Only load the parsers if necessary.

Check if Biopython already knows about our formats (which will be
the case starting from Biopython 1.75) before loading the parsers.

Emit a warning if the parsers are already available in Biopython
so that the user will know she no longer needs to load our module.

incenp/bio/seqio/__init__.py

@@ -27,28 +27,25 @@ formats in Biopython's SeqIO.
 
 from Bio import SeqIO
 
-from . import SnapGeneIO
-from . import XdnaIO
-from . import GckIO
-
-
-if not 'xdna' in SeqIO._FormatToIterator:
-    SeqIO._FormatToIterator['xdna'] = XdnaIO.XdnaIterator
+if not 'gck' in SeqIO._FormatToIterator:
 
-if not 'xdna' in SeqIO._FormatToWriter:
-    SeqIO._FormatToWriter['xdna'] = XdnaIO.XdnaWriter
+    from . import GckIO
+    from . import SnapGeneIO
+    from . import XdnaIO
 
-if not 'xdna' in SeqIO._BinaryFormats:
-    SeqIO._BinaryFormats.append('xdna')
+    SeqIO._FormatToIterator['gck'] = GckIO.GckIterator
+    SeqIO._BinaryFormats.append('gck')
 
-if not 'snapgene' in SeqIO._FormatToIterator:
     SeqIO._FormatToIterator['snapgene'] = SnapGeneIO.SnapGeneIterator
-
-if not 'snapgene' in SeqIO._BinaryFormats:
     SeqIO._BinaryFormats.append('snapgene')
 
-if not 'gck' in SeqIO._FormatToIterator:
-    SeqIO._FormatToIterator['gck'] = GckIO.GckIterator
+    SeqIO._FormatToIterator['xdna'] = XdnaIO.XdnaIterator
+    SeqIO._FormatToWriter['xdna'] = XdnaIO.XdnaWriter
+    SeqIO._BinaryFormats.append('xdna')
 
-if not 'gck' in SeqIO._BinaryFormats:
-    SeqIO._BinaryFormats.append('gck')
+else:
+    import warnings
+    warnings.warn("Your Biopython installation already has support for the "
+                  "Gck, SnapGene, and Xdna formats. You no longer need to "
+                  "load the incenp.bio.seqio module",
+                  DeprecationWarning)