Index: packages/Python/lldbsuite/test/functionalities/command_script_immediate_output/TestCommandScriptImmediateOutput.py
===================================================================
--- packages/Python/lldbsuite/test/functionalities/command_script_immediate_output/TestCommandScriptImmediateOutput.py
+++ packages/Python/lldbsuite/test/functionalities/command_script_immediate_output/TestCommandScriptImmediateOutput.py
@@ -26,7 +26,7 @@
     @expectedFailureAll(oslist=["freebsd","linux"], bugnumber="llvm.org/pr26139")
     def test_command_script_immediate_output (self):
         """Test that LLDB correctly allows scripted commands to set an immediate output file."""
-        self.launch(timeout=5)
+        self.launch(timeout=60)
 
         script = os.path.join(os.getcwd(), 'custom_command.py')
         prompt = "(lldb)"
@@ -35,4 +35,38 @@
         self.sendline('command script add -f custom_command.command_function mycommand', patterns=[prompt])
         self.sendline('mycommand', patterns='this is a test string, just a test string')
         self.sendline('command script delete mycommand', patterns=[prompt])
+
+        test_files = {os.path.join(os.getcwd(), 'read.txt')        :'r',
+                      os.path.join(os.getcwd(), 'write.txt')       :'w',
+                      os.path.join(os.getcwd(), 'append.txt')      :'a',
+                      os.path.join(os.getcwd(), 'write_plus.txt')  :'w+',
+                      os.path.join(os.getcwd(), 'read_plus.txt')   :'r+',
+                      os.path.join(os.getcwd(), 'append_plus.txt') :'a+'}
+
+        starter_string = 'Starter Garbage\n'
+        write_string   = 'writing to file with mode: '
+
+        for path, mode in test_files.iteritems():
+            with open(path, 'w+') as init:
+                init.write(starter_string)
+
+        self.sendline('command script add -f custom_command.write_file mywrite', patterns=[prompt])
+        for path, mode in test_files.iteritems():
+            command = 'mywrite ' + path + ' ' + mode
+
+            self.sendline(command, patterns=[prompt])
+
+        self.sendline('command script delete mywrite', patterns=[prompt])
+
         self.quit(gracefully=False)
+
+        for path, mode in test_files.iteritems():
+            with open(path, 'r') as result:
+                if mode in ['r', 'a', 'a+']:
+                    self.assertEquals(result.readline(), starter_string)
+                if mode in ['w', 'w+', 'r+', 'a', 'a+']:
+                    self.assertEquals(result.readline(), write_string + mode + '\n')
+
+            self.assertTrue(os.path.isfile(path))
+            os.remove(path)
+
Index: packages/Python/lldbsuite/test/functionalities/command_script_immediate_output/custom_command.py
===================================================================
--- packages/Python/lldbsuite/test/functionalities/command_script_immediate_output/custom_command.py
+++ packages/Python/lldbsuite/test/functionalities/command_script_immediate_output/custom_command.py
@@ -6,3 +6,12 @@
         result.SetImmediateOutputFile(sys.__stdout__)
         print('this is a test string, just a test string', file=result)
 
+def write_file(debugger, command, exe_ctx, result, internal_dict):
+        args = command.split(' ')
+        path = args[0]
+        mode = args[1]
+
+        with open(path, mode) as f:
+            result.SetImmediateOutputFile(f)
+            if not mode in ['r']:
+                print('writing to file with mode: ' + mode, file=result)
Index: source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp
===================================================================
--- source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp
+++ source/Plugins/ScriptInterpreter/Python/PythonDataObjects.cpp
@@ -1252,10 +1252,10 @@
     return llvm::StringSwitch<uint32_t>(mode.str().c_str())
     .Case("r",   File::eOpenOptionRead)
     .Case("w",   File::eOpenOptionWrite)
-    .Case("a",   File::eOpenOptionAppend|File::eOpenOptionCanCreate)
+    .Case("a",   File::eOpenOptionWrite|File::eOpenOptionAppend|File::eOpenOptionCanCreate)
     .Case("r+",  File::eOpenOptionRead|File::eOpenOptionWrite)
     .Case("w+",  File::eOpenOptionRead|File::eOpenOptionWrite|File::eOpenOptionCanCreate|File::eOpenOptionTruncate)
-    .Case("a+",  File::eOpenOptionRead|File::eOpenOptionWrite|File::eOpenOptionCanCreate)
+    .Case("a+",  File::eOpenOptionRead|File::eOpenOptionWrite|File::eOpenOptionAppend|File::eOpenOptionCanCreate)
     .Default(0);
 }