ENH: Allow save-time passing of on-disk dtype to to_* methods

Author: effigies

nibabel/analyze.py

@@ -992,23 +992,29 @@ 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(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
@@ -1048,6 +1054,7 @@ 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/cifti2/cifti2.py

@@ -1460,7 +1460,7 @@ def from_image(klass, img):
             return img
         raise NotImplementedError
 
-    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
@@ -1493,7 +1493,7 @@ def to_file_map(self, file_map=None):
         # If qform not set, reset pixdim values so Nifti2 does not complain
         if header['qform_code'] == 0:
             header['pixdim'][:4] = 1
-        img = Nifti2Image(data, None, header)
+        img = Nifti2Image(data, None, header, dtype=dtype)
         img.to_file_map(file_map or self.file_map)
 
     def update_headers(self):

nibabel/filebasedimages.py

@@ -299,24 +299,26 @@ def filespec_to_file_map(klass, filespec):
             file_map[key] = FileHolder(filename=fname)
         return file_map
 
-    def to_filename(self, filename):
-        """ Write image to files implied by filename string
+    def to_filename(self, filename, **kwargs):
+        r""" Write image to files implied by filename string
 
         Parameters
         ----------
         filename : str or os.PathLike
            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):
-        """ Return a ``bytes`` object with the contents of the file that would
+    def to_bytes(self, **kwargs):
+        r""" 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/spm99analyze.py

@@ -308,7 +308,7 @@ def from_file_map(klass, file_map, *, mmap=True, keep_file_open=None):
         ret._affine = np.dot(ret._affine, to_111)
         return ret
 
-    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``
 
         Extends Analyze ``to_file_map`` method by writing ``mat`` file
@@ -321,7 +321,7 @@ def to_file_map(self, file_map=None):
         """
         if file_map is None:
             file_map = self.file_map
-        super(Spm99AnalyzeImage, self).to_file_map(file_map)
+        super(Spm99AnalyzeImage, self).to_file_map(file_map, dtype=dtype)
         mat = self._affine
         if mat is None:
             return