ENH: Allow save-time passing of on-disk dtype to save functions

nibabel/analyze.py

@@ -989,23 +989,29 @@ def _get_fileholders(file_map):
         """
         return file_map['header'], file_map['image']
 
-    def to_file_map(self, file_map=None):
+    def to_file_map(self, file_map=None, dtype=None):
         """ Write image to `file_map` or contained ``self.file_map``
 
         Parameters
         ----------
         file_map : None or mapping, optional
            files mapping.  If None (default) use object's ``file_map``
            attribute instead
+        dtype : dtype-like, optional
+           The on-disk data type to coerce the data array.
         """
         if file_map is None:
             file_map = self.file_map
         data = np.asanyarray(self.dataobj)
         self.update_header()
         hdr = self._header
-        out_dtype = self.get_data_dtype()
         # Store consumable values for later restore
         offset = hdr.get_data_offset()
+        data_dtype = hdr.get_data_dtype()
+        # Override dtype conditionally
+        if dtype is not None:
+            hdr.set_data_dtype(out_dtype)
+        out_dtype = hdr.get_data_dtype()
         # Scalars of slope, offset to get immutable values
         slope = hdr['scl_slope'].item() if hdr.has_data_slope else np.nan
         inter = hdr['scl_inter'].item() if hdr.has_data_intercept else np.nan
@@ -1045,6 +1051,7 @@ def to_file_map(self, file_map=None):
         self.file_map = file_map
         # Restore any changed consumable values
         hdr.set_data_offset(offset)
+        hdr.set_data_dtype(data_dtype)
         if hdr.has_data_slope:
             hdr['scl_slope'] = slope
         if hdr.has_data_intercept:

nibabel/filebasedimages.py

@@ -299,7 +299,7 @@ def filespec_to_file_map(klass, filespec):
             file_map[key] = FileHolder(filename=fname)
         return file_map
 
-    def to_filename(self, filename):
+    def to_filename(self, filename, **kwargs):
         """ Write image to files implied by filename string
 
         Parameters
@@ -308,15 +308,17 @@ def to_filename(self, filename):
            filename to which to save image.  We will parse `filename`
            with ``filespec_to_file_map`` to work out names for image,
            header etc.
+        \*\*kwargs : keyword arguments
+           Keyword arguments to format-specific save
 
         Returns
         -------
         None
         """
         self.file_map = self.filespec_to_file_map(filename)
-        self.to_file_map()
+        self.to_file_map(**kwargs)
 
-    def to_file_map(self, file_map=None):
+    def to_file_map(self, file_map=None, **kwargs):
         raise NotImplementedError
 
     @classmethod
@@ -552,13 +554,14 @@ def from_bytes(klass, bytestring):
         file_map = klass.make_file_map({'image': bio, 'header': bio})
         return klass.from_file_map(file_map)
 
-    def to_bytes(self):
+    def to_bytes(self, **kwargs):
         """ Return a ``bytes`` object with the contents of the file that would
         be written if the image were saved.
 
         Parameters
         ----------
-        None
+        \*\*kwargs : keyword arguments
+            Keyword arguments that may be passed to ``img.to_file_map()``
 
         Returns
         -------
@@ -569,5 +572,5 @@ def to_bytes(self):
             raise NotImplementedError("to_bytes() is undefined for multi-file images")
         bio = io.BytesIO()
         file_map = self.make_file_map({'image': bio, 'header': bio})
-        self.to_file_map(file_map)
+        self.to_file_map(file_map, **kwargs)
         return bio.getvalue()

nibabel/loadsave.py

@@ -130,7 +130,7 @@ def guessed_image_type(filename):
     raise ImageFileError(f'Cannot work out file type of "{filename}"')
 
 
-def save(img, filename):
+def save(img, filename, **kwargs):
     """ Save an image to file adapting format to `filename`
 
     Parameters
@@ -139,6 +139,8 @@ def save(img, filename):
        image to save
     filename : str or os.PathLike
        filename (often implying filenames) to which to save `img`.
+    \*\*kwargs : keyword arguments
+        Keyword arguments to format-specific save
 
     Returns
     -------
@@ -148,7 +150,7 @@ def save(img, filename):
 
     # Save the type as expected
     try:
-        img.to_filename(filename)
+        img.to_filename(filename, **kwargs)
     except ImageFileError:
         pass
     else:
@@ -196,7 +198,7 @@ def save(img, filename):
     # Here, we either have a klass or a converted image.
     if converted is None:
         converted = klass.from_image(img)
-    converted.to_filename(filename)
+    converted.to_filename(filename, **kwargs)
 
 
 @deprecate_with_version('read_img_data deprecated. '