```
"""
Reimplementations of constructs introduced in later versions of Python than
we support. Also some functions that are needed SymPy-wide and are located
here for easy import.
"""
from __future__ import with_statement
from collections import defaultdict
from sympy.external import import_module
# These are in here because telling if something is an iterable just by calling
# hasattr(obj, "__iter__") behaves differently in Python 2 and Python 3. In
# particular, hasattr(str, "__iter__") is False in Python 2 and True in Python 3.
# I think putting them here also makes it easier to use them in the core.
[docs]def iterable(i, exclude=(basestring, dict)):
"""
Return a boolean indicating whether ``i`` is SymPy iterable.
When SymPy is working with iterables, it is almost always assuming
that the iterable is not a string or a mapping, so those are excluded
by default. If you want a pure Python definition, make exclude=None. To
exclude multiple items, pass them as a tuple.
See also: is_sequence
Examples
========
>>> from sympy.utilities.iterables import iterable
>>> from sympy import Tuple
>>> things = [[1], (1,), set([1]), Tuple(1), (j for j in [1, 2]), {1:2}, '1', 1]
>>> for i in things:
... print iterable(i), type(i)
True <... 'list'>
True <... 'tuple'>
True <... 'set'>
True <class 'sympy.core.containers.Tuple'>
True <... 'generator'>
False <... 'dict'>
False <... 'str'>
False <... 'int'>
>>> iterable({}, exclude=None)
True
>>> iterable({}, exclude=str)
True
>>> iterable("no", exclude=str)
False
"""
try:
iter(i)
except TypeError:
return False
if exclude:
return not isinstance(i, exclude)
return True
[docs]def is_sequence(i, include=None):
"""
Return a boolean indicating whether ``i`` is a sequence in the SymPy
sense. If anything that fails the test below should be included as
being a sequence for your application, set 'include' to that object's
type; multiple types should be passed as a tuple of types.
Note: although generators can generate a sequence, they often need special
handling to make sure their elements are captured before the generator is
exhausted, so these are not included by default in the definition of a
sequence.
See also: iterable
Examples
========
>>> from sympy.utilities.iterables import is_sequence
>>> from types import GeneratorType
>>> is_sequence([])
True
>>> is_sequence(set())
False
>>> is_sequence('abc')
False
>>> is_sequence('abc', include=str)
True
>>> generator = (c for c in 'abc')
>>> is_sequence(generator)
False
>>> is_sequence(generator, include=(str, GeneratorType))
True
"""
return (hasattr(i, '__getitem__') and
iterable(i) or
bool(include) and
isinstance(i, include))
"""
Wrapping some imports in try/except statements to allow the same code to
be used in Python 3+ as well.
"""
try:
callable = callable
except NameError:
import collections
def callable(obj):
return isinstance(obj, collections.Callable)
try:
from functools import reduce
except ImportError:
reduce = reduce
def cmp_to_key(mycmp):
"""
Convert a cmp= function into a key= function
This code is included in Python 2.7 and 3.2 in functools.
"""
class K(object):
def __init__(self, obj, *args):
self.obj = obj
def __lt__(self, other):
return mycmp(self.obj, other.obj) < 0
def __gt__(self, other):
return mycmp(self.obj, other.obj) > 0
def __eq__(self, other):
return mycmp(self.obj, other.obj) == 0
def __le__(self, other):
return mycmp(self.obj, other.obj) <= 0
def __ge__(self, other):
return mycmp(self.obj, other.obj) >= 0
def __ne__(self, other):
return mycmp(self.obj, other.obj) != 0
return K
try:
import __builtin__
cmp = __builtin__.cmp
except AttributeError:
def cmp(a, b):
return (a > b) - (a < b)
try:
from itertools import product
except ImportError: # Python 2.5
def product(*args, **kwargs):
"""
Cartesian product of input iterables.
Equivalent to nested for-loops in a generator expression. For example,
cartes(A, B) returns the same as ((x,y) for x in A for y in B).
