Make the serialize module cheap to import

src/calibre/utils/serialize.py

@@ -4,50 +4,55 @@
 
 from __future__ import absolute_import, division, print_function, unicode_literals
 
-import json, base64
-from functools import partial
-from datetime import datetime
-
-import msgpack
-
-from calibre.utils.iso8601 import parse_iso8601
 
 MSGPACK_MIME = 'application/x-msgpack'
 CANARY = 'jPoAv3zOyHvQ5JFNYg4hJ9'
 
 
-def encoded(typ, data, for_json):
-    if for_json:
+def encoded(typ, data, ExtType):
+    if ExtType is None:
         return {CANARY: typ, 'v': data}
-    return msgpack.ExtType(typ, msgpack_dumps(data))
+    return ExtType(typ, msgpack_dumps(data))
 
 
-def encoder(obj, for_json=False):
-    if isinstance(obj, datetime):
-        return encoded(0, unicode(obj.isoformat()), for_json)
-    if isinstance(obj, (set, frozenset)):
-        return encoded(1, tuple(obj), for_json)
-    if getattr(obj, '__calibre_serializable__', False):
-        from calibre.ebooks.metadata.book.base import Metadata
-        from calibre.library.field_metadata import FieldMetadata, fm_as_dict
-        from calibre.db.categories import Tag
-        if isinstance(obj, Metadata):
-            from calibre.ebooks.metadata.book.serialize import metadata_as_dict
-            return encoded(
-                2, metadata_as_dict(obj, encode_cover_data=for_json), for_json
-            )
-        elif isinstance(obj, FieldMetadata):
-            return encoded(3, fm_as_dict(obj), for_json)
-        elif isinstance(obj, Tag):
-            return encoded(4, obj.as_dict(), for_json)
-    raise TypeError('Cannot serialize objects of type {}'.format(type(obj)))
+def create_encoder(for_json=False):
+    from datetime import datetime
+    ExtType = None
+    if not for_json:
+        import msgpack
+        ExtType = msgpack.ExtType
+
+    def encoder(obj):
+        if isinstance(obj, datetime):
+            return encoded(0, unicode(obj.isoformat()), ExtType)
+        if isinstance(obj, (set, frozenset)):
+            return encoded(1, tuple(obj), ExtType)
+        if getattr(obj, '__calibre_serializable__', False):
+            from calibre.ebooks.metadata.book.base import Metadata
+            from calibre.library.field_metadata import FieldMetadata, fm_as_dict
+            from calibre.db.categories import Tag
+            if isinstance(obj, Metadata):
+                from calibre.ebooks.metadata.book.serialize import metadata_as_dict
+                return encoded(
+                    2, metadata_as_dict(obj, encode_cover_data=for_json), ExtType
+                )
+            elif isinstance(obj, FieldMetadata):
+                return encoded(3, fm_as_dict(obj), ExtType)
+            elif isinstance(obj, Tag):
+                return encoded(4, obj.as_dict(), ExtType)
+        raise TypeError('Cannot serialize objects of type {}'.format(type(obj)))
+
+    return encoder
 
 
-msgpack_dumps = partial(msgpack.packb, default=encoder, use_bin_type=True)
+def msgpack_dumps(obj):
+    import msgpack
+    return msgpack.packb(obj, default=create_encoder(), use_bin_type=True)
 
 
 def json_dumps(data, **kw):
-    kw['default'] = partial(encoder, for_json=True)
+    import json
+    kw['default'] = create_encoder(for_json=True)
     kw['ensure_ascii'] = False
     ans = json.dumps(data, **kw)
     if not isinstance(ans, bytes):
@@ -56,6 +61,7 @@ def json_dumps(data, **kw):
 
 
 def decode_metadata(x, for_json):
+    import base64
     from calibre.ebooks.metadata.book.serialize import metadata_from_dict
     obj = metadata_from_dict(x)
     if for_json and obj.cover_data and obj.cover_data[1]:
@@ -75,8 +81,14 @@ def decode_category_tag(x, for_json):
     return Tag.from_dict(x)
 
 
+def decode_datetime(x, fj):
+    from calibre.utils.iso8601 import parse_iso8601
+    return parse_iso8601(x, assume_utc=True)
+
+
 decoders = (
-    lambda x, fj: parse_iso8601(x, assume_utc=True), lambda x, fj: set(x),
+    decode_datetime,
+    lambda x, fj: set(x),
     decode_metadata, decode_field_metadata, decode_category_tag
 )
 
@@ -92,8 +104,11 @@ def msgpack_decoder(code, data):
     return decoders[code](msgpack_loads(data), False)
 
 
-msgpack_loads = partial(msgpack.unpackb, encoding='utf-8', ext_hook=msgpack_decoder, raw=False)
+def msgpack_loads(dump):
+    import msgpack
+    return msgpack.unpackb(dump, ext_hook=msgpack_decoder, raw=False)
 
 
 def json_loads(data):
+    import json
     return json.loads(data, object_hook=json_decoder)