diff --git a/Lib/idlelib/idle_test/mock_tk.py b/Lib/idlelib/idle_test/mock_tk.py
index ef18c3374cf..c21ff433f29 100644
--- a/Lib/idlelib/idle_test/mock_tk.py
+++ b/Lib/idlelib/idle_test/mock_tk.py
@@ -1,4 +1,5 @@
 """Classes that replace tkinter gui objects used by an object being tested.
+
 A gui object is anything with a master or parent paramenter, which is typically
 required in spite of what the doc strings say.
 """
@@ -15,8 +16,10 @@ class Var:
         return self.value
 
 class Mbox_func:
-    """Generic mock for messagebox functions. All have same call signature.
-    Mbox instantiates once for each function. Tester uses attributes.
+    """Generic mock for messagebox functions, which all have the same signature.
+
+    Instead of displaying a message box, the mock's call method saves the
+    arguments as instance attributes, which test functions can then examime.
     """
     def __init__(self):
         self.result = None  # The return for all show funcs
@@ -30,6 +33,7 @@ class Mbox_func:
 
 class Mbox:
     """Mock for tkinter.messagebox with an Mbox_func for each function.
+
     This module was 'tkMessageBox' in 2.x; hence the 'import as' in  3.x.
     Example usage in test_module.py for testing functios in module.py:
     ---
@@ -49,9 +53,9 @@ class Test(unittest.TestCase):
     def tearDownClass(cls):
         module.tkMessageBox = orig_mbox
     ---
-    When tkMessageBox functions are the only gui making calls in a method,
-    this replacement makes the method gui-free and unit-testable.
-    For 'ask' functions, set func.result return before calling method.
+    For 'ask' functions, set func.result return value before calling the method
+    that uses the message function. When tkMessageBox functions are the
+    only gui alls in a method, this replacement makes the method gui-free,
     """
     askokcancel = Mbox_func()     # True or False
     askquestion = Mbox_func()     # 'yes' or 'no'
@@ -61,3 +65,215 @@ class Test(unittest.TestCase):
     showerror = Mbox_func()    # None
     showinfo = Mbox_func()     # None
     showwarning = Mbox_func()  # None
+
+from _tkinter import TclError
+
+class Text:
+    """A semi-functional non-gui replacement for tkinter.Text text editors.
+
+    The mock's data model is that a text is a list of \n-terminated lines.
+    The mock adds an empty string at  the beginning of the list so that the
+    index of actual lines start at 1, as with Tk. The methods never see this.
+    Tk initializes files with a terminal \n that cannot be deleted. It is
+    invisible in the sense that one cannot move the cursor beyond it.
+
+    This class is only tested (and valid) with strings of ascii chars.
+    For testing, we are not concerned with Tk Text's treatment of,
+    for instance, 0-width characters or character + accent.
+   """
+    def __init__(self, master=None, cnf={}, **kw):
+        '''Initialize mock, non-gui, text-only Text widget.
+
+        At present, all args are ignored. Almost all affect visual behavior.
+        There are just a few Text-only options that affect text behavior.
+        '''
+        self.data = ['', '\n']
+
+    def index(self, index):
+        "Return string version of index decoded according to current text."
+        return "%s.%s" % self._decode(index, endflag=1)
+
+    def _decode(self, index, endflag=0):
+        """Return a (line, char) tuple of int indexes into self.data.
+
+        This implements .index without converting the result back to a string.
+        The result is contrained by the number of lines and linelengths of
+        self.data. For many indexes, the result is initally (1, 0).
+
+        The input index may have any of several possible forms:
+        * line.char float: converted to 'line.char' string;
+        * 'line.char' string, where line and char are decimal integers;
+        * 'line.char lineend', where lineend='lineend' (and char is ignored);
+        * 'line.end', where end='end' (same as above);
+        * 'insert', the positions before terminal \n;
+        * 'end', whose meaning depends on the endflag passed to ._endex.
+        * 'sel.first' or 'sel.last', where sel is a tag -- not implemented.
+        """
+        if isinstance(index, (float, bytes)):
+            index = str(index)
+        try:
+            index=index.lower()
+        except AttributeError:
+            raise TclError('bad text index "%s"' % index) from None
+
+        lastline =  len(self.data) - 1  # same as number of text lines
+        if index == 'insert':
+            return lastline, len(self.data[lastline]) - 1
+        elif index == 'end':
+            return self._endex(endflag)
+
+        line, char = index.split('.')
