RF: move create_empty_header logic to top level

Move the creation of the empty streamline header into the kinda-abstract
base class.  Adjust the Trk file header creation accordingly.

Author: matthew-brett

nibabel/streamlines/tck.py

@@ -72,9 +72,6 @@ def __init__(self, tractogram, header=None):
         This is in contrast with TRK's internal convention where it would
         have referred to a corner.
         """
-        if header is None:
-            header = self.create_empty_header()
-
         super(TckFile, self).__init__(tractogram, header)
 
     @classmethod
@@ -103,7 +100,7 @@ def is_correct_format(cls, fileobj):
 
     @classmethod
     def create_empty_header(cls):
-        """ Return an empty compliant TCK header. """
+        """ Return an empty compliant TCK header as dict """
         header = {}
 
         # Default values

nibabel/streamlines/tractogram_file.py

@@ -39,7 +39,7 @@ class TractogramFile(with_metaclass(ABCMeta)):
 
     def __init__(self, tractogram, header=None):
         self._tractogram = tractogram
-        self._header = {} if header is None else header
+        self._header = self.create_empty_header() if header is None else header
 
     @property
     def tractogram(self):
@@ -77,10 +77,10 @@ def is_correct_format(cls, fileobj):
         """
         raise NotImplementedError()
 
-    @abstractclassmethod
+    @classmethod
     def create_empty_header(cls):
         """ Returns an empty header for this streamlines file format. """
-        raise NotImplementedError()
+        return {}
 
     @abstractclassmethod
     def load(cls, fileobj, lazy_load=True):

nibabel/streamlines/trk.py

@@ -235,10 +235,6 @@ def __init__(self, tractogram, header=None):
         and *mm* space where coordinate (0,0,0) refers to the center
         of the voxel.
         """
-        if header is None:
-            header_rec = self.create_empty_header()
-            header = dict(zip(header_rec.dtype.names, header_rec[0]))
-
         super(TrkFile, self).__init__(tractogram, header)
 
     @classmethod
@@ -266,8 +262,9 @@ def is_correct_format(cls, fileobj):
             return magic_number == cls.MAGIC_NUMBER
 
     @classmethod
-    def create_empty_header(cls):
-        """ Return an empty compliant TRK header. """
+    def _default_structarr(cls):
+        """ Return an empty compliant TRK header as numpy structured array
+        """
         header = np.zeros(1, dtype=header_2_dtype)
 
         # Default values
@@ -281,6 +278,13 @@ def create_empty_header(cls):
 
         return header
 
+    @classmethod
+    def create_empty_header(cls):
+        """ Return an empty compliant TRK header as dict
+        """
+        header_rec = cls._default_structarr()
+        return dict(zip(header_rec.dtype.names, header_rec))
+
     @classmethod
     def load(cls, fileobj, lazy_load=False):
         """ Loads streamlines from a filename or file-like object.
@@ -388,7 +392,7 @@ def save(self, fileobj):
             of the TRK header data).
         """
         # Enforce little-endian byte order for header
-        header = self.create_empty_header().newbyteorder('<')
+        header = self._default_structarr().newbyteorder('<')
 
         # Override hdr's fields by those contained in `header`.
         for k, v in self.header.items():