Status: Accepted
Owner: ----
Labels: Type-Defect Priority-High
New issue 3174 by [email protected]: sum evalf is broken
http://code.google.com/p/sympy/issues/detail?id=3174
It is my impression that evalf really needs an overhaul, especially when it
comes to handling evaluation of things that need substitution. Here is a
case in point:
>>> Sum(x, (x, 1, y)).n(1,subs={y:0})
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "sympy\core\evalf.py", line 1181, in evalf
result = evalf(self, prec+4, options)
File "sympy\core\evalf.py", line 1089, in evalf
r = rf(x, prec, options)
File "sympy\core\evalf.py", line 1010, in evalf_sum
re, im, re_acc, im_acc = evalf(s, prec2, options)
File "sympy\core\evalf.py", line 1089, in evalf
r = rf(x, prec, options)
File "sympy\core\evalf.py", line 412, in evalf_add
terms = [evalf(arg, prec + 10, options) for arg in v.args]
File "sympy\core\evalf.py", line 1089, in evalf
r = rf(x, prec, options)
File "sympy\core\evalf.py", line 873, in evalf_integral
result = do_integral(expr, workprec, options)
File "sympy\core\evalf.py", line 798, in do_integral
xhigh = as_mpmath(xhigh, prec+15, options)
File "sympy\core\evalf.py", line 785, in as_mpmath
return mpf(re)
File "sympy\mpmath\ctx_mp_python.py", line 77, in __new__
v._mpf_ = mpf_pos(cls.mpf_convert_arg(val, prec, rounding), prec,
rounding)
File "sympy\mpmath\ctx_mp_python.py", line 96, in mpf_convert_arg
raise TypeError("cannot create mpf from " + repr(x))
TypeError: cannot create mpf from None
There are two approaches to evaluation in evalf_sum: 1) a fast
hypergeometric summation (for limit between int and oo) (this Sum fails
that criteria); 2) a euler_maclaurin summation (but that fails unless the
upper limit is specified).
The problem is that even if you raise an error if the limit is not literal,
there is no fallback to try the subs which have been provided and the Sum
will just come back unevaluated. This is also the sort of problem with trig
evaluation with complex subs as detailed in comment 6 of issue 2057 (
http://code.google.com/p/sympy/issues/detail?id=2057#c6 ).
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