+        line = int(line)
+
+        # Out of bounds line becomes first or last ('end') index
+        if line < 1:
+            return 1, 0
+        elif line > lastline:
+            return self._endex(endflag)
+
+        linelength = len(self.data[line])  -1  # position before/at \n
+        if char.endswith(' lineend') or char == 'end':
+            return line, linelength
+            # Tk requires that ignored chars before ' lineend' be valid int
+
+        # Out of bounds char becomes first or last index of line
+        char = int(char)
+        if char < 0:
+            char = 0
+        elif char > linelength:
+            char = linelength
+        return line, char
+
+    def _endex(self, endflag):
+        '''Return position for 'end' or line overflow corresponding to endflag.
+
+       -1: position before terminal \n; for .insert(), .delete
+       0: position after terminal \n; for .get, .delete index 1
+       1: same viewed as begininning of non-existent next line (for .index)
+       '''
+        n = len(self.data)
+        if endflag == 1:
+            return n, 0
+        else:
+            n -= 1
+            return n, len(self.data[n]) + endflag
+
+
+    def insert(self, index, chars):
+        "Insert chars before the character at index."
+
+        if not chars:  # ''.splitlines() is [], not ['']
+            return
+        chars = chars.splitlines(True)
+        if chars[-1][-1] == '\n':
+            chars.append('')
+        line, char = self._decode(index, -1)
+        before = self.data[line][:char]
+        after = self.data[line][char:]
+        self.data[line] = before + chars[0]
+        self.data[line+1:line+1] = chars[1:]
+        self.data[line+len(chars)-1] += after
+
+
+    def get(self, index1, index2=None):
+        "Return slice from index1 to index2 (default is 'index1+1')."
+
+        startline, startchar = self._decode(index1)
+        if index2 is None:
+            endline, endchar = startline, startchar+1
+        else:
+            endline, endchar = self._decode(index2)
+
+        if startline == endline:
+            return self.data[startline][startchar:endchar]
+        else:
+            lines = [self.data[startline][startchar:]]
+            for i in range(startline+1, endline):
+                lines.append(self.data[i])
+            lines.append(self.data[endline][:endchar])
+            return ''.join(lines)
+
+
+    def delete(self, index1, index2=None):
+        '''Delete slice from index1 to index2 (default is 'index1+1').
+
+        Adjust default index2 ('index+1) for line ends.
+        Do not delete the terminal \n at the very end of self.data ([-1][-1]).
+        '''
+        startline, startchar = self._decode(index1, -1)
+        if index2 is None:
+            if startchar < len(self.data[startline])-1:
+                # not deleting \n
+                endline, endchar = startline, startchar+1
+            elif startline < len(self.data) - 1:
+                # deleting non-terminal \n, convert 'index1+1 to start of next line
+                endline, endchar = startline+1, 0
+            else:
+                # do not delete terminal \n if index1 == 'insert'
+               return
+        else:
+            endline, endchar = self._decode(index2, -1)
+            # restricting end position to insert position excludes terminal \n
+
+        if startline == endline and startchar < endchar:
+            self.data[startline] = self.data[startline][:startchar] + \
+                                             self.data[startline][endchar:]
+        elif startline < endline:
+                self.data[startline] = self.data[startline][:startchar] + \
+                                       self.data[endline][endchar:]
+                startline += 1
+                for i in range(startline, endline+1):
+                    del self.data[startline]
+
+    def compare(self, index1, op, index2):
+        line1, char1 = self._decode(index1)
+        line2, char2 = self._decode(index2)
+        if op == '<':
+            return line1 < line2 or line1 == line2 and char1 < char2
+        elif op == '<=':
+            return line1 < line2 or line1 == line2 and char1 <= char2
+        elif op == '>':
+            return line1 > line2 or line1 == line2 and char1 > char2
+        elif op == '>=':
+            return line1 > line2 or line1 == line2 and char1 >= char2
+        elif op == '==':
+            return line1 == line2 and char1 == char2
+        elif op == '!=':
+            return line1 != line2 or  char1 != char2
+        else:
+            raise TclError('''bad comparison operator "%s":'''
+                                  '''must be <, <=, ==, >=, >, or !=''' % op)
+
+    # The following Text methods normally do something and return None.
+    # Whether doing nothing is sufficient for a test will depend on the test.
+
+    def mark_set(self, name, index):
+        "Set mark *name* before the character at index."
+        pass
+
+    def mark_unset(self, *markNames):
+        "Delete all marks in markNames."
+
+    def tag_remove(self, tagName, index1, index2=None):
+        "Remove tag tagName from all characters between index1 and index2."