The nested loops cycle like an odometer with the rightmost element
advancing on every iteration. This pattern creates a lexicographic
ordering so that if the input's iterables are sorted, the product
tuples are emitted in sorted order.
To compute the product of an iterable with itself, specify the number
of repetitions with the optional repeat keyword argument. For example,
product(A, repeat=4) means the same as product(A, A, A, A).
Examples
========
>>> from sympy.utilities.iterables import cartes
>>> [''.join(p) for p in list(cartes('ABC', 'xy'))]
['Ax', 'Ay', 'Bx', 'By', 'Cx', 'Cy']
>>> list(cartes(range(2), repeat=2))
[(0, 0), (0, 1), (1, 0), (1, 1)]
See Also
========
variations
"""
pools = map(tuple, args) * kwargs.get('repeat', 1)
result = [[]]
for pool in pools:
result = [x + [y] for x in result for y in pool]
for prod in result:
yield tuple(prod)
try:
from itertools import permutations
except ImportError: # Python 2.5
def permutations(iterable, r=None):
"""
Return successive r length permutations of elements in the iterable.
If r is not specified or is None, then r defaults to the length of
the iterable and all possible full-length permutations are generated.
Permutations are emitted in lexicographic sort order. So, if the input
iterable is sorted, the permutation tuples will be produced in sorted
order.
Elements are treated as unique based on their position, not on their
value. So if the input elements are unique, there will be no repeat
values in each permutation.
Examples;
>>> from sympy.core.compatibility import permutations
>>> [''.join(p) for p in list(permutations('ABC', 2))]
['AB', 'AC', 'BA', 'BC', 'CA', 'CB']
>>> list(permutations(range(3)))
[(0, 1, 2), (0, 2, 1), (1, 0, 2), (1, 2, 0), (2, 0, 1), (2, 1, 0)]
"""
pool = tuple(iterable)
n = len(pool)
r = n if r is None else r
if r > n:
return
indices = range(n)
cycles = range(n, n - r, -1)
yield tuple(pool[i] for i in indices[:r])
while n:
for i in reversed(range(r)):
cycles[i] -= 1
if cycles[i] == 0:
indices[i:] = indices[i + 1:] + indices[i:i + 1]
cycles[i] = n - i
else:
j = cycles[i]
indices[i], indices[-j] = indices[-j], indices[i]
yield tuple(pool[i] for i in indices[:r])
break
else:
return
try:
from itertools import combinations, combinations_with_replacement
except ImportError: # < python 2.6
def combinations(iterable, r):
"""
Return r length subsequences of elements from the input iterable.
Combinations are emitted in lexicographic sort order. So, if the
input iterable is sorted, the combination tuples will be produced
in sorted order.
Elements are treated as unique based on their position, not on their
value. So if the input elements are unique, there will be no repeat
values in each combination.
See also: combinations_with_replacement
Examples
========
>>> from sympy.core.compatibility import combinations
>>> list(combinations('ABC', 2))
[('A', 'B'), ('A', 'C'), ('B', 'C')]
>>> list(combinations(range(4), 3))
[(0, 1, 2), (0, 1, 3), (0, 2, 3), (1, 2, 3)]
"""
pool = tuple(iterable)
n = len(pool)
if r > n:
return
indices = range(r)
yield tuple(pool[i] for i in indices)
while True:
for i in reversed(range(r)):
if indices[i] != i + n - r:
break
else:
return
indices[i] += 1
for j in range(i + 1, r):
indices[j] = indices[j - 1] + 1
yield tuple(pool[i] for i in indices)
def combinations_with_replacement(iterable, r):
"""Return r length subsequences of elements from the input iterable
allowing individual elements to be repeated more than once.
Combinations are emitted in lexicographic sort order. So, if the
input iterable is sorted, the combination tuples will be produced
in sorted order.
Elements are treated as unique based on their position, not on their
value. So if the input elements are unique, the generated combinations
will also be unique.
