move 'memoize' decorator to kwant/common.py

this decorator will be needed in kwant/builder.py in the next
commit.

kwant/_common.py

@@ -13,6 +13,7 @@ import inspect
 import warnings
 import importlib
 import functools
+import collections
 from contextlib import contextmanager
 
 __all__ = ['KwantDeprecationWarning', 'UserCodeError']
@@ -191,3 +192,27 @@ class lazy_import:
         package = sys.modules[self.__package]
         setattr(package, self.__module, mod)
         return getattr(mod, name)
+
+
+def _hashable(obj):
+    return isinstance(obj, collections.abc.Hashable)
+
+
+def memoize(f):
+    """Decorator to memoize a function that works even with unhashable args.
+
+    This decorator will even work with functions whose args are not hashable.
+    The cache key is made up by the hashable arguments and the ids of the
+    non-hashable args.  It is up to the user to make sure that non-hashable
+    args do not change during the lifetime of the decorator.
+
+    This decorator will keep reevaluating functions that return None.
+    """
+    def lookup(*args):
+        key = tuple(arg if _hashable(arg) else id(arg) for arg in args)
+        result = cache.get(key)
+        if result is None:
+            cache[key] = result = f(*args)
+        return result
+    cache = {}
+    return lookup

kwant/wraparound.py

@@ -20,36 +20,12 @@ from . import builder, system, plotter
 from .linalg import lll
 from .builder import herm_conj, HermConjOfFunc
 from .lattice import TranslationalSymmetry
-from ._common import get_parameters
+from ._common import get_parameters, memoize
 
 
 __all__ = ['wraparound', 'plot_2d_bands']
 
 
-def _hashable(obj):
-    return isinstance(obj, collections.abc.Hashable)
-
-
-def _memoize(f):
-    """Decorator to memoize a function that works even with unhashable args.
-
-    This decorator will even work with functions whose args are not hashable.
-    The cache key is made up by the hashable arguments and the ids of the
-    non-hashable args.  It is up to the user to make sure that non-hashable
-    args do not change during the lifetime of the decorator.
-
-    This decorator will keep reevaluating functions that return None.
-    """
-    def lookup(*args):
-        key = tuple(arg if _hashable(arg) else id(arg) for arg in args)
-        result = cache.get(key)
-        if result is None:
-            cache[key] = result = f(*args)
-        return result
-    cache = {}
-    return lookup
-
-
 def _set_signature(func, params):
     """Set the signature of 'func'.
 
@@ -103,7 +79,7 @@ def wraparound(builder, keep=None, *, coordinate_names='xyz'):
     format. It will be deprecated in the 2.0 release of Kwant.
     """
 
-    @_memoize
+    @memoize
     def bind_site(val):
         def f(*args):
             a, *args = args
@@ -113,7 +89,7 @@ def wraparound(builder, keep=None, *, coordinate_names='xyz'):
         _set_signature(f, get_parameters(val) + momenta)
         return f
 
-    @_memoize
+    @memoize
     def bind_hopping_as_site(elem, val):
         def f(*args):
             a, *args = args
@@ -128,7 +104,7 @@ def wraparound(builder, keep=None, *, coordinate_names='xyz'):
         _set_signature(f, params + momenta)
         return f
 
-    @_memoize
+    @memoize
     def bind_hopping(elem, val):
         def f(*args):
             a, b, *args = args
@@ -142,7 +118,7 @@ def wraparound(builder, keep=None, *, coordinate_names='xyz'):
         _set_signature(f, params + momenta)
         return f
 
-    @_memoize
+    @memoize
     def bind_sum(num_sites, *vals):
         """Construct joint signature for all 'vals'."""