Fix for TypeError: can only concatenate str (not "type") to str (#288)

* Fixed type error (cannot concatenate string and type)

* Added unit test (not testing variable removal atm)

Author: Markus28

mip/model.py

@@ -1427,8 +1427,7 @@ def remove(
         else:
             raise TypeError(
                 "Cannot handle removal of object of type "
-                + type(objects)
-                + " from model."
+                "{} from model".format(type(objects))
             )
 
     def translate(self: "Model", ref) -> Union[List[Any], Dict[Any, Any], "mip.Var"]:

test/test_model.py

@@ -1,4 +1,5 @@
 import os
+import re
 
 import pytest
 
@@ -1260,3 +1261,20 @@ def test_query_attributes_of_lin_expr(solver):
     assert constr_expr.violation is None
 
     m.optimize()
+
+
+@pytest.mark.parametrize("solver", SOLVERS)
+def test_remove(solver):
+    m = Model(solver_name=solver)
+    x = m.add_var("x")
+    constr = m.add_constr(x >= 0)
+    m.objective = x
+
+    with pytest.raises(TypeError, match=re.escape("Cannot handle removal of object of type <class 'NoneType'> from model")):
+        m.remove(None)
+
+    with pytest.raises(TypeError, match=re.escape("Cannot handle removal of object of type <class 'NoneType'> from model")):
+        m.remove([None])
+
+    m.remove(constr)
+    # TODO: Test the removal of variables (currently failing, see https://github.com/coin-or/python-mip/pull/288#discussion_r919215654)