diff --git a/ebook_converter/__init__.py b/ebook_converter/__init__.py
index 3af6217..aeda223 100644
--- a/ebook_converter/__init__.py
+++ b/ebook_converter/__init__.py
@@ -9,10 +9,9 @@ import urllib.parse
 import urllib.request
 import warnings
 
-from ebook_converter.polyglot.builtins import hasenv
 from functools import partial
 
-if not hasenv('CALIBRE_SHOW_DEPRECATION_WARNINGS'):
+if os.getenv('CALIBRE_SHOW_DEPRECATION_WARNINGS') is None:
     warnings.simplefilter('ignore', DeprecationWarning)
 try:
     os.getcwd()
@@ -155,7 +154,7 @@ def prints(*args, **kwargs):
     '''
     file = kwargs.get('file', sys.stdout)
     file = getattr(file, 'buffer', file)
-    enc = 'utf-8' if hasenv('CALIBRE_WORKER') else preferred_encoding
+    enc = 'utf-8' if os.getenv('CALIBRE_WORKER') else preferred_encoding
     sep  = kwargs.get('sep', ' ')
     if not isinstance(sep, bytes):
         sep = sep.encode(enc)
@@ -219,7 +218,7 @@ class CommandLineError(Exception):
 
 def setup_cli_handlers(logger, level):
     import logging
-    if hasenv('CALIBRE_WORKER') and logger.handlers:
+    if os.getenv('CALIBRE_WORKER') and logger.handlers:
         return
     logger.setLevel(level)
     if level == logging.WARNING:
diff --git a/ebook_converter/constants.py b/ebook_converter/constants.py
index 2abc793..b77da5a 100644
--- a/ebook_converter/constants.py
+++ b/ebook_converter/constants.py
@@ -5,7 +5,7 @@ import locale
 import os
 import sys
 
-from ebook_converter.polyglot.builtins import environ_item, hasenv, as_unicode
+from ebook_converter.polyglot.builtins import environ_item, as_unicode
 
 __appname__   = 'calibre'
 numeric_version = (4, 12, 0)
@@ -30,14 +30,15 @@ ishaiku = 'haiku1' in _plat
 islinux   = not(iswindows or isosx or isbsd or ishaiku)
 isfrozen  = hasattr(sys, 'frozen')
 isunix = isosx or islinux or ishaiku
-isportable = hasenv('CALIBRE_PORTABLE_BUILD')
+isportable = os.getenv('CALIBRE_PORTABLE_BUILD') is not None
 isxp = isoldvista = False
 if iswindows:
     wver = sys.getwindowsversion()
     isxp = wver.major < 6
     isoldvista = wver.build < 6002
 is64bit = sys.maxsize > (1 << 32)
-isworker = hasenv('CALIBRE_WORKER') or hasenv('CALIBRE_SIMPLE_WORKER')
+isworker = any([os.getenv('CALIBRE_WORKER') or
+                os.getenv('CALIBRE_SIMPLE_WORKER')])
 if isworker:
     os.environ.pop(environ_item('CALIBRE_FORCE_ANSI'), None)
 FAKE_PROTOCOL, FAKE_HOST = 'clbr', 'internal.invalid'
@@ -92,7 +93,7 @@ else:
         filesystem_encoding = 'utf-8'
 
 
-DEBUG = hasenv('CALIBRE_DEBUG')
+DEBUG = os.getenv('CALIBRE_DEBUG') is not None
 
 
 def debug():
diff --git a/ebook_converter/customize/ui.py b/ebook_converter/customize/ui.py
index 3ea692f..90a3de4 100644
--- a/ebook_converter/customize/ui.py
+++ b/ebook_converter/customize/ui.py
@@ -9,7 +9,6 @@ from ebook_converter.customize import (CatalogPlugin, FileTypePlugin, PluginNotF
                               StoreBase as Store, EditBookToolPlugin,
                               LibraryClosedPlugin)
 from ebook_converter.customize.conversion import InputFormatPlugin, OutputFormatPlugin
-from ebook_converter.customize.zipplugin import loader
 from ebook_converter.customize.profiles import InputProfile, OutputProfile
 from ebook_converter.customize.builtins import plugins as builtin_plugins
 # from ebook_converter.devices.interface import DevicePlugin
diff --git a/ebook_converter/customize/zipplugin.py b/ebook_converter/customize/zipplugin.py
index 29e3c01..77ae2d0 100644
--- a/ebook_converter/customize/zipplugin.py
+++ b/ebook_converter/customize/zipplugin.py
@@ -294,7 +294,7 @@ class PluginLoader(object):
 
 
 loader = PluginLoader()
-sys.meta_path.insert(0, loader)
+#sys.meta_path.insert(0, loader)
 
 
 if __name__ == '__main__':
diff --git a/ebook_converter/polyglot/builtins.py b/ebook_converter/polyglot/builtins.py
index 204e464..8575ba7 100644
--- a/ebook_converter/polyglot/builtins.py
+++ b/ebook_converter/polyglot/builtins.py
@@ -1,9 +1,4 @@
 import importlib
-import os
-
-
-def hasenv(x):
-    return os.getenv(x) is not None
 
 
 def as_bytes(x, encoding='utf-8'):