将 SymPy 字符串列表转换为符号并在求解集中使用

Question

The post " How can I convert a list of strings into sympy variables? " discusses how to generate SymPy symbols from a list of strings. My question is what steps are needed to use these symbols x, y and z in SymPy calculations?

I tried something along the lines

from sympy import symbols, solveset

var_as_strings = ['x', 'y', 'z']
var_as_symbol_objects = [sympy.symbols(v) for v  in var_as_strings]
var_as_symbol_objects

for x1, x2 in zip(var_as_symbol_objects, var_as_strings):
    x1 = symbols(x2)

soln = solveset(x-y-z, x)

but I get the error "NameError: name 'x' is not defined". Any help is greatly appreciated.

Answer 1

This is answered by the first listed gotcha in the documentation .

You need to store your symbols into the required variables manually ( x = symbols('x') ), or import them from sympy.abc ( from sympy.abc import x ).

The name error is happening because you haven't defined what x , y or z are in your code.

Answer 2

You can create Python variables in the current namespace from a list of strings (or a string of space or comma separated names) with var :

>>> names = 'x y z'  # or 'x, y, z'
>>> var(names)
(x, y, z)
>>> var(list('xyz'))
[x, y, z]
>>> var(set('xyz'))
{x, y, z}

In all cases, after issuing the var command you will be able to use variables that have been assigned SymPy symbols with the same name.

Answer 3

that I have a lot of items in my list that I wish to define as symbols

Just FYI, a snippet which can automatically define the symbols in an enhanced fashion

nmax = 7
names = ["phi" + str(i) for i in range(nmax)]
sympy_vars = [symbols("\\" + name.replace("phi", "phi_")) for name in names]
globals().update(dict(zip(names, sympy_vars)))
phi0

This yields a nice LaTeX type output in, eg, jupyter notebooks.