[IPython-User] ipython parallelism returning user-defined types

John Reid j.reid@mail.cryst.bbk.ac...
Tue Nov 9 04:18:47 CST 2010


I'm having some problems returning named tuples from a multi-engine 
client. I do something like:

from IPython.kernel import client
mec = client.MultiEngineClient()

def parallel_get_starts(arg):
     <do some work>
     return <some named tuples>

results = parallel_get_starts(<some list>)

but I always get errors like:
TypeError: __new__() takes exactly 8 arguments (2 given)

whenever the results of the parallel_get_starts() function contain named 
tuples. I'm using a namedtuple as given by Raymond Hettinger in the 
Python Cookbook:
See below.

Does anyone know how I might get it to work? presumably I could move to 
python 2.6 and use the collections.namedtuple type.


def namedtuple(typename, field_names, verbose=False):
     """Returns a new subclass of tuple with named fields.

     >>> Point = namedtuple('Point', 'x y')
     >>> Point.__doc__                   # docstring for the new class
     'Point(x, y)'
     >>> p = Point(11, y=22)             # instantiate with positional 
args or keywords
     >>> p[0] + p[1]                     # indexable like a plain tuple
     >>> x, y = p                        # unpack like a regular tuple
     >>> x, y
     (11, 22)
     >>> p.x + p.y                       # fields also accessable by name
     >>> d = p._asdict()                 # convert to a dictionary
     >>> d['x']
     >>> Point(**d)                      # convert from a dictionary
     Point(x=11, y=22)
     >>> p._replace(x=100)               # _replace() is like 
str.replace() but targets named fields
     Point(x=100, y=22)


     # Parse and validate the field names.  Validation serves two purposes,
     # generating informative error messages and preventing template 
injection attacks.
     if isinstance(field_names, basestring):
         field_names = field_names.replace(',', ' ').split() # names 
separated by whitespace and/or commas
     field_names = tuple(field_names)
     for name in (typename,) + field_names:
         if not min(c.isalnum() or c=='_' for c in name):
             raise ValueError('Type names and field names can only 
contain alphanumeric characters and underscores: %r' % name)
         if _iskeyword(name):
             raise ValueError('Type names and field names cannot be a 
keyword: %r' % name)
         if name[0].isdigit():
             raise ValueError('Type names and field names cannot start 
with a number: %r' % name)
     seen_names = set()
     for name in field_names
         if name.startswith('_'):
             raise ValueError('Field names cannot start with an 
underscore: %r' % name)
         if name in seen_names:
             raise ValueError('Encountered duplicate field name: %r' % name)

     # Create and fill-in the class template
     numfields = len(field_names)
     argtxt = repr(field_names).replace("'", "")[1:-1]   # tuple repr 
without parens or quotes
     reprtxt = ', '.join('%s=%%r' % name for name in field_names)
     dicttxt = ', '.join('%r: t[%d]' % (name, pos) for pos, name in 
     template = '''class %(typename)s(tuple):
         '%(typename)s(%(argtxt)s)' \n
         __slots__ = () \n
         _fields = %(field_names)r \n
         def __new__(cls, %(argtxt)s):
             return tuple.__new__(cls, (%(argtxt)s)) \n
         def _make(cls, iterable, new=tuple.__new__, len=len):
             'Make a new %(typename)s object from a sequence or iterable'
             result = new(cls, iterable)
             if len(result) != %(numfields)d:
                 raise TypeError('Expected %(numfields)d arguments, got 
%%d' %% len(result))
             return result \n
         def __repr__(self):
             return '%(typename)s(%(reprtxt)s)' %% self \n
         def _asdict(t):
             'Return a new dict which maps field names to their values'
             return {%(dicttxt)s} \n
         def _replace(self, **kwds):
             'Return a new %(typename)s object replacing specified 
fields with new values'
             result = self._make(map(kwds.pop, %(field_names)r, self))
             if kwds:
                 raise ValueError('Got unexpected field names: %%r' %% 
             return result \n\n''' % locals()
     for i, name in enumerate(field_names):
         template += '        %s = property(itemgetter(%d))\n' % (name, i)
     if verbose:
         print template

     # Execute the template string in a temporary namespace
     namespace = dict(itemgetter=_itemgetter)
         exec template in namespace
     except SyntaxError, e:
         raise SyntaxError(e.message + ':\n' + template)
     result = namespace[typename]

     # For pickling to work, the __module__ variable needs to be set to 
the frame
     # where the named tuple is created.  Bypass this step in 
enviroments where
     # sys._getframe is not defined (Jython for example).
     if hasattr(_sys, '_getframe'):
         result.__module__ = _sys._getframe(1).f_globals['__name__']

     return result

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