See also: combinations
Examples
========
>>> from sympy.core.compatibility import combinations_with_replacement
>>> list(combinations_with_replacement('AB', 2))
[('A', 'A'), ('A', 'B'), ('B', 'B')]
"""
pool = tuple(iterable)
n = len(pool)
if not n and r:
return
indices = [0] * r
yield tuple(pool[i] for i in indices)
while True:
for i in reversed(range(r)):
if indices[i] != n - 1:
break
else:
return
indices[i:] = [indices[i] + 1] * (r - i)
yield tuple(pool[i] for i in indices)
[docs]def set_intersection(*sets):
"""Return the intersection of all the given sets.
As of Python 2.6 you can write ``set.intersection(*sets)``.
Examples
========
>>> from sympy.core.compatibility import set_intersection
>>> set_intersection(set([1, 2]), set([2, 3]))
set([2])
>>> set_intersection()
set()
"""
if not sets:
return set()
rv = sets[0]
for s in sets:
rv &= s
return rv
[docs]def set_union(*sets):
"""Return the union of all the given sets.
As of Python 2.6 you can write ``set.union(*sets)``.
>>> from sympy.core.compatibility import set_union
>>> set_union(set([1, 2]), set([2, 3]))
set([1, 2, 3])
>>> set_union()
set()
"""
rv = set()
for s in sets:
rv |= s
return rv
try:
from collections import namedtuple
except ImportError:
# code from http://code.activestate.com/recipes/500261-named-tuples/
# PSF license
# code is Copyright 2007-2013 Raymond Hettinger
from operator import itemgetter as _itemgetter
from keyword import iskeyword as _iskeyword
import sys as _sys
# For some reason, doctest will test namedtuple's docstring if we simply
# have a def namedtuple() here
def _namedtuple(typename, field_names, verbose=False, rename=False):
if isinstance(field_names, basestring):
field_names = field_names.replace(',', ' ').split()
field_names = tuple(map(str, field_names))
if rename:
names = list(field_names)
seen = set()
for i, name in enumerate(names):
if (not min(c.isalnum() or c=='_' for c in name) or _iskeyword(name)
or not name or name[0].isdigit() or name.startswith('_')
or name in seen):
names[i] = '_%d' % i
seen.add(name)
field_names = tuple(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('_') and not rename:
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]
reprtxt = ', '.join('%s=%%r' % name for name in 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(self):
'Return a new dict which maps field names to their values'
return dict(zip(self._fields, self)) \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
def __getnewargs__(self):
return tuple(self) \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, __name__='namedtuple_%s' % typename,
_property=property, _tuple=tuple)
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) or sys._getframe is not
# defined for arguments greater than 0 (IronPython).
try:
result.__module__ = _sys._getframe(1).f_globals.get('__name__', '__main__')
except (AttributeError, ValueError):
pass
return result
namedtuple = _namedtuple
try:
bin = bin
except NameError: # Python 2.5
def bin(x):
"""
bin(number) -> string
Stringifies an int or long in base 2.
"""
if x < 0:
return '-' + bin(-x)
out = []
if x == 0:
out.append('0')
while x > 0:
out.append('01'[x & 1])
x >>= 1
pass
return '0b' + ''.join(reversed(out))
try:
next = next
except NameError: # Python 2.5
def next(*args):
"""
next(iterator[, default])
Return the next item from the iterator. If default is given and the
iterator is exhausted, it is returned instead of raising StopIteration.
"""
if len(args) == 1:
return args[0].next()
elif len(args) == 2:
try:
return args[0].next()
except StopIteration:
return args[1]
else:
raise TypeError('Expected 1 or 2 arguments, got %s' % len(args))
try:
from __builtin__ import bin
except ImportError: # Python 2.5
_hexDict = {
'0': '0000', '1': '0001', '2': '0010', '3': '0011', '4': '0100', '5': '0101',
'6': '0110', '7': '0111', '8': '1000', '9': '1001', 'a': '1010', 'b': '1011',
'c': '1100', 'd': '1101', 'e': '1110', 'f': '1111', 'L': ''}
def bin(n):
"""Return the equivalent to Python 2.6's bin function.
