log(Abs)

[proy87]

log(Abs) returns zoo, but the argument of log is FunctionClass.

[oscarbenjamin]

The problem is that Basic subclasses pass through sympify so this can be fixed with:

diff --git a/sympy/functions/elementary/exponential.py b/sympy/functions/elementary/exponential.py
index bdedc455c2..8bf8972000 100644
--- a/sympy/functions/elementary/exponential.py
+++ b/sympy/functions/elementary/exponential.py
@@ -1,4 +1,5 @@
 from sympy.core import sympify
+from sympy.core.sympify import _sympify
 from sympy.core.add import Add
 from sympy.core.cache import cacheit
 from sympy.core.function import (
@@ -641,7 +642,7 @@ def eval(cls, arg, base=None):
         from sympy.sets.setexpr import SetExpr
         from sympy.functions.elementary.complexes import Abs
 
-        arg = sympify(arg)
+        arg = _sympify(arg)
 
         if base is not None:
             base = sympify(base)

That kind of change should be used throughout much of the codebase.

It is probably also good to change it so that sympify rejects Basic subclasses.

[asmeurer]

I had thought we had an issue about replacing sympify with _sympify everywhere in library code, but I'm not finding it right now.

This is also related to #17280. Really FunctionClass the superclass should handle calling sympify. Classes shouldn't need to do this themselves in eval.

[oscarbenjamin]

The proper fix would be to make sympify more strict:

diff --git a/sympy/core/sympify.py b/sympy/core/sympify.py
index ed5ba267d9..242cd2e6f3 100644
--- a/sympy/core/sympify.py
+++ b/sympy/core/sympify.py
@@ -346,10 +346,7 @@ def sympify(a, locals=None, convert_xor=True, strict=False, rational=False,
     if is_sympy is True:
         return a
     elif is_sympy is not None:
-        if not strict:
-            return a
-        else:
-            raise SympifyError(a)
+        raise SympifyError(a)
 
     if isinstance(a, CantSympify):
         raise SympifyError(a)

That leads to these failures in core:

================================================================== short test summary info ===================================================================
FAILED sympy/core/tests/test_containers.py::test_Tuple_Eq - assert Eq((1,), f(1)) is True
FAILED sympy/core/tests/test_expr.py::test_has_tuple - sympy.core.sympify.SympifyError: SympifyError: f
FAILED sympy/core/tests/test_function.py::test_diff_wrt_func_subs - assert Derivative(f(g(x)), g(x))*Derivative(g(x), x) == 2*Subs(Derivative(f(_xi_1), _xi...
FAILED sympy/core/tests/test_function.py::test_subs_in_derivative - assert Derivative(f(x, f(x, x)), f(x, x)) == Subs(Derivative(x + z, z), z, 2*x)
FAILED sympy/core/tests/test_function.py::test_Subs_Derivative - assert False
FAILED sympy/core/tests/test_function.py::test_issue_15947 - sympy.core.sympify.SympifyError: SympifyError: f
FAILED sympy/core/tests/test_match.py::test_derivative_bug1 - sympy.core.sympify.SympifyError: SympifyError: f
FAILED sympy/core/tests/test_match.py::test_derivative2 - sympy.core.sympify.SympifyError: SympifyError: f
FAILED sympy/core/tests/test_match.py::test_exclude - sympy.core.sympify.SympifyError: SympifyError: sin
FAILED sympy/core/tests/test_subs.py::test_trigonometric - assert exp(pi) == 0
FAILED sympy/core/tests/test_subs.py::test_functions_subs - assert g(y, x) + cos(x) == ((sin(y) + x) + cos(x))
FAILED sympy/core/tests/test_subs.py::test_derivative_subs - assert Derivative(g(x), g(x)) == Derivative(f(x), f(x))
FAILED sympy/core/tests/test_subs.py::test_Function_subs - assert Piecewise((g(f(x, y)), x < -1), (g(x), x <= 1)) == Piecewise((h(f(x, y)), x < -1), (h(x),...
FAILED sympy/core/tests/test_subs.py::test_recurse_Application_args - assert f(x, f(x, x)) == exp(2*x + 3*exp(5*x))
FAILED sympy/core/tests/test_sympify.py::test__sympify - sympy.core.sympify.SympifyError: SympifyError: f
================================================= 15 failed, 1193 deselected, 1 warning in 130.65s (0:02:10) =================================================

The main problem here seems to be that with that change things like sympify(sin) or sympify(Function('f')) fail

In [1]: sin(x).subs(sin, cos)
Out[1]: sin(x)

In [2]: f = Function('f')

In [3]: f(x).subs(f, cos)
Out[3]: f(x)

In [4]: sympify(f)
---------------------------------------------------------------------------
SympifyError                              Traceback (most recent call last)
<ipython-input-4-f6adfff4c072> in <module>
----> 1 sympify(f)

~/current/sympy/sympy/sympy/core/sympify.py in sympify(a, locals, convert_xor, strict, rational, evaluate)
    347         return a
    348     elif is_sympy is not None:
--> 349         raise SympifyError(a)
    350 
    351     if isinstance(a, CantSympify):

SympifyError: SympifyError: f: