Do not update the unders if they are user defined

Closes jupyter/notebook#1628 (once there are tests).

Still shift the internal reference to self._,__,___ but do not assign it
in user ns.

I would expect this to be a change of behavior and have the (slight)
side effect that if any of the _, __, or __ are defined, then none of
these are updated.

We could update the logic to do the right think on leapfrog _, __ or __
if user-set, but is it worth it.

If _ is in builtins, then _ will not update the history and _ will be
the builtins, unless user set it explicitly in which case it takes
precedence.

Summary
    if user have set _
       _ == value set by the user
    elif _ in builtins:
        _ == value in builtins._
    else:
        _ == previous result.

Note that this logic may fall down if the use set _ to a specific value,
and have this save value returned while _ is also in builtin:

    In [1]: import gettext ; gettext.install('foo')

    In [2]: _ = 'Example'

    In [3]: _
    Out[3]: 'Example'

    In [4]: _
    Out[4]: 'Example'

    In [5]: _
    Out[5]: <bound method NullTranslations.gettext of <...>>

IPython/core/displayhook.py

@@ -1,295 +1,308 b''
 # -*- coding: utf-8 -*-
 """Displayhook for IPython.
 
 This defines a callable class that IPython uses for `sys.displayhook`.
 """
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 from __future__ import print_function
 
 import sys
 import io as _io
 import tokenize
 
 from traitlets.config.configurable import Configurable
 from IPython.utils.py3compat import builtin_mod, cast_unicode_py2
 from traitlets import Instance, Float
 from warnings import warn
 
 # TODO: Move the various attributes (cache_size, [others now moved]). Some
 # of these are also attributes of InteractiveShell. They should be on ONE object
 # only and the other objects should ask that one object for their values.
 
 class DisplayHook(Configurable):
     """The custom IPython displayhook to replace sys.displayhook.
 
     This class does many things, but the basic idea is that it is a callable
     that gets called anytime user code returns a value.
     """
 
     shell = Instance('IPython.core.interactiveshell.InteractiveShellABC',
                      allow_none=True)
     exec_result = Instance('IPython.core.interactiveshell.ExecutionResult',
                            allow_none=True)
     cull_fraction = Float(0.2)
 
     def __init__(self, shell=None, cache_size=1000, **kwargs):
         super(DisplayHook, self).__init__(shell=shell, **kwargs)
         cache_size_min = 3
         if cache_size <= 0:
             self.do_full_cache = 0
             cache_size = 0
         elif cache_size < cache_size_min:
             self.do_full_cache = 0
             cache_size = 0
             warn('caching was disabled (min value for cache size is %s).' %
                  cache_size_min,level=3)
         else:
             self.do_full_cache = 1
 
         self.cache_size = cache_size
 
         # we need a reference to the user-level namespace
         self.shell = shell
         
         self._,self.__,self.___ = '','',''
 
         # these are deliberately global:
         to_user_ns = {'_':self._,'__':self.__,'___':self.___}
         self.shell.user_ns.update(to_user_ns)
 
     @property
     def prompt_count(self):
         return self.shell.execution_count
 
     #-------------------------------------------------------------------------
     # Methods used in __call__. Override these methods to modify the behavior
     # of the displayhook.
     #-------------------------------------------------------------------------
 
     def check_for_underscore(self):
         """Check if the user has set the '_' variable by hand."""
         # If something injected a '_' variable in __builtin__, delete
         # ipython's automatic one so we don't clobber that.  gettext() in
         # particular uses _, so we need to stay away from it.
         if '_' in builtin_mod.__dict__:
             try:
+                user_value = self.shell.user_ns['_']
+                if user_value is not self._:
+                    return
                 del self.shell.user_ns['_']
             except KeyError:
                 pass
 
     def quiet(self):
         """Should we silence the display hook because of ';'?"""
         # do not print output if input ends in ';'
         
         try:
             cell = cast_unicode_py2(self.shell.history_manager.input_hist_parsed[-1])
         except IndexError:
             # some uses of ipshellembed may fail here
             return False
         
         sio = _io.StringIO(cell)
         tokens = list(tokenize.generate_tokens(sio.readline))
 
         for token in reversed(tokens):
             if token[0] in (tokenize.ENDMARKER, tokenize.NL, tokenize.NEWLINE, tokenize.COMMENT):
                 continue
             if (token[0] == tokenize.OP) and (token[1] == ';'):
                 return True
             else:
                 return False
 
     def start_displayhook(self):
         """Start the displayhook, initializing resources."""
         pass
 
     def write_output_prompt(self):
         """Write the output prompt.
 