Examples
========
>>> from sympy.core.compatibility import bin
>>> bin(-123)
'-0b1111011'
>>> bin(0) # this is the only time a 0 will be to the right of 'b'
'0b0'
See Also
========
sympy.physics.quantum.shor.arr
Modified from http://code.activestate.com/recipes/576847/
"""
# =========================================================
# create hex of int, remove '0x'. now for each hex char,
# look up binary string, append in list and join at the end.
# =========================================================
if n < 0:
return '-%s' % bin(-n)
return '0b%s' % (''.join([_hexDict[hstr] for hstr in hex(n)[2:].lower()
]).lstrip('0') or '0')
[docs]def as_int(n):
"""
Convert the argument to a builtin integer.
The return value is guaranteed to be equal to the input. ValueError is
raised if the input has a non-integral value.
Examples
========
>>> from sympy.core.compatibility import as_int
>>> from sympy import sqrt
>>> 3.0
3.0
>>> as_int(3.0) # convert to int and test for equality
3
>>> int(sqrt(10))
3
>>> as_int(sqrt(10))
Traceback (most recent call last):
...
ValueError: ... is not an integer
"""
try:
result = int(n)
if result != n:
raise TypeError
except TypeError:
raise ValueError('%s is not an integer' % n)
return result
def default_sort_key(item, order=None):
"""Return a key that can be used for sorting.
The key has the structure:
(class_key, (len(args), args), exponent.sort_key(), coefficient)
This key is supplied by the sort_key routine of Basic objects when
``item`` is a Basic object or an object (other than a string) that
sympifies to a Basic object. Otherwise, this function produces the
key.
The ``order`` argument is passed along to the sort_key routine and is
used to determine how the terms *within* an expression are ordered.
(See examples below) ``order`` options are: 'lex', 'grlex', 'grevlex',
and reversed values of the same (e.g. 'rev-lex'). The default order
value is None (which translates to 'lex').
Examples
========
>>> from sympy import S, I, default_sort_key
>>> from sympy.core.function import UndefinedFunction
>>> from sympy.abc import x
The following are eqivalent ways of getting the key for an object:
>>> x.sort_key() == default_sort_key(x)
True
Here are some examples of the key that is produced:
>>> default_sort_key(UndefinedFunction('f'))
((0, 0, 'UndefinedFunction'), (1, ('f',)), ((1, 0, 'Number'),
(0, ()), (), 1), 1)
>>> default_sort_key('1')
((0, 0, 'str'), (1, ('1',)), ((1, 0, 'Number'), (0, ()), (), 1), 1)
>>> default_sort_key(S.One)
((1, 0, 'Number'), (0, ()), (), 1)
>>> default_sort_key(2)
((1, 0, 'Number'), (0, ()), (), 2)
While sort_key is a method only defined for SymPy objects,
default_sort_key will accept anything as an argument so it is
more robust as a sorting key. For the following, using key=
lambda i: i.sort_key() would fail because 2 doesn't have a sort_key
method; that's why default_sort_key is used. Note, that it also
handles sympification of non-string items likes ints:
>>> a = [2, I, -I]
>>> sorted(a, key=default_sort_key)
[2, -I, I]
The returned key can be used anywhere that a key can be specified for
a function, e.g. sort, min, max, etc...:
>>> a.sort(key=default_sort_key); a[0]
2
>>> min(a, key=default_sort_key)
2
Note
----
The key returned is useful for getting items into a canonical order
that will be the same across platforms. It is not directly useful for
sorting lists of expressions:
>>> a, b = x, 1/x
Since ``a`` has only 1 term, its value of sort_key is unaffected by
``order``:
>>> a.sort_key() == a.sort_key('rev-lex')
True
If ``a`` and ``b`` are combined then the key will differ because there
are terms that can be ordered:
>>> eq = a + b
>>> eq.sort_key() == eq.sort_key('rev-lex')
False
>>> eq.as_ordered_terms()
[x, 1/x]
>>> eq.as_ordered_terms('rev-lex')
[1/x, x]
But since the keys for each of these terms are independent of ``order``'s
value, they don't sort differently when they appear separately in a list:
>>> sorted(eq.args, key=default_sort_key)
[1/x, x]
>>> sorted(eq.args, key=lambda i: default_sort_key(i, order='rev-lex'))
[1/x, x]
The order of terms obtained when using these keys is the order that would
be obtained if those terms were *factors* in a product.
