CODEWITHSUNDEEP
pythonsympy
LNK
LNK

Reputation: 191

Cannot lambdify sympy expression with derivative

I cannot lambdify expression that contains derivatives. Here is the minimum code that explains the problem:

import sympy
x = sympy.symbols('x', real=True)
a = sympy.Function('a')(x)
b = a**2
r = sympy.diff(b, x)
sympy.lambdify((a), b)  # works
r.evalf(subs={diff(a, x): 1, a: 2})  # works 
b_x = sympy.lambdify((diff(a, x), a), r)  # throws an error

The last line of code throws the following error:

Traceback (most recent call last):   File "C:\Anaconda2\lib\site-packages\IPython\core\interactiveshell.py", line 2882, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)   File "<ipython-input-29-a0069a059795>", line 2, in <module>
    gx = sympy.lambdify((diff(a, x), a), r)  # gives an error   File "C:\Anaconda2\lib\site-packages\sympy\utilities\lambdify.py", line 434, in lambdify
    func = eval(lstr, namespace)   File "<string>", line 1
    lambda Derivative(a(x), x),_Dummy_29: (2*Derivative(_Dummy_29, x)*_Dummy_29)
                     ^ SyntaxError: invalid syntax

Upvotes: 1

Views: 1738

Answers (1)

unutbu
unutbu

Reputation: 879511

lambdify attempts to replace symbolic functions with numerical equivalents and return a function which can accept numeric values or arrays and return a number or array.

The error message 

lambda Derivative(a(x), x),_Dummy_29: (2*Derivative(_Dummy_29, x)*_Dummy_29)
                     ^ SyntaxError: invalid syntax

shows that lambdify is trying to build a lambda function whose first argument is Derivative(a(x), x). Clearly that's not going to work. We need a bare variable name instead of the symbolic expression Derivative(a(x), x).

So replace Derivative(a(x), x) with a bare symbol, da:

import sympy
x = sympy.symbols('x', real=True)
a = sympy.Function('a')(x)
b = a**2
r = sympy.diff(b, x)
da = sympy.symbols('da')
b_x = sympy.lambdify((da, a), r.subs([(sympy.diff(a, x), da)]))
assert b_x(1, 2) == r.evalf(subs={sympy.diff(a, x): 1, a: 2})
# 4

Upvotes: 4

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