         The default implementation simply writes the prompt to
         ``sys.stdout``.
         """
         # Use write, not print which adds an extra space.
         sys.stdout.write(self.shell.separate_out)
         outprompt = 'Out[{}]: '.format(self.shell.execution_count)
         if self.do_full_cache:
             sys.stdout.write(outprompt)
 
     def compute_format_data(self, result):
         """Compute format data of the object to be displayed.
 
         The format data is a generalization of the :func:`repr` of an object.
         In the default implementation the format data is a :class:`dict` of
         key value pair where the keys are valid MIME types and the values
         are JSON'able data structure containing the raw data for that MIME
         type. It is up to frontends to determine pick a MIME to to use and
         display that data in an appropriate manner.
 
         This method only computes the format data for the object and should
         NOT actually print or write that to a stream.
 
         Parameters
         ----------
         result : object
             The Python object passed to the display hook, whose format will be
             computed.
 
         Returns
         -------
         (format_dict, md_dict) : dict
             format_dict is a :class:`dict` whose keys are valid MIME types and values are
             JSON'able raw data for that MIME type. It is recommended that
             all return values of this should always include the "text/plain"
             MIME type representation of the object.
             md_dict is a :class:`dict` with the same MIME type keys
             of metadata associated with each output.
             
         """
         return self.shell.display_formatter.format(result)
 
     # This can be set to True by the write_output_prompt method in a subclass
     prompt_end_newline = False
 
     def write_format_data(self, format_dict, md_dict=None):
         """Write the format data dict to the frontend.
 
         This default version of this method simply writes the plain text
         representation of the object to ``sys.stdout``. Subclasses should
         override this method to send the entire `format_dict` to the
         frontends.
 
         Parameters
         ----------
         format_dict : dict
             The format dict for the object passed to `sys.displayhook`.
         md_dict : dict (optional)
             The metadata dict to be associated with the display data.
         """
         if 'text/plain' not in format_dict:
             # nothing to do
             return
         # We want to print because we want to always make sure we have a
         # newline, even if all the prompt separators are ''. This is the
         # standard IPython behavior.
         result_repr = format_dict['text/plain']
         if '\n' in result_repr:
             # So that multi-line strings line up with the left column of
             # the screen, instead of having the output prompt mess up
             # their first line.
             # We use the prompt template instead of the expanded prompt
             # because the expansion may add ANSI escapes that will interfere
             # with our ability to determine whether or not we should add
             # a newline.
             if not self.prompt_end_newline:
                 # But avoid extraneous empty lines.
                 result_repr = '\n' + result_repr
 
         print(result_repr)
 
     def update_user_ns(self, result):
         """Update user_ns with various things like _, __, _1, etc."""
 
         # Avoid recursive reference when displaying _oh/Out
         if result is not self.shell.user_ns['_oh']:
             if len(self.shell.user_ns['_oh']) >= self.cache_size and self.do_full_cache:
                 self.cull_cache()
-            # Don't overwrite '_' and friends if '_' is in __builtin__ (otherwise
-            # we cause buggy behavior for things like gettext).
 
-            if '_' not in builtin_mod.__dict__:
-                self.___ = self.__
-                self.__ = self._
-                self._ = result
+            # Don't overwrite '_' and friends if '_' is in __builtin__
+            # (otherwise we cause buggy behavior for things like gettext). and
+            # do not overwrite _, __ or ___ if one of these has been assigned
+            # by the user.
+            update_unders = True
+            for unders in ['_'*i for i in range(1,4)]:
+                if not unders in self.shell.user_ns:
+                    continue
+                if getattr(self, unders) is not self.shell.user_ns.get(unders):
+                    update_unders = False
+
+            self.___ = self.__
+            self.__ = self._
+            self._ = result
+
+            if ('_' not in builtin_mod.__dict__) and (update_unders):
                 self.shell.push({'_':self._,
                                  '__':self.__,
                                 '___':self.___}, interactive=False)
 
             # hackish access to top-level  namespace to create _1,_2... dynamically
             to_main = {}
             if self.do_full_cache:
-                new_result = '_'+repr(self.prompt_count)
+                new_result = '_%s' % self.prompt_count
                 to_main[new_result] = result
                 self.shell.push(to_main, interactive=False)
                 self.shell.user_ns['_oh'][self.prompt_count] = result
 
     def fill_exec_result(self, result):
         if self.exec_result is not None:
             self.exec_result.result = result
 
     def log_output(self, format_dict):
         """Log the output."""
         if 'text/plain' not in format_dict:
             # nothing to do
             return
         if self.shell.logger.log_output:
             self.shell.logger.log_write(format_dict['text/plain'], 'output')
         self.shell.history_manager.output_hist_reprs[self.prompt_count] = \
                                                     format_dict['text/plain']
 
     def finish_displayhook(self):
         """Finish up all displayhook activities."""
         sys.stdout.write(self.shell.separate_out2)
         sys.stdout.flush()
 
     def __call__(self, result=None):
         """Printing with history cache management.
 
