diff --git a/opt_einsum/contract.py b/opt_einsum/contract.py
index b837b21..6e7550a 100644
--- a/opt_einsum/contract.py
+++ b/opt_einsum/contract.py
@@ -214,7 +214,7 @@ def contract_path(*operands, **kwargs):
     indices = set(input_subscripts.replace(',', ''))
 
     # Get length of each unique dimension and ensure all dimensions are correct
-    dimension_dict = {}
+    size_dict = {}
     for tnum, term in enumerate(input_list):
         sh = input_shps[tnum]
 
@@ -224,18 +224,18 @@ def contract_path(*operands, **kwargs):
         for cnum, char in enumerate(term):
             dim = int(sh[cnum])
 
-            if char in dimension_dict:
+            if char in size_dict:
                 # For broadcasting cases we always want the largest dim size
-                if dimension_dict[char] == 1:
-                    dimension_dict[char] = dim
-                elif dim not in (1, dimension_dict[char]):
+                if size_dict[char] == 1:
+                    size_dict[char] = dim
+                elif dim not in (1, size_dict[char]):
                     raise ValueError("Size of label '{}' for operand {} ({}) does not match previous "
-                                     "terms ({}).".format(char, tnum, dimension_dict[char], dim))
+                                     "terms ({}).".format(char, tnum, size_dict[char], dim))
             else:
-                dimension_dict[char] = dim
+                size_dict[char] = dim
 
     # Compute size of each input array plus the output array
-    size_list = [helpers.compute_size_by_dict(term, dimension_dict) for term in input_list + [output_subscript]]
+    size_list = [helpers.compute_size_by_dict(term, size_dict) for term in input_list + [output_subscript]]
     memory_arg = _choose_memory_arg(memory_limit, size_list)
 
     num_ops = len(input_list)
@@ -245,7 +245,7 @@ def contract_path(*operands, **kwargs):
     # indices_in_input = input_subscripts.replace(',', '')
 
     inner_product = (sum(len(x) for x in input_sets) - len(indices)) > 0
-    naive_cost = helpers.flop_count(indices, inner_product, num_ops, dimension_dict)
+    naive_cost = helpers.flop_count(indices, inner_product, num_ops, size_dict)
 
     # Compute the path
     if not isinstance(path_type, (str, paths.PathOptimizer)):
@@ -256,10 +256,10 @@ def contract_path(*operands, **kwargs):
         path = [tuple(range(num_ops))]
     elif isinstance(path_type, paths.PathOptimizer):
         # Custom path optimizer supplied
-        path = path_type(input_sets, output_set, dimension_dict, memory_arg)
+        path = path_type(input_sets, output_set, size_dict, memory_arg)
     else:
         path_optimizer = paths.get_path_fn(path_type)
-        path = path_optimizer(input_sets, output_set, dimension_dict, memory_arg)
+        path = path_optimizer(input_sets, output_set, size_dict, memory_arg)
 
     cost_list = []
     scale_list = []
@@ -275,10 +275,10 @@ def contract_path(*operands, **kwargs):
         out_inds, input_sets, idx_removed, idx_contract = contract_tuple
 
         # Compute cost, scale, and size
-        cost = helpers.flop_count(idx_contract, idx_removed, len(contract_inds), dimension_dict)
+        cost = helpers.flop_count(idx_contract, idx_removed, len(contract_inds), size_dict)
         cost_list.append(cost)
         scale_list.append(len(idx_contract))
-        size_list.append(helpers.compute_size_by_dict(out_inds, dimension_dict))
+        size_list.append(helpers.compute_size_by_dict(out_inds, size_dict))
 
         tmp_inputs = [input_list.pop(x) for x in contract_inds]
         tmp_shapes = [input_shps.pop(x) for x in contract_inds]
@@ -312,7 +312,7 @@ def contract_path(*operands, **kwargs):
         return operands, contraction_list
 
     path_print = PathInfo(contraction_list, input_subscripts, output_subscript, indices, path, scale_list, naive_cost,
-                          opt_cost, size_list, dimension_dict)
+                          opt_cost, size_list, size_dict)
 