See Also
========
sympy.core.expr.as_ordered_factors, sympy.core.expr.as_ordered_terms
"""
from sympy.core import S, Basic
from sympy.core.sympify import sympify, SympifyError
from sympy.core.compatibility import iterable
if isinstance(item, Basic):
return item.sort_key(order=order)
if iterable(item, exclude=basestring):
if isinstance(item, dict):
args = item.items()
unordered = True
elif isinstance(item, set):
args = item
unordered = True
else:
# e.g. tuple, list
args = list(item)
unordered = False
args = [default_sort_key(arg, order=order) for arg in args]
if unordered:
# e.g. dict, set
args = sorted(args)
cls_index, args = 10, (len(args), tuple(args))
else:
if not isinstance(item, basestring):
try:
item = sympify(item)
except SympifyError:
# e.g. lambda x: x
pass
else:
if isinstance(item, Basic):
# e.g int -> Integer
return default_sort_key(item)
# e.g. UndefinedFunction
# e.g. str
cls_index, args = 0, (1, (str(item),))
return (cls_index, 0, item.__class__.__name__
), args, S.One.sort_key(), S.One
def _nodes(e):
"""
A helper for ordered() which returns the node count of ``e`` which
for Basic object is the number of Basic nodes in the expression tree
but for other object is 1 (unless the object is an iterable or dict
for which the sum of nodes is returned).
"""
from basic import Basic
if isinstance(e, Basic):
return e.count(Basic)
elif iterable(e):
return 1 + sum(_nodes(ei) for ei in e)
elif isinstance(e, dict):
return 1 + sum(_nodes(k) + _nodes(v) for k, v in e.iteritems())
else:
return 1
def ordered(seq, keys=None, default=True, warn=False):
"""Return an iterator of the seq where keys are used to break ties.
Two default keys will be applied after and provided unless ``default``
is False. The two keys are _nodes and default_sort_key which will
place smaller expressions before larger ones (in terms of Basic nodes)
and where there are ties, they will be broken by the default_sort_key.
If ``warn`` is True then an error will be raised if there were no
keys remaining to break ties. This can be used if it was expected that
there should be no ties.
Examples
========
>>> from sympy.utilities.iterables import ordered
>>> from sympy import count_ops
>>> from sympy.abc import x, y
The count_ops is not sufficient to break ties in this list and the first
two items appear in their original order (i.e. the sorting is stable):
>>> list(ordered([y + 2, x + 2, x**2 + y + 3],
... count_ops, default=False, warn=False))
...
[y + 2, x + 2, x**2 + y + 3]
The default_sort_key allows the tie to be broken:
>>> list(ordered([y + 2, x + 2, x**2 + y + 3]))
...
[x + 2, y + 2, x**2 + y + 3]
Here, sequences are sorted by length, then sum:
>>> seq, keys = [[[1, 2, 1], [0, 3, 1], [1, 1, 3], [2], [1]], [
... lambda x: len(x),
... lambda x: sum(x)]]
...
>>> list(ordered(seq, keys, default=False, warn=False))
[[1], [2], [1, 2, 1], [0, 3, 1], [1, 1, 3]]
If ``warn`` is True, an error will be raised if there were not
enough keys to break ties:
>>> list(ordered(seq, keys, default=False, warn=True))
Traceback (most recent call last):
...
