Faster substitutions with smart_subs_dict (#3025)

* Disable evaluation during substitution
* Use xreplace instead of subs

For my current benchmark, this reduces the time for one `smart_subs_dict` call from over 21h (still running...) to ~12min.

Disabling evaluation during substitution may prevent simplification of some expressions during substitution, but those can be simplified later on.

Author: dweindl

python/sdist/amici/importers/utils.py

@@ -421,17 +421,21 @@ def smart_subs_dict(
     :return:
         Substituted symbolic expression
     """
-    s = [
-        (eid, expr[field] if field is not None else expr)
-        for eid, expr in subs.items()
-    ]
+    if field is None:
+        s = [(eid, expr) for eid, expr in subs.items()]
+    else:
+        s = [(eid, expr[field]) for eid, expr in subs.items()]
+
     if reverse:
         s.reverse()
-    for substitution in s:
-        # note that substitution may change free symbols, so we have to do
-        # this recursively
-        if sym.has(substitution[0]):
-            sym = sym.subs(*substitution)
+
+    with sp.evaluate(False):
+        for old, new in s:
+            # note that substitution may change free symbols, so we have to do
+            # this recursively
+            if sym.has(old):
+                sym = sym.xreplace({old: new})
+
     return sym