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pythonsympysymbolic-mathderivative

Can't convert expression to float

I'm trying to learn the ins and outs of symbolic manipulation in python (I'm a beginner).

I have the following basic code, and the output is giving me an error telling me that it "can't convert expression to float".

What's wrong with this code:

from sympy import *
from math import *

def h(x):
    return log(0.75392 * x)

x = symbols('x')
hprime = h(x).diff(x)

print(hprime)
Solution

  • This is a classic example of what is said in PEP-8 about wildcard imports:

    Wildcard imports ( from <module> import * ) should be avoided, as they make it unclear which names are present in the namespace, confusing both readers and many automated tools.

    The problem is that you need to work with sympy.log class, but using math.log function instead which works on float objects, not Symbol objects.

    When you write

    from sympy import *

    you are importing in your module namespace everything that sympy package providing at the top level (and there are a lot of stuff, much of that you don't need at all), including sympy.log class.

    After next statement

    from math import *

    you are importing everything in math module, including math.log, which overwrites previously imported sympy.log class.

    Considering this your example may be written like

    import sympy


def h(x):
    return sympy.log(0.485022 * x)


x = sympy.symbols('x')
h_x = h(x)
hprime = h_x.diff(x)

print(hprime)

    gives us

    1.0/x

    P. S.: I've removed math import since it is not used in given example.