     return path, path_print
 
@@ -393,15 +393,26 @@ def contract(*operands, **kwargs):
         - ``'optimal'`` An algorithm that explores all possible ways of
           contracting the listed tensors. Scales factorially with the number of
           terms in the contraction.
+        - ``'dp'`` A faster (but essentially optimal) algorithm that uses
+          dynamic programming to exhaustively search all contraction paths
+          without outer-products.
+        - ``'greedy'`` An cheap algorithm that heuristically chooses the best
+          pairwise contraction at each step. Scales linearly in the number of
+          terms in the contraction.
+        - ``'random-greedy'`` Run a randomized version of the greedy algorithm
+          32 times and pick the best path.
+        - ``'random-greedy-128'`` Run a randomized version of the greedy
+          algorithm 128 times and pick the best path.
         - ``'branch-all'`` An algorithm like optimal but that restricts itself
           to searching 'likely' paths. Still scales factorially.
         - ``'branch-2'`` An even more restricted version of 'branch-all' that
           only searches the best two options at each step. Scales exponentially
           with the number of terms in the contraction.
-        - ``'greedy'`` An algorithm that heuristically chooses the best pair
-          contraction at each step.
         - ``'auto'`` Choose the best of the above algorithms whilst aiming to
           keep the path finding time below 1ms.
+        - ``'auto-hq'`` Aim for a high quality contraction, choosing the best
+          of the above algorithms whilst aiming to keep the path finding time
+          below 1sec.
 
     memory_limit : {None, int, 'max_input'} (default: None)
         Give the upper bound of the largest intermediate tensor contract will build.
@@ -412,7 +423,7 @@ def contract(*operands, **kwargs):
 
         The default is None. Note that imposing a limit can make contractions
         exponentially slower to perform.
-    backend : str, optional (default: ``numpy``)
+    backend : str, optional (default: ``auto``)
         Which library to use to perform the required ``tensordot``, ``transpose``
         and ``einsum`` calls. Should match the types of arrays supplied, See
         :func:`contract_expression` for generating expressions which convert
diff --git a/opt_einsum/path_random.py b/opt_einsum/path_random.py
index a49c06c..94b29a8 100644
--- a/opt_einsum/path_random.py
+++ b/opt_einsum/path_random.py
@@ -163,6 +163,8 @@ class RandomOptimizer(paths.PathOptimizer):
         raise NotImplementedError
 
     def __call__(self, inputs, output, size_dict, memory_limit):
+        self._check_args_against_first_call(inputs, output, size_dict)
+
         # start a timer?
         if self.max_time is not None:
             t0 = time.time()
diff --git a/opt_einsum/paths.py b/opt_einsum/paths.py
index 4103224..b952bc2 100644
--- a/opt_einsum/paths.py
+++ b/opt_einsum/paths.py
@@ -43,6 +43,19 @@ class PathOptimizer(object):
 
     where ``path`` is a list of int-tuples specifiying a contraction order.
     """
+
+    def _check_args_against_first_call(self, inputs, output, size_dict):
+        """Utility that stateful optimizers can use to ensure they are not
+        called with different contractions across separate runs.
+        """
+        args = (inputs, output, size_dict)
+        if not hasattr(self, '_first_call_args'):
+            # simply set the attribute as currently there is no global PathOptimizer init
+            self._first_call_args = args
+        elif args != self._first_call_args:
+            raise ValueError("The arguments specifiying the contraction that this path optimizer "
+                             "instance was called with have changed - try creating a new instance.")
+
     def __call__(self, inputs, output, size_dict, memory_limit=None):
         raise NotImplementedError
 
