move part of image parsing to its own function to avoid issue with cl…

…assmethod

specreduce/background.py

@@ -9,7 +9,7 @@
 from astropy.utils.decorators import deprecated_attribute
 from specutils import Spectrum1D
 
-from specreduce.core import _ImageParser
+from specreduce.core import _ImageParser, _get_data_from_image
 from specreduce.extract import _ap_weight_image
 from specreduce.tracing import Trace, FlatTrace
 
@@ -183,7 +183,7 @@ def two_sided(cls, image, trace_object, separation, **kwargs):
         crossdisp_axis : int
             cross-dispersion axis
         """
-        image = cls._parse_image(cls, image)
+        image = _get_data_from_image(image) if image is not None else cls.image
         kwargs['traces'] = [trace_object-separation, trace_object+separation]
         return cls(image=image, **kwargs)
 
@@ -220,7 +220,7 @@ def one_sided(cls, image, trace_object, separation, **kwargs):
         crossdisp_axis : int
             cross-dispersion axis
         """
-        image = cls._parse_image(cls, image)
+        image = _get_data_from_image(image) if image is not None else cls.image
         kwargs['traces'] = [trace_object+separation]
         return cls(image=image, **kwargs)
 

specreduce/core.py

@@ -10,6 +10,17 @@
 
 __all__ = ['SpecreduceOperation']
 
+def _get_data_from_image(image):
+    """Extract data array from various input types for `image`."""
+
+    if isinstance(image, np.ndarray):
+        img = image
+    elif isinstance(image, u.quantity.Quantity):
+        img = image.value
+    else:  # NDData, including CCDData and Spectrum1D
+        img = image.data
+    return img
+
 
 class _ImageParser:
     """
@@ -50,12 +61,7 @@ def _parse_image(self, image, disp_axis=1):
             # useful for Background's instance methods
             return self.image
 
-        if isinstance(image, np.ndarray):
-            img = image
-        elif isinstance(image, u.quantity.Quantity):
-            img = image.value
-        else:  # NDData, including CCDData and Spectrum1D
-            img = image.data
+        img = _get_data_from_image(image)
 
         # mask and uncertainty are set as None when they aren't specified upon
         # creating a Spectrum1D object, so we must check whether these