Sympy 突然无法识别 varialbe

Question

Last night I wrote a bit of code which ran just fine. Today it won't:

sigma = symbols('x1:4')
D = [1]
for d in D:
    for s in sigma:
        D.append(s*d)
    if len(D) > 20:
        break
print(D)
print(D[19].subs([(x1,4),(x2,2),(x3,3)]))

This, and, in fact anything I write defining symbols xi (ia natural number) returns an error. For the code above specifically, I get:

>>> for d in D:
...     for s in sigma:
...         D.append(s*d)
...     if len(D) > 20:
...         break
... 
>>> print(D)
[1, x1, x2, x3, x1**2, x1*x2, x1*x3, x1*x2, x2**2, x2*x3, x1*x3, x2*x3, x3**2, x1**3, x1**2*x2, x1**2*x3, x1**2*x2, x1*x2**2, x1*x2*x3, x1**2*x3, x1*x2*x3, x1*x3**2]
>>> print(D[19].subs([(x1,4),(x2,2),(x3,3)]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x1' is not defined

Defining variable names without the 'subscript' works fine. I'm on Ubuntu 19.04 and Visual Studio Code, which, and this is the only thing I can think of, I updated last night.

Answer 1

Python thinks you're trying to access a variable called x1, x2, or x3, when instead, you want to tell the subs function to replace instances of "x1" in your expression with 4.

To fix this, wrap them in quotes to make them a string (explicit is better than implicit):

print(D[19].subs([("x1",4),("x2",2),("x3",3)]))
>>> 48

Otherwise, you're left with the ambiguity of if you want to replace instances of the string "x1" or if you have a string variable defined somewhere in your code called x1 and want to replace the value x1 in your expression (which is what python thinks you are trying to do).

What you might've had yesterday (maybe you were working in a jupyter notebook or interactive shell), is somewhere in your code you had:

x1,x2,x3 = symbols('x1:4')

Which is the same as:

x1 = 'x1'
x2 = 'x2'
x3 = 'x3'

and would explain why your code worked yesterday.