tweak lambdify

Author: jverzani

src/lambdify.jl

@@ -219,11 +219,17 @@ function  lambdify(ex::Sym, vars=free_symbols(ex);
                    invoke_latest=true)
     if isempty(vars)
         # can't call N(ex) here...
-        v = ex.evalf()
-        if v.is_real == True
-            val = convert(Real, v)
+        flag = pygetattr(ex, "evalf", nothing)
+        if isnothing(flag)
+            val = pyconvert(Real, ex)
         else
-            val = Complex(convert(Real, real(v)), convert(Real, imag(v)))
+            v = ex.evalf()
+            if pyconvert(Bool, v.is_real)
+                val = pyconvert(Real, v)
+            else
+                a,b = pyconvert.(Real, (real(v), imag(v)))
+                val = Complex(a, b)
+            end
         end
         return (ts...) -> val
     end

test/tests.jl

@@ -633,6 +633,13 @@ end
     @test u(.5) == 1
     @test u(1.5) == 0
 
+    # SymPy issue 567; constants
+    u = lambdify(Sym(1//2))
+    @test u() == u(1,2,3) == 1/2
+    @syms x
+    ex = integrate(sqrt(1 + (1/x)^2), (x, 1/sympy.E, sympy.E))
+    @test lambdify(ex)() ≈ 3.1961985135995072
+
 #    i2 = SymPy.lambdify_expr(x^2,name=:square)
 #    @test i2.head == :function
 #    @test i2.args[1].args[1] == :square