Python 如何在sympy的表达式中替换多个符号？
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How to substitute multiple symbols in an expression in sympy?
提问by Wesley
Assigning a variable directly does not modify expressions that used the variable retroactively.
直接分配变量不会修改追溯使用该变量的表达式。
>>> from sympy import Symbol
>>> x = Symbol('x')
>>> y = Symbol('y')
>>> f = x + y
>>> x = 0
>>> f
x + y
采纳答案by Wesley
To substitute several values:
替换多个值：
>>> from sympy import Symbol
>>> x, y = Symbol('x y')
>>> f = x + y
>>> f.subs({x:10, y: 20})
>>> f
30
回答by MSeifert
Actually sympy is designed not to substitute values until you really want to substitute them with subs(see http://docs.sympy.org/latest/tutorial/basic_operations.html)
实际上，sympy 旨在不替换值，直到您真的想替换它们subs（请参阅http://docs.sympy.org/latest/tutorial/basic_operations.html）
Try
尝试
f.subs({x:0})
f.subs(x, 0) # as alternative
instead of
代替
x = 0
回答by Adrien
The command x = Symbol('x')stores Sympy's Symbol('x')into Python's variable x. The Sympy expression fthat you create afterwards does contain Symbol('x'), not the Python variable x.
该命令x = Symbol('x')将 Sympy 存储Symbol('x')到 Python 的变量中x。f您之后创建的 Sympy 表达式确实包含Symbol('x')，而不是 Python 变量x。
When you reassign x = 0, the Python variable xis set to zero, and is no longer related to Symbol('x'). This has no effect on the Sympy expression, which still contains Symbol('x').
当您重新分配 时x = 0，Python 变量x设置为零，并且不再与 相关Symbol('x')。这对 Sympy 表达式没有影响，它仍然包含Symbol('x').
This is best explained in this page of the Sympy documentation: http://docs.sympy.org/latest/gotchas.html#variables
这在 Sympy 文档的这一页中得到了最好的解释：http://docs.sympy.org/latest/gotchas.html#variables
What you want to do is f.subs(x,0), as said in other answers.
f.subs(x,0)正如其他答案中所说，您想要做的是。
