Hello all,
At PyCon UK Raymond Hettinger showed off the Named Tuple; a very useful
recipe for creating tuples with named fields. It is becoming part of the
standard library in Python 2.6.
http://code.activestate.com/recipes/500261/
>>> from namedtuple import namedtuple
>>> thing = namedtuple('thing', ('whizz', 'pop'))
>>> thing
<class '__builtin__.thing'>
>>> i = thing(1, 2)
>>> i.whizz
1
>>> i.pop
2
>>> w, p = i
>>> w, p
(1, 2)
>>> i
thing(whizz=1, pop=2)
>>>
I've attached a modified version that works with IronPython 2 B5 (it
doesn't work with 2B4 due to a very odd bug that is now fixed).
The only change needed was to comment out the use of 'sys._getframe'
which is there for pickle support.
It is only conditionally used, because Jython didn't support this either
- but on Jython _getframe wasn't defined at all so the conditional check
fails on IronPython:
# 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__']
I'll suggest a modification to the recipe that works with IronPython. A
better solution would be to implement '_getframe' of course... :-)
Michael Foord
--
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from operator import itemgetter as _itemgetter
from keyword import iskeyword as _iskeyword
import sys as _sys
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
33
>>> x, y = p # unpack like a regular tuple
>>> x, y
(11, 22)
>>> p.x + p.y # fields also accessable by name
33
>>> d = p._asdict() # convert to a dictionary
>>> d['x']
11
>>> 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)
seen_names.add(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
enumerate(field_names))
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
@classmethod
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' %%
kwds.keys())
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)
try:
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
if __name__ == '__main__':
# verify that instances can be pickled
from cPickle import loads, dumps
Point = namedtuple('Point', 'x, y', True)
p = Point(x=10, y=20)
assert p == loads(dumps(p))
# test and demonstrate ability to override methods
class Point(namedtuple('Point', 'x y')):
@property
def hypot(self):
return (self.x ** 2 + self.y ** 2) ** 0.5
def __str__(self):
return 'Point: x=%6.3f y=%6.3f hypot=%6.3f' % (self.x, self.y,
self.hypot)
for p in Point(3,4), Point(14,5), Point(9./7,6):
print p
class Point(namedtuple('Point', 'x y')):
'Point class with optimized _make() and _replace() without
error-checking'
_make = classmethod(tuple.__new__)
def _replace(self, _map=map, **kwds):
return self._make(_map(kwds.get, ('x', 'y'), self))
print Point(11, 22)._replace(x=100)
import doctest
TestResults = namedtuple('TestResults', 'failed attempted')
print TestResults(*doctest.testmod())
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