Why is sympy.diff not distinguishing between simplex polynomials as expected?
I'm trying to figure out why sympy.diff
doesn't differentiate polynomials sympy
as expected. Usually sympy.diff
works just fine if a symbolic variable is defined and the polynomial is NOT defined with sympy.Poly
. However, if a function is defined using sympy.Poly
, sympy.diff
apparently, does not compute the derivative. Below is some sample code that shows what I mean:
import sympy as sy
# define symbolic variables
x = sy.Symbol('x')
y = sy.Symbol('y')
# define function WITHOUT using sy.Poly
f1 = x + 1
# define function WITH using sy.Poly
f2 = sy.Poly(x + 1, x, domain='QQ')
# compute derivatives and return results
df1 = sy.diff(f1,x)
df2 = sy.diff(f2,x)
print('f1: ',f1)
print('f2: ',f2)
print('df1: ',df1)
print('df2: ',df2)
This outputs the following results:
f1: x + 1
f2: Poly(x + 1, x, domain='QQ')
df1: 1
df2: Derivative(Poly(x + 1, x, domain='QQ'), x)
Why sympy.diff
doesn't he know how to tell the difference between the version of the polynomial sympy.Poly
? Is there a way to differentiate the polynomial, sympy
or a way to transform the polynomial sympy
into a form that allows it to be differentiated?
Note . I've tried with different domains (i.e. domain='RR'
instead of domain='QQ'
) and the output doesn't change.
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It seems like a mistake. You can work around it by calling diff
the instance directly Poly
. Ideally, calling a function diff
from a top-level sympy module should produce the same result as calling a method diff
.
In [1]: from sympy import *
In [2]: from sympy.abc import x
In [3]: p = Poly(x+1, x, domain='QQ')
In [4]: p.diff(x)
Out[4]: Poly(1, x, domain='QQ')
In [5]: diff(p, x)
Out[5]: Derivative(Poly(x + 1, x, domain='QQ'), x)
In [6]: diff(p, x).doit()
Out[6]: Derivative(Poly(x + 1, x, domain='ZZ'), x)
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