tests: Add tests for differentiation

This ensures that the zero-th derivative returns the correct answer.

The sympy python library returns the 'correct' answer. There are still
some other deviations which are documented further in the next commit.

Signed-off-by: Garming Sam <[email protected]>

Author: GSam

symengine/tests/test_functions.py

@@ -62,6 +62,8 @@ def test_derivative():
     assert f.diff(y) == 0
     assert f.diff(x).args == (f, x)
     assert f.diff(x).diff(x).args == (f, x, x)
+    assert f.diff(x, 0) == f
+    assert f.diff(x, 0) == Derivative(function_symbol("f", x), x, 0)
 
     g = function_symbol("f", y)
     assert g.diff(x) == 0
@@ -84,6 +86,17 @@ def test_derivative():
     assert g == fxy.diff(y, 1, x, 2)
     assert g == fxy.diff(y, x, 2)
 
+    h = Derivative(Function("f")(x, y), x, 0, y, 1)
+    assert h == fxy.diff(x, 0, y)
+    assert h == fxy.diff(y, x, 0)
+
+    i = Derivative(Function("f")(x, y), x, 0, y, 1, x, 1)
+    assert i == fxy.diff(x, 0, y, x, 1)
+    assert i == fxy.diff(x, 0, y, x)
+    assert i == fxy.diff(y, x)
+    assert i == fxy.diff(y, 1, x, 1)
+    assert i == fxy.diff(y, 1, x)
+
 
 def test_abs():
     x = Symbol("x")