subs() TypeError: symbolic boolean expression has no truth value.

[Manuel Schleiffelder]

hello everyone,

i am working on plots in the complex-plane and got stuck trying to sutbstitue values to the symbols of a certain class of equations. the wired thing is, that i only get the "TypeError" the first time i substitute(In [3]:). second try it works(In [4]:) like a charm until i alter the equation or start over again.

i was able to track the error down to a simpler form of nested functions  (mathematically senseless, just for demonstration purposes):

>>> re(acos(re(x))).subs(x,5)

where i can exchange the complex functions arbitrary by another one (re(), im(), Abs(), arg()) and the trigonometric function "acos()" can also be a "asin()" still producing the same error. this also occurs in python 2.7.

any idea anybody?  is this a bug?

great thanks,
manuel

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In [1]: from sympy import *

In [2]: x = symbols('x')

In [3]: re(acos(re(x))).subs(x,5)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-515558ca2309> in <module>()
----> 1 re(acos(re(x))).subs(x,5)

/usr/local/lib/python3.2/dist-packages/sympy/core/cache.py in wrapper(*args, **kw_args)
     91         except (KeyError, TypeError):
     92             pass
---> 93         r = func(*args, **kw_args)
     94         try:
     95             func_cache_it_cache[k] = r

/usr/local/lib/python3.2/dist-packages/sympy/core/function.py in __new__(cls, *args, **options)
    366
    367         evaluate = options.get('evaluate', True)
--> 368         result = super(Function, cls).__new__(cls, *args, **options)
    369         if not evaluate or not isinstance(result, cls):
    370             return result

/usr/local/lib/python3.2/dist-packages/sympy/core/cache.py in wrapper(*args, **kw_args)
     91         except (KeyError, TypeError):
     92             pass
---> 93         r = func(*args, **kw_args)
     94         try:
     95             func_cache_it_cache[k] = r

/usr/local/lib/python3.2/dist-packages/sympy/core/function.py in __new__(cls, *args, **options)
    196
    197         if evaluate:
--> 198             evaluated = cls.eval(*args)
    199             if evaluated is not None:
    200                 return evaluated

/usr/local/lib/python3.2/dist-packages/sympy/functions/elementary/complexes.py in eval(cls, arg)
     50         if arg is S.NaN:
     51             return S.NaN
---> 52         elif arg.is_real:
     53             return arg
     54         elif arg.is_imaginary:

/usr/local/lib/python3.2/dist-packages/sympy/core/assumptions.py in getit(self)
    142             if self._assumptions is self.default_assumptions:
    143                 self._assumptions = self.default_assumptions.copy()
--> 144             return _ask(fact, self)
    145
    146     getit.func_name = as_property(fact)

/usr/local/lib/python3.2/dist-packages/sympy/core/assumptions.py in _ask(fact, obj)
    185         pass
    186     else:
--> 187         a = evaluate(obj)
    188         if a is not None:
    189             assumptions.deduce_all_facts(((fact, a),))

/usr/local/lib/python3.2/dist-packages/sympy/functions/elementary/trigonometric.py in _eval_is_real(self)
   1546
   1547     def _eval_is_real(self):
-> 1548         return self.args[0].is_real and (self.args[0] >= -1 and self.args[0] <= 1)
   1549
   1550     def _eval_rewrite_as_log(self, x):

/usr/local/lib/python3.2/dist-packages/sympy/core/relational.py in __nonzero__(self)
    109
    110     def __nonzero__(self):
--> 111         raise TypeError("symbolic boolean expression has no truth value.")
    112
    113     __bool__ = __nonzero__

TypeError: symbolic boolean expression has no truth value.

In [4]: re(acos(re(x))).subs(x,5)
Out[4]: re(acos(5))

In [5]:

[Aaron Meurer]

It's a bug. The code in trigonometric.py should be using (self.args[0]
+ 1).is_nonnegative and (self.args[0] - 1).is_nonpositive.

Aaron Meurer

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[Aaron Meurer]

We should do an audit of this. Check for all instances of git grep '^
*if.*[<>]' to see make sure the comparison cannot be symbolic.

Aaron Meurer

[Julien Rioux]

On Friday, 7 March 2014 20:43:48 UTC+1, Manuel Schleiffelder wrote:

hello everyone,

i am working on plots in the complex-plane and got stuck trying to sutbstitue values to the symbols of a certain class of equations. the wired thing is, that i only get the "TypeError" the first time i substitute(In [3]:). second try it works(In [4]:) like a charm until i alter the equation or start over again.

i was able to track the error down to a simpler form of nested functions  (mathematically senseless, just for demonstration purposes):

>>> re(acos(re(x))).subs(x,5)

Thanks for the bug report. Can you open an issue at https://github.com/sympy/sympy/issues?state=open ?
Cheers,
Julien