Speed up py2mat and mat2py by avoiding repeated Python package imports

- For details, see:
    - Apress/python-for-matlab-development#4
    - Apress/python-for-matlab-development#5

bittrue/models/matlab/helpers/mat2py.m

@@ -36,13 +36,8 @@
 % FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 % DEALINGS IN THE SOFTWARE.
 % }}}
-
-    Im = @py.importlib.import_module;    
-    np = Im('numpy');
-    sp = Im('scipy.sparse');
-    dt = Im('datetime');
-    tz = Im('dateutil.tz');
-    x_py = np.array({});
+
+    x_py = py.numpy.array({});
     switch class(x_mat)
         case 'char'
             if strcmp(char_to,'bytes')
@@ -57,11 +52,11 @@
             frac_sec = x_mat.Second - double(int_sec);
             micro_sec = int64(round(1e6 * frac_sec));
             if ~isempty(x_mat.TimeZone)
-                tzinfo = tz.gettz(x_mat.TimeZone);
+                tzinfo = py.dateutil.tz.gettz(x_mat.TimeZone);
             else
                 tzinfo = py.None;
             end
-            x_py = dt.datetime(int64(x_mat.Year), int64(x_mat.Month), ...
+            x_py = py.datetime.datetime(int64(x_mat.Year), int64(x_mat.Month), ...
                                int64(x_mat.Day) , int64(x_mat.Hour) , ...
                                int64(x_mat.Minute), int64(x_mat.Second), ...
                                micro_sec, tzinfo);
@@ -76,20 +71,20 @@
                 [nR,nC] = size(x_mat);
                 [i,j,vals] = find(x_mat);
                 % subtract 1 to go from 1-based to 0-based indices
-                py_I    = np.array(int64(i)-1);
-                py_J    = np.array(int64(j)-1);
+                py_I    = py.numpy.array(int64(i)-1);
+                py_J    = py.numpy.array(int64(j)-1);
                 py_vals = mat2py(vals);
                 py_dims = py.tuple({int64(nR), int64(nC)});
                 py_IJ   = py.tuple({py_I, py_J});
                 V_IJ    = py.tuple({py_vals, py_IJ});
-                x_py = sp.coo_matrix(V_IJ,py_dims);
+                x_py = py.scipy.sparse.coo_matrix(V_IJ,py_dims);
             elseif ismatrix(x_mat)
                 if numel(x_mat) == 1
                     x_py = x_mat;  % scalar numeric value
                 elseif isreal(x_mat)
-                    x_py = np.array(x_mat);
+                    x_py = py.numpy.array(x_mat);
                 else
-                    x_py = np.array(real(x_mat)) + 1j*np.array(imag(x_mat));
+                    x_py = py.numpy.array(real(x_mat)) + 1j*py.numpy.array(imag(x_mat));
                 end
             end
         case 'logical'

bittrue/models/matlab/helpers/py2mat.m

@@ -29,8 +29,6 @@
 % DEALINGS IN THE SOFTWARE.
 % }}}
 function [x_mat] = py2mat(x_py)
-  Im = @py.importlib.import_module;
-  np = Im('numpy');
   switch class(x_py)
       % Python dictionaries
       case 'py.dict'
@@ -77,25 +75,25 @@
           x_mat = x_py.single;
         case "float16"
           % doesn't exist in matlab, upcast to float32
-          x_mat = single(x_py.astype(np.float32));
+          x_mat = single(x_py.astype('float32'));
         % only reals and logicals can be cast to arrays so
         % have to cast the rest to either single or double
         case "uint8"
-          x_mat = uint8(x_py.astype(np.float32));
+          x_mat = uint8(x_py.astype('float32'));
         case "int8"
-          x_mat = int8(x_py.astype(np.float32));
+          x_mat = int8(x_py.astype('float32'));
         case "uint16"
-          x_mat = uint16(x_py.astype(np.float32));
+          x_mat = uint16(x_py.astype('float32'));
         case "int16"
-          x_mat = int16(x_py.astype(np.float32));
+          x_mat = int16(x_py.astype('float32'));
         case "uint32"
-          x_mat = uint32(x_py.astype(np.float32));
+          x_mat = uint32(x_py.astype('float32'));
         case "int32"
-          x_mat = int32(x_py.astype(np.float32));
+          x_mat = int32(x_py.astype('float32'));
         case "uint64"
-          x_mat = uint64(x_py.astype(np.float64));
+          x_mat = uint64(x_py.astype('float64'));
         case "int64"
-          x_mat = int64(x_py.astype(np.float64));
+          x_mat = int64(x_py.astype('float64'));
         case "bool"
           x_mat = logical(x_py);
         % Complex types require a math operation to coerce the