diff --git a/easybuild/easyblocks/p/python.py b/easybuild/easyblocks/p/python.py
index 700c726cd77..c436eb05a38 100644
--- a/easybuild/easyblocks/p/python.py
+++ b/easybuild/easyblocks/p/python.py
@@ -180,7 +180,7 @@ def _get_pip_ext_version(self):
     def prepare_step(self, *args, **kwargs):
         super(EB_Python, self).prepare_step(*args, **kwargs)
 
-        if self.cfg['ebpythonprefixes']:
+        if self.cfg.get('ebpythonprefixes'):
             easybuild_subdir = log_path()
             self.pythonpath = os.path.join(easybuild_subdir, 'python')
         else:
@@ -459,7 +459,7 @@ def install_step(self):
             if not os.path.isfile(pip_binary_path):
                 symlink('pip' + self.pyshortver, pip_binary_path, use_abspath_source=False)
 
-        if self.cfg['ebpythonprefixes']:
+        if self.cfg.get('ebpythonprefixes'):
             write_file(os.path.join(self.installdir, self.pythonpath, 'sitecustomize.py'), SITECUSTOMIZE)
 
         # symlink lib/python*/lib-dynload to lib64/python*/lib-dynload if it doesn't exist;
@@ -535,7 +535,7 @@ def sanity_check_step(self):
         else:
             self.log.info("No errors found in output of %s: %s", cmd, out)
 
-        if self.cfg['ebpythonprefixes']:
+        if self.cfg.get('ebpythonprefixes'):
             self._sanity_check_ebpythonprefixes()
 
         pyver = 'python' + self.pyshortver
@@ -587,7 +587,7 @@ def make_module_extra(self, *args, **kwargs):
         """Add path to sitecustomize.py to $PYTHONPATH"""
         txt = super(EB_Python, self).make_module_extra()
 
-        if self.cfg['ebpythonprefixes']:
+        if self.cfg.get('ebpythonprefixes'):
             txt += self.module_generator.prepend_paths('PYTHONPATH', self.pythonpath)
 
         return txt
diff --git a/easybuild/easyblocks/t/tkinter.py b/easybuild/easyblocks/t/tkinter.py
index 5a9088ea210..c8a5a4e7702 100644
--- a/easybuild/easyblocks/t/tkinter.py
+++ b/easybuild/easyblocks/t/tkinter.py
@@ -55,10 +55,15 @@ def extra_options():
         """Disable EBPYTHONPREFIXES."""
         extra_vars = EB_Python.extra_options()
         # Not used for Tkinter
-        extra_vars['ebpythonprefixes'][0] = False
+        del extra_vars['ebpythonprefixes']
 
         return extra_vars
 
+    def __init__(self, *args, **kwargs):
+        """Initialize Tkinter-specific variables."""
+        super(EB_Tkinter, self).__init__(*args, **kwargs)
+        self.tkinter_so_basename = None
+
     def configure_step(self):
         """Check for Tk before configuring"""
         tk = get_software_root('Tk')
@@ -78,11 +83,11 @@ def install_step(self):
 
         tmpdir = tempfile.mkdtemp(dir=self.builddir)
 
-        tkinter_so_basename = self.get_tkinter_so_basename(False)
+        self.tkinter_so_basename = self.get_tkinter_so_basename(False)
         if LooseVersion(self.version) >= LooseVersion('3'):
-            tkparts = ["tkinter", os.path.join("lib-dynload", tkinter_so_basename)]
+            tkparts = ["tkinter", os.path.join("lib-dynload", self.tkinter_so_basename)]
         else:
-            tkparts = ["lib-tk", os.path.join("lib-dynload", tkinter_so_basename)]
+            tkparts = ["lib-tk", os.path.join("lib-dynload", self.tkinter_so_basename)]
 
         pylibdir = os.path.join(self.installdir, det_pylibdir())
         copy([os.path.join(os.path.dirname(pylibdir), x) for x in tkparts], tmpdir)
@@ -90,7 +95,7 @@ def install_step(self):
         remove_dir(self.installdir)
 
         move_file(os.path.join(tmpdir, tkparts[0]), os.path.join(pylibdir, tkparts[0]))
-        move_file(os.path.join(tmpdir, tkinter_so_basename), os.path.join(pylibdir, tkinter_so_basename))
+        move_file(os.path.join(tmpdir, self.tkinter_so_basename), os.path.join(pylibdir, self.tkinter_so_basename))
 
     def get_tkinter_so_basename(self, in_final_dir):
         pylibdir = os.path.join(self.installdir, det_pylibdir())
@@ -115,10 +120,11 @@ def sanity_check_step(self):
             tkinter = 'Tkinter'
         custom_commands = ["python -c 'import %s'" % tkinter]
 
-        tkinter_so_basename = self.get_tkinter_so_basename(True)
+        if not self.tkinter_so_basename:
+            self.tkinter_so_basename = self.get_tkinter_so_basename(True)
 
         custom_paths = {
-            'files': [os.path.join(det_pylibdir(), tkinter_so_basename)],
+            'files': [os.path.join(det_pylibdir(), self.tkinter_so_basename)],
             'dirs': ['lib']
         }
         super(EB_Python, self).sanity_check_step(custom_commands=custom_commands, custom_paths=custom_paths)