In sympy, evaluating an expression using .subs() when that expression has been defined in another fu-CodePudding

If I define a symbolic expression inside a function, how do I evaluate that expression using .subs() outside that function. The following code demonstrates the issue:

import sympy as sp

def myFun():
    x = sp.symbols("x", positive = True)
    y = 3*x

    return y

expr = myFun()
print(expr.subs({x:10}))

Running the code above gives NameError: name 'x' is not defined.

How can I perform this evaluation? Two things I could think of is declaring x to be a global variable using global x inside myFun(). However, I want to avoid this method. Another way is to lambdify the expression in myFun() before returning the expression. But I would prefer not to use this method because in the real problem I am working with, I have multiple variables in my expression and would like to substitute only some of the variables at one stage, and the remaining variables at a later stage. Would you adopt one of the two approaches I mentioned or is it possible to evaluate the expression using .subs() using some other approach?

CodePudding user response：

@cards answer is not going to work, the reason is that you have defined x to be positive, instead when calling print(expr.subs({'x':10})) the string 'x' will generate a generic symbol, without assumptions.

You either create your symbols outside of the function, like this:

import sympy as sp

x = sp.symbols("x", positive = True)
def myFun():
    y = 3*x
    return y

expr = myFun()
print(expr.subs({x:10}))
# out: 30

Or you can retrieve the symbols that make up a symbolic expression with the free_symbol attribute, like this:

import sympy as sp

def myFun():
    x = sp.symbols("x", positive = True)
    y = 3*x
    return y

expr = myFun()
x = expr.free_symbols.pop()
print(expr.subs({x:10}))
# out: 30

CodePudding user response：

Let's create an expression with two symbols in it:

In [44]: x, y = symbols('x, y')

In [45]: expr = sin(y)   x**2

Suppose that we only have the expression as a local variable. We can find the symbols it contains using free_symbols. We can use that to build a dict that maps symbol names (as strings) to the symbols themselves:

In [46]: expr.free_symbols
Out[46]: {x, y}

In [47]: syms = {s.name: s for s in expr.free_symbols}

In [48]: syms
Out[48]: {'y': y, 'x': x}

This mapping is important because different symbols with the same names but different assumptions are not interchangeable. With that we can substitute the symbol whose name is 'x' like this:

In [49]: expr.subs(syms['x'], 2)
Out[49]: sin(y)   4

CodePudding user response：

x as a Symbol object is only declared in the function's body, so it's not accessible from the outside. Use 'x' as a string.

x = list(expr.free_symbols)[0]
print(expr.subs({x: 2}))