created type_helpers.py with isstr

diff --git a/type_helpers.py b/type_helpers.py
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+++ b/type_helpers.py
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+# The software in this package is distributed under the GNU General
+# Public License version 2 (with a special exception described below).
+#
+# A copy of GNU General Public License (GPL) is included in this distribution,
+# in the file COPYING.GPL.
+#
+# As a special exception, if other files instantiate templates or use macros
+# or inline functions from this file, or you compile this file and link it
+# with other works to produce a work based on this file, this file
+# does not by itself cause the resulting work to be covered
+# by the GNU General Public License.
+#
+# However the source code for this file must still be made available
+# in accordance with section (3) of the GNU General Public License.
+#
+# This exception does not invalidate any other reasons why a work based
+# on this file might be covered by the GNU General Public License.
+
+"""
+Helpers for types, like isstr(x)
+
+Christian Herdtweck, Intra2net, August 2015
+(c) Intra2net AG 2015
+"""
+
+from __future__ import print_function
+import sys
+
+# determine python version
+PY3 = sys.version_info.major == 3
+PY2 = sys.version_info.major == 2
+
+
+def isstr(var):
+    """ determines if the given var is a (regular/unicode/raw) string or not
+
+    in python2, u'a' is not a subclass of str, so to get a True as result, you
+      have to test for basestring as superclass.
+
+    In python3 that is no longer the case
+
+    @returns True if the input is a regular string, a unicode or a raw string,
+       False for everything else (including byte literals like b'abc')
+
+    For more complex py2/py3 compatibility issues, consider using six
+      (https://pythonhosted.org/six)
+    """
+
+    if PY3:
+        return isinstance(var, str)
+    else:
+        return isinstance(var, basestring)
+# end: function isstr
+
+
+def test_isstr():
+    """ test function isstr
+
+    TODO: should probably be moved to some unittest or so ...
+    """
+
+    tests = [
+        ('abc', True), (u'abc', True), (r'abc', True),
+        (1, False), (['a', 'b', 'c'], False), (('a', 'b', 'c'), False)]
+    if PY3:
+        tests.append((b'abc', False))  # b'' does not exist in py2
+
+    for test_var, expected_result in tests:
+        assert isstr(test_var) == expected_result, 'isstr of var {0} ' \
+            '(which is of type {1}) returned {2} but expected {3}!'.format(
+                test_var, type(test_var), isstr(test_var), expected_result)
+    print('all string tests finished successfully')
+# end: test_isstr
+
+
+def main():
+    """ Main function, called when running file as script
+
+    just tells you what python version you are running (2 or 3)
+    """
+    if PY3:
+        print('is python3')
+    else:
+        print('is python2')
+
+    # test_isstr()
+# end: function main
+
+
+if __name__ == '__main__':
+    main()