+        pass
+
+    # The following Text methods affect the graphics screen and return None.
+    # Doing nothing should always be sufficient for tests.
+
+    def scan_dragto(self, x, y):
+        "Adjust the view of the text according to scan_mark"
+
+    def scan_mark(self, x, y):
+        "Remember the current X, Y coordinates."
+
+    def see(self, index):
+        "Scroll screen to make the character at INDEX is visible."
+        pass
+
+    #  The following is a Misc method inheritet by Text.
+    # It should properly go in a Misc mock, but is included here for now.
+
+    def bind(sequence=None, func=None, add=None):
+        "Bind to this widget at event sequence a call to function func."
+        pass
diff --git a/Lib/idlelib/idle_test/test_text.py b/Lib/idlelib/idle_test/test_text.py
new file mode 100644
index 00000000000..69419c1b0ca
--- /dev/null
+++ b/Lib/idlelib/idle_test/test_text.py
@@ -0,0 +1,228 @@
+# Test mock_tk.Text class against tkinter.Text class by running same tests with both.
+import unittest
+from test.support import requires
+
+from _tkinter import TclError
+import tkinter as tk
+
+class TextTest(object):
+
+    hw = 'hello\nworld'  # usual initial insert after initialization
+    hwn = hw+'\n'  # \n present at initialization, before insert
+
+    Text = None
+    def setUp(self):
+        self.text = self.Text()
+
+    def test_init(self):
+        self.assertEqual(self.text.get('1.0'), '\n')
+        self.assertEqual(self.text.get('end'), '')
+
+    def test_index_empty(self):
+        index = self.text.index
+
+        for dex in (-1.0, 0.3, '1.-1', '1.0', '1.0 lineend', '1.end', '1.33',
+                'insert'):
+            self.assertEqual(index(dex), '1.0')
+
+        for dex in 'end', 2.0, '2.1', '33.44':
+            self.assertEqual(index(dex), '2.0')
+
+    def test_index_data(self):
+        index = self.text.index
+        self.text.insert('1.0', self.hw)
+
+        for dex in -1.0, 0.3, '1.-1', '1.0':
+            self.assertEqual(index(dex), '1.0')
+
+        for dex in '1.0 lineend', '1.end', '1.33':
+             self.assertEqual(index(dex), '1.5')
+
+        for dex in 'end',  '33.44':
+            self.assertEqual(index(dex), '3.0')
+
+    def test_get(self):
+        get = self.text.get
+        Equal = self.assertEqual
+        self.text.insert('1.0', self.hw)
+
+        Equal(get('end'), '')
+        Equal(get('end', 'end'), '')
+        Equal(get('1.0'), 'h')
+        Equal(get('1.0', '1.1'), 'h')
+        Equal(get('1.0', '1.3'), 'hel')
+        Equal(get('1.1', '1.3'), 'el')
+        Equal(get('1.0', '1.0 lineend'), 'hello')
+        Equal(get('1.0', '1.10'), 'hello')
+        Equal(get('1.0 lineend'), '\n')
+        Equal(get('1.1', '2.3'), 'ello\nwor')
+        Equal(get('1.0', '2.5'), self.hw)
+        Equal(get('1.0', 'end'), self.hwn)
+        Equal(get('0.0', '5.0'), self.hwn)
+
+    def test_insert(self):
+        insert = self.text.insert
+        get = self.text.get
+        Equal = self.assertEqual
+
+        insert('1.0', self.hw)
+        Equal(get('1.0', 'end'), self.hwn)
+
+        insert('1.0', '')  # nothing
+        Equal(get('1.0', 'end'), self.hwn)
+
+        insert('1.0', '*')
+        Equal(get('1.0', 'end'), '*hello\nworld\n')
+
+        insert('1.0 lineend', '*')
+        Equal(get('1.0', 'end'), '*hello*\nworld\n')
+
+        insert('2.3', '*')
+        Equal(get('1.0', 'end'), '*hello*\nwor*ld\n')
+
+        insert('end', 'x')
+        Equal(get('1.0', 'end'), '*hello*\nwor*ldx\n')
+
+        insert('1.4', 'x\n')
+        Equal(get('1.0', 'end'), '*helx\nlo*\nwor*ldx\n')
+
+    def test_no_delete(self):
+        # if index1 == 'insert' or 'end' or >= end, there is no deletion
+        delete = self.text.delete
+        get = self.text.get
+        Equal = self.assertEqual
+        self.text.insert('1.0', self.hw)
+
+        delete('insert')
+        Equal(get('1.0', 'end'), self.hwn)
+
