It seems that evalf of atan2 currently expects that its arguments are real.
For symbols like x and y, this
means that they have to be initialized with `real=True`. It does not
suffice that evalf would return a real
value as assumption code does not try to run evalf.
Kalevi Suominen
On Tuesday, August 20, 2019 at 5:34:21 PM UTC+3, Ville Bergholm wrote:
>
> I've been trying to use sympy.Symbol to implement a system where the
> numerical value of a symbolic quantity is only fixed when it is evaluated
> using evalf().
> To accomplish this I've redefined _eval_evalf() to return the
> aforementioned value (a fixed 5.0 in the toy example below).
>
> This works fine with basic algebraic operations and most SymPy functions.
> However, for some reason atan2 seems to behave differently and _eval_evalf
> is never called.
> Is this a bug, or have I misunderstood the purpose of _eval_evalf?
>
>
> import sympy
> import sympy.functions as syf
>
> class A(sympy.Symbol):
> def _eval_evalf(self, prec):
> print('*')
> return sympy.Float(5.0)
>
> x = A('X')
> y = A('Y')
> print('polynomial:')
> print((2*x-4+y).evalf()) # works fine
> print('atan:')
> print(syf.atan(y / x).evalf()) # works fine
> print('beta:')
> print(syf.beta(y, x).evalf()) # works fine
> print('atan2:')
> print(syf.atan2(y, x).evalf()) # prints "atan2(Y, X)", does not call
> A._eval_evalf
>
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