@@ -336,6 +349,7 @@ class BranchBound(PathOptimizer):
         >>> optimal(isets, oset, idx_sizes, 5000)
         [(0, 2), (0, 1)]
         """
+        self._check_args_against_first_call(inputs, output, size_dict)
 
         inputs = tuple(map(frozenset, inputs))
         output = frozenset(output)

diff --git a/opt_einsum/tests/test_paths.py b/opt_einsum/tests/test_paths.py
index 5105bbe..04769f6 100644
--- a/opt_einsum/tests/test_paths.py
+++ b/opt_einsum/tests/test_paths.py
@@ -299,6 +299,12 @@ def test_custom_random_greedy():
     assert path_info.largest_intermediate == optimizer.best['size']
     assert path_info.opt_cost == optimizer.best['flops']
 
+    # check error if we try and reuse the optimizer on a different expression
+    eq, shapes = oe.helpers.rand_equation(10, 4, seed=41)
+    views = list(map(np.ones, shapes))
+    with pytest.raises(ValueError):
+        path, path_info = oe.contract_path(eq, *views, optimize=optimizer)
+
 
 def test_custom_branchbound():
     eq, shapes = oe.helpers.rand_equation(8, 4, seed=42)
@@ -320,6 +326,12 @@ def test_custom_branchbound():
     assert path_info.largest_intermediate == optimizer.best['size']
     assert path_info.opt_cost == optimizer.best['flops']
 
+    # check error if we try and reuse the optimizer on a different expression
+    eq, shapes = oe.helpers.rand_equation(8, 4, seed=41)
+    views = list(map(np.ones, shapes))
+    with pytest.raises(ValueError):
+        path, path_info = oe.contract_path(eq, *views, optimize=optimizer)
+
 
 @pytest.mark.skipif(sys.version_info < (3, 2), reason="requires python3.2 or higher")
 def test_parallel_random_greedy():

Possible bug in RandomGreedy path
I have found an interesting quirk of the RandomGreedy optimizer that may not have been intended. In my application, I need to repeat two contractions several times. I use the same optimizer to compute the paths in advance and cache the result. If I use RandomGreedy, then I get the wrong shape for the second contraction, which leads me to believe that there's some internal state of RandomGreedy that persists into computing the second path. Below is a MWE.

```python
import torch
import opt_einsum as oe

eqn1 = 'abc,bef,ahi,ehk,uvc,vwf,uxi,wxk->'
eqn2 = 'abd,beg,ahj,ehl,mndo,npgq,mrjs,prlt,uvo,vwq,uxs,wxt->'
shapes1 = [(1, 1, 2), (1, 1, 2), (1, 1, 2), (1, 1, 2), 
           (1, 1, 2), (1, 1, 2), (1, 1, 2), (1, 1, 2)] 
shapes2 = [(1, 1, 2), (1, 1, 2), (1, 1, 2), (1, 1, 2), (1, 1, 2, 2), (1, 1, 2, 2), 
           (1, 1, 2, 2), (1, 1, 2, 2), (1, 1, 2), (1, 1, 2), (1, 1, 2), (1, 1, 2)] 
tens1 = [torch.randn(*sh) for sh in shapes1]
tens2 = [torch.randn(*sh) for sh in shapes2]

for Opt in [oe.DynamicProgramming, oe.RandomGreedy]:
    opt = Opt(minimize='flops')
                                                                                                                                 
    expr1 = oe.contract_expression(eqn1, *[ten.shape for ten in tens1], optimize=opt)
    expr2 = oe.contract_expression(eqn2, *[ten.shape for ten in tens2], optimize=opt)

    print(expr1(*tens1))
    print(expr2(*tens2))
```

The output of this program, using the latest PyPI release, is:

```
tensor(-0.0187)
tensor(0.4303)
tensor(-0.0187)
tensor([[[[[-0.4418]],

          [[ 1.2737]]]]])
```

