allow IPython to run without writable home dir

get_ipython_dir() ensures that the *IPython* dir is writable, which is more relevant, but the home dir need not be writable. Some optional behaviors (e.g. `%logstart global`) will not work if the home dir is not writable, but IPython should not crash.  Approximately no other operations actually depend on writing directly to $HOME.

IPython/core/interactiveshell.py

@@ -552,10 +552,7 b' class InteractiveShell(SingletonConfigurable, Magic):'
 
     def init_pushd_popd_magic(self):
         # for pushd/popd management
-        try:
-            self.home_dir = get_home_dir()
-        except HomeDirError, msg:
-            fatal(msg)
+        self.home_dir = get_home_dir()
 
         self.dir_stack = []
 
@@ -1751,12 +1748,10 b' class InteractiveShell(SingletonConfigurable, Magic):'
             # Or if libedit is used, load editrc.
             inputrc_name = os.environ.get('INPUTRC')
             if inputrc_name is None:
-                home_dir = get_home_dir()
-                if home_dir is not None:
-                    inputrc_name = '.inputrc'
-                    if readline.uses_libedit:
-                        inputrc_name = '.editrc'
-                    inputrc_name = os.path.join(home_dir, inputrc_name)
+                inputrc_name = '.inputrc'
+                if readline.uses_libedit:
+                    inputrc_name = '.editrc'
+                inputrc_name = os.path.join(self.home_dir, inputrc_name)
             if os.path.isfile(inputrc_name):
                 try:
                     readline.read_init_file(inputrc_name)

IPython/utils/path.py

@@ -167,15 +167,24 b' class HomeDirError(Exception):'
     pass
 
 
-def get_home_dir():
-    """Return the closest possible equivalent to a 'home' directory.
+def get_home_dir(require_writable=False):
+    """Return the 'home' directory, as a unicode string.
 
     * First, check for frozen env in case of py2exe
-    * Otherwise, defer to os.path.expanduser('~'), ensuring unicode
+    * Otherwise, defer to os.path.expanduser('~')
     
     See stdlib docs for how this is determined.
-    
     $HOME is first priority on *ALL* platforms.
+    
+    Parameters
+    ----------
+    
+    require_writable : bool [default: False]
+        if True:
+            guarantees the return value is a writable directory, otherwise
+            raises HomeDirError
+        if False:
+            The path is resolved, but it is not guaranteed to exist or be writable.
     """
 
     # first, check py2exe distribution root directory for _ipython.
@@ -192,10 +201,10 b' def get_home_dir():'
         return py3compat.cast_unicode(root, fs_encoding)
     
     homedir = os.path.expanduser('~')
-    if _writable_dir(homedir):
+    if (not require_writable) or _writable_dir(homedir):
         return py3compat.cast_unicode(homedir, fs_encoding)
     else:
-        raise HomeDirError('%s not a writable dir, set $HOME env to override' % homedir)
+        raise HomeDirError('%s is not a writable dir, set $HOME env to override' % homedir)
 
 def get_xdg_dir():
     """Return the XDG_CONFIG_HOME, if it is defined and exists, else None.
@@ -207,7 +216,7 b' def get_xdg_dir():'
 
     if os.name == 'posix':
         # Linux, Unix, AIX, OS X
-        # use ~/.config if not set OR empty
+        # use ~/.config if empty OR not set
         xdg = env.get("XDG_CONFIG_HOME", None) or os.path.join(get_home_dir(), '.config')
         if xdg and _writable_dir(xdg):
             return py3compat.cast_unicode(xdg, fs_encoding)
@@ -231,6 +240,7 b' def get_ipython_dir():'
 
     home_dir = get_home_dir()
     xdg_dir = get_xdg_dir()
+    
     # import pdb; pdb.set_trace()  # dbg
     ipdir = env.get('IPYTHON_DIR', env.get('IPYTHONDIR', None))
     if ipdir is None:

IPython/utils/tests/test_path.py

@@ -141,7 +141,7 b' def test_get_home_dir_2():'
     #fake filename for IPython.__init__
     IPython.__file__ = abspath(join(HOME_TEST_DIR, "Library.zip/IPython/__init__.py")).lower()
 
-    home_dir = path.get_home_dir()
+    home_dir = path.get_home_dir(True)
     nt.assert_equal(home_dir, abspath(HOME_TEST_DIR).lower())
 
 
@@ -149,7 +149,7 b' def test_get_home_dir_2():'
 def test_get_home_dir_3():
     """get_home_dir() uses $HOME if set"""
     env["HOME"] = HOME_TEST_DIR
-    home_dir = path.get_home_dir()
+    home_dir = path.get_home_dir(True)
     nt.assert_equal(home_dir, env["HOME"])
 
 
@@ -159,14 +159,14 b' def test_get_home_dir_4():'
 
     if 'HOME' in env: del env['HOME']
     # this should still succeed, but we don't know what the answer should be
-    home = path.get_home_dir()
+    home = path.get_home_dir(True)
     nt.assert_true(path._writable_dir(home))
 
 @with_environment
 def test_get_home_dir_5():
     """raise HomeDirError if $HOME is specified, but not a writable dir"""
     env['HOME'] = abspath(HOME_TEST_DIR+'garbage')
-    nt.assert_raises(path.HomeDirError, path.get_home_dir)
+    nt.assert_raises(path.HomeDirError, path.get_home_dir, True)
 
 @with_environment
 def test_get_ipython_dir_1():