+        delete('end')
+        Equal(get('1.0', 'end'), self.hwn)
+
+        delete('insert', 'end')
+        Equal(get('1.0', 'end'), self.hwn)
+
+        delete('insert', '5.5')
+        Equal(get('1.0', 'end'), self.hwn)
+
+        delete('1.4', '1.0')
+        Equal(get('1.0', 'end'), self.hwn)
+
+        delete('1.4', '1.4')
+        Equal(get('1.0', 'end'), self.hwn)
+
+    def test_delete_char(self):
+        delete = self.text.delete
+        get = self.text.get
+        Equal = self.assertEqual
+        self.text.insert('1.0', self.hw)
+
+        delete('1.0')
+        Equal(get('1.0', '1.end'), 'ello')
+
+        delete('1.0', '1.1')
+        Equal(get('1.0', '1.end'), 'llo')
+
+        # delete \n and combine 2 lines into 1
+        delete('1.end')
+        Equal(get('1.0', '1.end'), 'lloworld')
+
+        self.text.insert('1.3', '\n')
+        delete('1.10')
+        Equal(get('1.0', '1.end'), 'lloworld')
+
+        self.text.insert('1.3', '\n')
+        delete('1.3', '2.0')
+        Equal(get('1.0', '1.end'), 'lloworld')
+
+    def test_delete_slice(self):
+        delete = self.text.delete
+        get = self.text.get
+        Equal = self.assertEqual
+        self.text.insert('1.0', self.hw)
+
+        delete('1.0', '1.0 lineend')
+        Equal(get('1.0', 'end'), '\nworld\n')
+
+        delete('1.0', 'end')
+        Equal(get('1.0', 'end'), '\n')
+
+        self.text.insert('1.0', self.hw)
+        delete('1.0', '2.0')
+        Equal(get('1.0', 'end'), 'world\n')
+
+        delete('1.0', 'end')
+        Equal(get('1.0', 'end'), '\n')
+
+        self.text.insert('1.0', self.hw)
+        delete('1.2', '2.3')
+        Equal(get('1.0', 'end'), 'held\n')
+
+    def test_multiple_lines(self):  # insert and delete
+        self.text.insert('1.0', 'hello')
+
+        self.text.insert('1.3', '1\n2\n3\n4\n5')
+        self.assertEqual(self.text.get('1.0', 'end'), 'hel1\n2\n3\n4\n5lo\n')
+
+        self.text.delete('1.3', '5.1')
+        self.assertEqual(self.text.get('1.0', 'end'), 'hello\n')
+
+    def test_compare(self):
+        compare = self.text.compare
+        Equal = self.assertEqual
+        # need data so indexes not squished to 1,0
+        self.text.insert('1.0', 'First\nSecond\nThird\n')
+
+        self.assertRaises(TclError, compare, '2.2', 'op', '2.2')
+
+        for op, less1, less0, equal, greater0, greater1 in (
+                ('<', True, True, False, False, False),
+                ('<=', True, True, True, False, False),
+                ('>', False, False, False, True, True),
+                ('>=', False, False, True, True, True),
+                ('==', False, False, True, False, False),
+                ('!=', True, True, False, True, True),
+                ):
+            Equal(compare('1.1', op, '2.2'), less1, op)
+            Equal(compare('2.1', op, '2.2'), less0, op)
+            Equal(compare('2.2', op, '2.2'), equal, op)
+            Equal(compare('2.3', op, '2.2'), greater0, op)
+            Equal(compare('3.3', op, '2.2'), greater1, op)
+
+
+class MockTextTest(TextTest, unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        from idlelib.idle_test.mock_tk import Text
+        cls.Text = Text
+
+    def test_decode(self):
+        # test endflags (-1, 0) not tested by test_index (which uses +1)
+        decode = self.text._decode
+        Equal = self.assertEqual
+        self.text.insert('1.0', self.hw)
+
+        Equal(decode('end', -1), (2, 5))
+        Equal(decode('3.1', -1), (2, 5))
+        Equal(decode('end',  0), (2, 6))
+        Equal(decode('3.1', 0), (2, 6))
+
+
+class TkTextTest(TextTest, unittest.TestCase):
+
+    @classmethod
+    def setUpClass(cls):
+        requires('gui')
+        from tkinter import Tk, Text
+        cls.Text = Text
+        cls.root = Tk()
+
+    @classmethod
+    def tearDownClass(cls):
+        cls.root.destroy()
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2, exit=False)
+