Add a copy argument to MultiVector.__init__

The behavior of `MultiVector(layout, some_array)` has always been to copy `some_array`.

This change makes that copy opt-out, via `MultiVector(layout, some_array, copy=False)`.
This is used internally by all of the arithmetic operators to avoid unnecessary copies.

clifford/_multivector.py

@@ -15,10 +15,22 @@ class MultiVector(object):
     Parameters
     -------------
     layout: instance of :class:`clifford.Layout`
-        the layout of the algebra
+        The layout of the algebra
 
     value : sequence of length ``layout.gaDims``
-        the coefficients of the base blades
+        The coefficients of the base blades
+
+    dtype : numpy.dtype
+        The datatype to use for the multivector, if no
+        value was passed.
+
+        .. versionadded:: 1.1.0
+
+    copy : bool
+        If True, the default, create a copy of `value` to store in this
+        multivector.
+
+        .. versionadded:: 1.3.0
 
     Notes
     ------
@@ -33,7 +45,7 @@ class MultiVector(object):
     * ``M[N]`` : blade projection
     """
 
-    def __init__(self, layout, value=None, string=None, *, dtype: np.dtype = np.float64) -> None:
+    def __init__(self, layout, value=None, string=None, *, dtype: np.dtype = np.float64, copy: bool = True) -> None:
         """Constructor."""
 
         self.layout = layout
@@ -45,7 +57,7 @@ def __init__(self, layout, value=None, string=None, *, dtype: np.dtype = np.floa
             else:
                 self.value = layout.parse_multivector(string).value
         else:
-            self.value = np.array(value)
+            self.value = np.array(value, copy=copy)
             if self.value.shape != (self.layout.gaDims,):
                 raise ValueError(
                     "value must be a sequence of length %s" %
@@ -85,7 +97,7 @@ def _newMV(self, newValue=None, *, dtype: np.dtype = None) -> 'MultiVector':
         if newValue is None and dtype is None:
             raise TypeError("Must specify either a type or value")
 
-        return self.__class__(self.layout, newValue, dtype=dtype)
+        return self.__class__(self.layout, newValue, dtype=dtype, copy=False)
 
     # numeric special methods
     # binary

docs/changelog.rst

@@ -18,6 +18,8 @@ Changes in 1.3.x
  * Faster algebra construction. ``Cl(3)`` is now 100\ |times| faster, and
    ``Cl(6)`` is 20\ |times| faster. This is achieved by deferring product JIT
    compilation until the product is used for the first time.
+ * The :class:`MultiVector` constructor gained a ``copy`` argument, which can be
+   set to ``False`` to allow construction without copying.
 
 Bugs fixed
 ----------