         This is invoked everytime the interpreter needs to print, and is
         activated by setting the variable sys.displayhook to it.
         """
         self.check_for_underscore()
         if result is not None and not self.quiet():
             self.start_displayhook()
             self.write_output_prompt()
             format_dict, md_dict = self.compute_format_data(result)
             self.update_user_ns(result)
             self.fill_exec_result(result)
             if format_dict:
                 self.write_format_data(format_dict, md_dict)
                 self.log_output(format_dict)
             self.finish_displayhook()
 
     def cull_cache(self):
         """Output cache is full, cull the oldest entries"""
         oh = self.shell.user_ns.get('_oh', {})
         sz = len(oh)
         cull_count = max(int(sz * self.cull_fraction), 2)
         warn('Output cache limit (currently {sz} entries) hit.\n'
              'Flushing oldest {cull_count} entries.'.format(sz=sz, cull_count=cull_count))
         
         for i, n in enumerate(sorted(oh)):
             if i >= cull_count:
                 break
             self.shell.user_ns.pop('_%i' % n, None)
             oh.pop(n, None)
         
 
     def flush(self):
         if not self.do_full_cache:
             raise ValueError("You shouldn't have reached the cache flush "
                              "if full caching is not enabled!")
         # delete auto-generated vars from global namespace
 
         for n in range(1,self.prompt_count + 1):
             key = '_'+repr(n)
             try:
                 del self.shell.user_ns[key]
             except: pass
         # In some embedded circumstances, the user_ns doesn't have the
         # '_oh' key set up.
         oh = self.shell.user_ns.get('_oh', None)
         if oh is not None:
             oh.clear()
 
         # Release our own references to objects:
         self._, self.__, self.___ = '', '', ''
 
         if '_' not in builtin_mod.__dict__:
             self.shell.user_ns.update({'_':None,'__':None, '___':None})
         import gc
         # TODO: Is this really needed?
         # IronPython blocks here forever
         if sys.platform != "cli":
             gc.collect()

IPython/core/tests/test_displayhook.py

@@ -1,28 +1,55 b''
 from IPython.testing.tools import AssertPrints, AssertNotPrints
 
 ip = get_ipython()
 
 def test_output_displayed():
     """Checking to make sure that output is displayed"""
   
     with AssertPrints('2'):
        ip.run_cell('1+1', store_history=True)
       
     with AssertPrints('2'):
         ip.run_cell('1+1 # comment with a semicolon;', store_history=True)
 
     with AssertPrints('2'):
         ip.run_cell('1+1\n#commented_out_function();', store_history=True)
 
       
 def test_output_quiet():
     """Checking to make sure that output is quiet"""
   
     with AssertNotPrints('2'):
         ip.run_cell('1+1;', store_history=True)
       
     with AssertNotPrints('2'):
         ip.run_cell('1+1; # comment with a semicolon', store_history=True)
 
     with AssertNotPrints('2'):
         ip.run_cell('1+1;\n#commented_out_function()', store_history=True)
+
+def test_underscore_no_overrite_user():
+    ip.run_cell('_ = 42', store_history=True)
+    ip.run_cell('1+1', store_history=True)
+
+    with AssertPrints('42'):
+        ip.run_cell('print(_)', store_history=True)
+
+    ip.run_cell('del _', store_history=True)
+    ip.run_cell('6+6', store_history=True)
+    with AssertPrints('12'):
+        ip.run_cell('_', store_history=True)
+
+
+def test_underscore_no_overrite_builtins():
+    ip.run_cell("import gettext ; gettext.install('foo')", store_history=True)
+    ip.run_cell('3+3', store_history=True)
+
+    with AssertPrints('gettext'):
+        ip.run_cell('print(_)', store_history=True)
+
+    ip.run_cell('_ = "userset"', store_history=True)
+
+    with AssertPrints('userset'):
+        ip.run_cell('print(_)', store_history=True)
+    ip.run_cell('import builtins; del builtins._')
+