import math, os  ## Removing this line causes a NameError: global name 'math'
                     ## is not defined.
    folder = inputs['folder']
    zfill_amt = int(math.floor(math.log10(inputs['num_iters'])))
    for i in range(inputs['num_iters']):
        run_num_str = str(i).zfill(zfill_amt)
        if not os.path.exists(folder + '/'):
            os.mkdir(folder)
        dimensionless_run(inputs)
    return

if __name__ == "__main__":
    inputs = [input1,input2,...]
    client = Client()
    lbview = client.load_balanced_view()
    lbview.block = True
    for x in sorted(globals().items()):
        print x
    lbview.map(parallel_run,inputs)

Executing this code after ipcluster start --n=6 yields the sorted global dictionary, including the math and os modules, and the parallel_run and dimensionless_run functions. This is followed by an IPython.parallel.error.CompositeError: one or more exceptions from call to method: parallel_run, which is composed of a large number of [n:apply]: NameError: global name 'dimensionless_run' is not defined, where n runs from 0-5.

There are two things I don’t understand, and they’re clearly linked.

1. Why doesn’t the code identify dimensionless_run in the global namespace?
2. Why is import math, os necessary inside the definition of parallel_run?

Edited: This turned out not be much of a namespace error at all–I was executing ipcluster start --n=6 in a directory that didn’t contain the code. To fix it, all I needed to do was execute the start command in my code’s directory. I also fixed it by adding the lines:

    inputs = input_pairs
    os.system("ipcluster start -n 6") #NEW
    client = Client()
    ...
    lbview.map(parallel_run,inputs)
    os.system("ipcluster stop")       #NEW

which start the required cluster in the right place.

Answer 1

This is mostly a duplicate of Python name space issues with IPython.parallel, which has a more detailed answer, but the gist:

When the Client sends parallel_run to the engine, it just sends that function, not the entire namespace in which the function is defined (the __main__ module). So when running the remote parallel_run, lookups to math or os or dimensionless_run will look first in locals() (what has been defined already in the function, i.e. your in-function imports), then in the globals(), which is the __main__ module on the engine.

There are various approaches to making sure names available on the engines, but perhaps the simplest is to explicitly define/send them to the engines (the interactive namespace is __main__ on the engines, just like it is locally in IPython):

client[:].execute("import os, math")
client[:]['dimensionless_run'] = dimensionless_run

prior to making your run, in which case everything should work as you expect.

This is an issue unique to modules defined interactively / in a script – It does not come up if this file is a module instead of a script, e.g.

from mymod import parallel_run
lbview.map(parallel_run, inputs)

In which case the globals() is the module globals, which are generally the same everywhere.