ValueError: not enough keys to break ties
Notes
=====
The decorated sort is one of the fastest ways to sort a sequence for
which special item comparison is desired: the sequence is decorated,
sorted on the basis of the decoration (e.g. making all letters lower
case) and then undecorated. If one wants to break ties for items that
have the same decorated value, a second key can be used. But if the
second key is expensive to compute then it is inefficient to decorate
all items with both keys: only those items having identical first key
values need to be decorated. This function applies keys successively
only when needed to break ties. By yielding an iterator, use of the
tie-breaker is delayed as long as possible.
This function is best used in cases when use of the first key is
expected to be a good hashing function; if there are no unique hashes
from application of a key then that key should not have been used. The
exception, however, is that even if there are many collisions, if the
first group is small and one does not need to process all items in the
list then time will not be wasted sorting what one was not interested
in. For example, if one were looking for the minimum in a list and
there were several criteria used to define the sort order, then this
function would be good at returning that quickly if the first group
of candidates is small relative to the number of items being processed.
"""
d = defaultdict(list)
if keys:
if not isinstance(keys, (list, tuple)):
keys = [keys]
keys = list(keys)
f = keys.pop(0)
for a in seq:
d[f(a)].append(a)
else:
if not default:
raise ValueError('if default=False then keys must be provided')
d[None].extend(seq)
for k in sorted(d.keys()):
if len(d[k]) > 1:
if keys:
d[k] = ordered(d[k], keys, default, warn)
elif default:
d[k] = ordered(d[k], (_nodes, default_sort_key,),
default=False, warn=warn)
elif warn:
raise ValueError('not enough keys to break ties')
for v in d[k]:
yield v
d.pop(k)
try:
next = next
except NameError:
def next(x):
return x.next()
# If HAS_GMPY is 0, no supported version of gmpy is available. Otherwise,
# HAS_GMPY contains the major version number of gmpy; i.e. 1 for gmpy, and
# 2 for gmpy2.
# Versions of gmpy prior to 1.03 do not work correctly with int(largempz)
# For example, int(gmpy.mpz(2**256)) would raise OverflowError.
# See issue 1881.
# Minimum version of gmpy changed to 1.13 to allow a single code base to also
# work with gmpy2.
def _getenv(key, default=None):
from os import getenv
return getenv(key, default)
GROUND_TYPES = _getenv('SYMPY_GROUND_TYPES', 'auto').lower()
HAS_GMPY = 0
if GROUND_TYPES != 'python':
# Don't try to import gmpy2 if ground types is set to gmpy1. This is
# primarily intended for testing.
if GROUND_TYPES != 'gmpy1':
gmpy = import_module('gmpy2', min_module_version='2.0.0',
module_version_attr='version', module_version_attr_call_args=())
if gmpy:
HAS_GMPY = 2
else:
GROUND_TYPES = 'gmpy'
if not HAS_GMPY:
gmpy = import_module('gmpy', min_module_version='1.13',
module_version_attr='version', module_version_attr_call_args=())
if gmpy:
HAS_GMPY = 1
if GROUND_TYPES == 'auto':
if HAS_GMPY:
GROUND_TYPES = 'gmpy'
else:
GROUND_TYPES = 'python'
if GROUND_TYPES == 'gmpy' and not HAS_GMPY:
from warnings import warn
warn("gmpy library is not installed, switching to 'python' ground types")
GROUND_TYPES = 'python'
# SYMPY_INTS is a tuple containing the base types for valid integer types.
import sys
if sys.version_info[0] == 2:
SYMPY_INTS = (int, long)
else:
SYMPY_INTS = (int,)
if GROUND_TYPES == 'gmpy':
SYMPY_INTS += (type(gmpy.mpz(0)),)
# check_output() is new in python 2.7
import os
try:
from subprocess import CalledProcessError
try:
from subprocess import check_output
except ImportError:
from subprocess import check_call
def check_output(*args, **kwargs):
with open(os.devnull, 'w') as fh:
kwargs['stdout'] = fh
try:
return check_call(*args, **kwargs)
except CalledProcessError, e:
e.output = ("program output is not available for "
"python 2.5.x and 2.6.x")
raise e
except ImportError:
# running on platform like App Engine, no subprocess at all
pass
```