Add support for append/replace mode after discussion with Evan.

Also increase test coverage.

IPython/core/blockbreaker.py

@@ -91,16 +91,35 b' class BlockBreaker(object):'
     code = None
     # Boolean indicating whether the current block is complete
     is_complete = None
+    # Input mode
+    input_mode = 'append'
 
     # Private attributes
     
     # List
     _buffer = None
     
-    def __init__(self):
+    def __init__(self, input_mode=None):
+        """Create a new BlockBreaker instance.
+
+        Parameters
+        ----------
+        input_mode : str
+
+          One of 'append', 'replace', default is 'append'.  This controls how
+          new inputs are used: in 'append' mode, they are appended to the
+          existing buffer and the whole buffer is compiled; in 'replace' mode,
+          each new input completely replaces all prior inputs.  Replace mode is
+          thus equivalent to prepending a full reset() to every push() call.
+
+          In practice, line-oriented clients likely want to use 'append' mode
+          while block-oriented ones will want to use 'replace'.
+        """
         self._buffer = []
         self.compile = codeop.CommandCompiler()
         self.encoding = get_input_encoding()
+        self.input_mode = BlockBreaker.input_mode if input_mode is None \
+                          else input_mode
 
     def reset(self):
         """Reset the input buffer and associated state."""
@@ -143,6 +162,9 b' class BlockBreaker(object):'
         this value is also stored as an attribute so it can be queried at any
         time.
         """
+        if self.input_mode == 'replace':
+            self.reset()
+        
         # If the source code has leading blanks, add 'if 1:\n' to it
         # this allows execution of indented pasted code. It is tempting
         # to add '\n' at the end of source to run commands like ' a=1'

IPython/core/tests/test_blockbreaker.py

@@ -137,6 +137,14 b' class BlockBreakerTestCase(unittest.TestCase):'
         bb.push('  y=2')
         self.assertEqual(bb.source, 'if 1:\n  x=1\n  y=2\n')
 
+    def test_replace_mode(self):
+        bb = self.bb
+        bb.input_mode = 'replace'
+        bb.push('x=1')
+        self.assertEqual(bb.source, 'x=1\n')
+        bb.push('x=2')
+        self.assertEqual(bb.source, 'x=2\n')
+
     def test_interactive_block_ready(self):
         bb = self.bb
         bb.push('x=1')
@@ -144,7 +152,9 b' class BlockBreakerTestCase(unittest.TestCase):'
 
     def test_interactive_block_ready2(self):
         bb = self.bb
-        bb.push('if 1:\n  x=1')
+        bb.push('if 1:')
+        self.assertFalse(bb.interactive_block_ready())
+        bb.push('  x=1')
         self.assertFalse(bb.interactive_block_ready())
         bb.push('')
         self.assertTrue(bb.interactive_block_ready())