Lines 1 and 3 should match (and do), but so should 2 and 4 (and don't).
opt_einsum/tests/test_paths.py::test_custom_random_greedy opt_einsum/tests/test_paths.py::test_custom_branchbound
opt_einsum/tests/test_paths.py::test_size_by_dict opt_einsum/tests/test_paths.py::test_flop_cost opt_einsum/tests/test_paths.py::test_bad_path_option opt_einsum/tests/test_paths.py::test_explicit_path opt_einsum/tests/test_paths.py::test_path_optimal opt_einsum/tests/test_paths.py::test_path_greedy opt_einsum/tests/test_paths.py::test_memory_paths opt_einsum/tests/test_paths.py::test_path_edge_cases[greedy-eb,cb,fb->cef-order0] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-all-eb,cb,fb->cef-order1] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-2-eb,cb,fb->cef-order2] opt_einsum/tests/test_paths.py::test_path_edge_cases[optimal-eb,cb,fb->cef-order3] opt_einsum/tests/test_paths.py::test_path_edge_cases[dp-eb,cb,fb->cef-order4] opt_einsum/tests/test_paths.py::test_path_edge_cases[greedy-dd,fb,be,cdb->cef-order5] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-all-dd,fb,be,cdb->cef-order6] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-2-dd,fb,be,cdb->cef-order7] opt_einsum/tests/test_paths.py::test_path_edge_cases[optimal-dd,fb,be,cdb->cef-order8] opt_einsum/tests/test_paths.py::test_path_edge_cases[optimal-dd,fb,be,cdb->cef-order9] opt_einsum/tests/test_paths.py::test_path_edge_cases[dp-dd,fb,be,cdb->cef-order10] opt_einsum/tests/test_paths.py::test_path_edge_cases[greedy-bca,cdb,dbf,afc->-order11] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-all-bca,cdb,dbf,afc->-order12] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-2-bca,cdb,dbf,afc->-order13] opt_einsum/tests/test_paths.py::test_path_edge_cases[optimal-bca,cdb,dbf,afc->-order14] opt_einsum/tests/test_paths.py::test_path_edge_cases[dp-bca,cdb,dbf,afc->-order15] opt_einsum/tests/test_paths.py::test_path_edge_cases[greedy-dcc,fce,ea,dbf->ab-order16] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-all-dcc,fce,ea,dbf->ab-order17] opt_einsum/tests/test_paths.py::test_path_edge_cases[branch-2-dcc,fce,ea,dbf->ab-order18] opt_einsum/tests/test_paths.py::test_path_edge_cases[optimal-dcc,fce,ea,dbf->ab-order19] opt_einsum/tests/test_paths.py::test_path_edge_cases[dp-dcc,fce,ea,dbf->ab-order20] opt_einsum/tests/test_paths.py::test_optimal_edge_cases opt_einsum/tests/test_paths.py::test_greedy_edge_cases opt_einsum/tests/test_paths.py::test_dp_edge_cases_dimension_1 opt_einsum/tests/test_paths.py::test_dp_edge_cases_all_singlet_indices opt_einsum/tests/test_paths.py::test_custom_dp_can_optimize_for_outer_products opt_einsum/tests/test_paths.py::test_custom_dp_can_optimize_for_size opt_einsum/tests/test_paths.py::test_custom_dp_can_set_cost_cap opt_einsum/tests/test_paths.py::test_can_optimize_outer_products[greedy] opt_einsum/tests/test_paths.py::test_can_optimize_outer_products[branch-2] opt_einsum/tests/test_paths.py::test_can_optimize_outer_products[branch-all] opt_einsum/tests/test_paths.py::test_can_optimize_outer_products[optimal] opt_einsum/tests/test_paths.py::test_can_optimize_outer_products[dp] opt_einsum/tests/test_paths.py::test_large_path[2] opt_einsum/tests/test_paths.py::test_large_path[3] opt_einsum/tests/test_paths.py::test_large_path[26] opt_einsum/tests/test_paths.py::test_large_path[52] opt_einsum/tests/test_paths.py::test_large_path[116] opt_einsum/tests/test_paths.py::test_large_path[300] opt_einsum/tests/test_paths.py::test_parallel_random_greedy opt_einsum/tests/test_paths.py::test_custom_path_optimizer opt_einsum/tests/test_paths.py::test_custom_random_optimizer opt_einsum/tests/test_paths.py::test_optimizer_registration