Non-sequential substitution in SymPy

Question

I\'m trying to use [SymPy][1] to substitute multiple terms in an expression at the same time. I tried the [subs function][2] with a dictionary as parameter, but found out that i

Answer 1

The subs(self,*args) method is defined (in part) this way:

In [11]: x.subs??
...
sequence = args[0]
if isinstance(sequence, dict):
    return self._subs_dict(sequence)
elif isinstance(sequence, (list, tuple)):
    return self._subs_list(sequence)

If you pass subs a dict, you lose control over the order of the substitutions. While if you pass subs a list or tuple, you can control the order.

This doesn't allow you to do simultaneous substitutions. That would lead to difficulties if the user were to pass stuff like x.subs([(x,y),(y,x)]). So I doubt sympy has a method for doing simultaneous substitutions. Instead I believe all substutions are either unordered (if you pass a dict) or, at best, done by a 1-pass ordered substitution (if you pass a list or tuple):

In [17]: x.subs([(x,y),(y,z)])
Out[18]: z

In [19]: x.subs([(y,z),(x,y)])
Out[19]: y

PS. _subs_list(self, sequence) is defined (in part) like this:

In [14]: x._subs_list??
...
    for old, new in sequence:
        result = result.subs(old, new)

This nails down the order in which the subs are done.