Alternative PR for: Some bytes/str issues in py3 w/ zlib and json (#2558)

* sql_lab.py: compress via utils

* utils.py: added zlib_compress and zlib_compress_to_string

* core.py: converted to use zlib_decompress_to_string; renamed uncompress to decompress in utils.py

* utils_tests.py: added test for compress/decompress

* fixed broken utils test; removed redundant code and empty lines from utils.py

* utils.py: corrected docstrings, removed unnecessary 'else'

* removed yet another superfluous else

superset/utils.py

@@ -16,6 +16,8 @@ import smtplib
 import sqlalchemy as sa
 import signal
 import uuid
+import sys
+import zlib
 
 from builtins import object
 from datetime import date, datetime, time
@@ -41,7 +43,7 @@ from sqlalchemy.types import TypeDecorator, TEXT
 
 logging.getLogger('MARKDOWN').setLevel(logging.INFO)
 
-
+PY3K = sys.version_info >= (3, 0)
 EPOCH = datetime(1970, 1, 1)
 DTTM_ALIAS = '__timestamp'
 
@@ -572,3 +574,34 @@ def setup_cache(app, cache_config):
     """Setup the flask-cache on a flask app"""
     if cache_config and cache_config.get('CACHE_TYPE') != 'null':
         return Cache(app, config=cache_config)
+
+
+def zlib_compress(data):
+    """
+    Compress things in a py2/3 safe fashion
+    >>> json_str = '{"test": 1}'
+    >>> blob = zlib_compress(json_str)
+    """
+    if PY3K:
+        if isinstance(data, str):
+            return zlib.compress(bytes(data, "utf-8"))
+        return zlib.compress(data)
+    return zlib.compress(data)
+
+
+def zlib_decompress_to_string(blob):
+    """
+    Decompress things to a string in a py2/3 safe fashion
+    >>> json_str = '{"test": 1}'
+    >>> blob = zlib_compress(json_str)
+    >>> got_str = zlib_decompress_to_string(blob)
+    >>> got_str == json_str
+    True
+    """
+    if PY3K:
+        if isinstance(blob, bytes):
+            decompressed = zlib.decompress(blob)
+        else:
+            decompressed = zlib.decompress(bytes(blob, "utf-8"))
+        return decompressed.decode("utf-8")
+    return zlib.decompress(blob)