Add support for segment.expand

Closes #154

docs/source/configuration/reference.rst

@@ -369,6 +369,8 @@ ascii                       Theme without any unicode characters at all
 
     Each segment dictionary has the following options:
 
+    .. _config-themes-seg-type:
+
     ``type``
         The segment type. Can be one of ``function`` (default), ``string`` or 
         ``segments_list``:
@@ -439,9 +441,13 @@ ascii                       Theme without any unicode characters at all
     ``args``
         A dict of arguments to be passed to a ``function`` segment.
 
+    .. _config-themes-seg-align:
+
     ``align``
         Aligns the segments contents to the left (``l``), center (``c``) or 
-        right (``r``). Has no sense if ``width`` key was not specified.
+        right (``r``). Has no sense if ``width`` key was not specified or if 
+        segment provides its own function for ``auto`` ``width`` handling and 
+        does not care about this option.
 
     .. _config-themes-seg-width:
 
@@ -454,6 +460,8 @@ ascii                       Theme without any unicode characters at all
         either returned by a function or a static string, and the contents 
         can be aligned with the ``align`` property.
 
+    .. _config-themes-seg-priority:
+
     ``priority``
         Optional segment priority. Segments with priority ``None`` (the default 
         priority, represented by ``null`` in json) will always be included, 
@@ -482,6 +490,8 @@ ascii                       Theme without any unicode characters at all
         segments. Only applicable for functions returning multiple segments. 
         Defaults to ``False``.
 
+    .. _config-themes-seg-exclude_modes:
+
     ``exclude_modes``
         A list of modes where this segment will be excluded: The segment is 
         included in all modes, *except* for the modes in this list.

docs/source/develop/listers.rst

@@ -36,8 +36,11 @@ Listers must return a sequence of pairs. First item in the pair must contain
 a ``segment_info`` dictionary specific to one of the listed entities.
 
 Second item must contain another dictionary: it will be used to modify the 
-resulting segment. In addition to usual keys that describe segment the following 
+resulting segment. In addition to :ref:`usual keys that describe segment 
-keys may be present (it is advised that *only* the following keys will be used):
+<dev-segments-segment>` the following keys may be present (it is advised that 
+*only* the following keys will be used):
+
+.. _dev-listers-mode:
 
 ``mode``
     Segment-specific mode. Used to alter segment highlighting.

docs/source/develop/segments.rst

@@ -51,6 +51,8 @@ powerline:
     theme-specific values go directly to :ref:`top-level themes 
     <config-themes>`.
 
+.. _dev-segments-startup:
+
 ``startup``
     This attribute must be a callable which accepts the following keyword 
     arguments:
@@ -68,6 +70,8 @@ powerline:
     used (more specific: when :py:class:`powerline.Powerline` constructor 
     received true ``run_once`` argument).
 
+.. _dev-segments-shutdown:
+
 ``shutdown``
     This attribute must be a callable that accepts no arguments and shuts down 
     threads and frees any other resources allocated in ``startup`` method of the 
@@ -75,6 +79,29 @@ powerline:
 
     This function is not called when ``startup`` method is not called.
 
+.. _dev-segments-expand:
+
+``expand``
+    This attribute must be a callable that accepts the following keyword 
+    arguments:
+
+    * ``pl``: :py:class:`powerline.PowerlineLogger` instance which is to be used 
+      for logging.
+    * ``amount``: integer number representing amount of display cells result 
+      must occupy.
+
+      .. warning::
+         “Amount of display cells” is *not* number of Unicode codepoints, string 
+         length, or byte count. It is suggested that your function should look 
+         something like ``return (' ' * amount) + segment['contents']`` where 
+         ``' '`` may be replaced with anything that is known to occupy exactly 
+         one display cell.
+    * ``segment``: :ref:`segment dictionary <dev-segments-segment>`.
+    * Any arguments found in user configuration for the given segment (i.e. 
+      :ref:`args key <config-themes-seg-args>`).
+
+    It must return new value of :ref:`contents <dev-segments-seg-contents>` key.
+
 This callable object should may return either a string (``unicode`` in Python2 
 or ``str`` in Python3, *not* ``str`` in Python2 or ``bytes`` in Python3) object 
 or a list of dictionaries. String object is a short form of the following return 
@@ -87,6 +114,8 @@ value:
         'highlight_group': [segment_name],
     }]
 
+.. _dev-segments-return:
+
 Returned list is a list of segments treated independently, except for 
 :ref:`draw_inner_divider key <dev-segments-draw_inner_divider>`.
 
@@ -158,6 +187,86 @@ Detailed description of used dictionary keys:
     No error occurs if segment has this key, but no used highlight groups use 
     gradient color.
 
+``_*``
+    Keys starting with underscore are reserved for powerline and must not be 
+    returned.
+
+``__*``
+    Keys starting with two underscores are reserved for the segment functions, 
+    specifically for :ref:`expand function <dev-segments-expand>`.
+
+.. _dev-segments-segment:
+
+Segment dictionary
+==================
+
+Segment dictionary contains the following keys:
+
+* All keys returned by segment function (if it was used).
+
+* All of the following keys:
+
+  ``name``
+    Segment name: value of the :ref:`name key <config-themes-seg-name>` or 
+    function name (last component of the :ref:`function key 
+    <config-themes-seg-function>`). May be ``None``.
+
+  ``type``
+    :ref:`Segment type <config-themes-seg-type>`. Always represents actual type 
+    and is never ``None``.
+
+  ``highlight_group``, ``divider_highlight_group``
+    Used highlight groups. May be ``None``.
+
+  .. _dev-segments-seg-around:
+
+  ``before``, ``after``
+    Value of :ref:`before <config-themes-seg-before>` or :ref:`after 
+    <config-themes-seg-after>` configuration options. May be ``None`` as well as 
+    an empty string.
+
+  ``contents_func``
+    Function used to get segment contents. May be ``None``.
+
+  .. _dev-segments-seg-contents:
+
+  ``contents``
+    Actual segment contents, excluding dividers and :ref:`before/after 
+    <dev-segments-seg-around>`. May be ``None``.
+
+  ``priority``
+    :ref:`Segment priority <config-themes-seg-priority>`. May be ``None`` for no 
+    priority (such segments are always shown).
+
+  ``draw_soft_divider``, ``draw_hard_divider``, ``draw_inner_divider``
+    :ref:`Divider control flags <dev-segments-draw_inner_divider>`.
+
+  ``side``
+    Segment side: ``right`` or ``left``.
+
+  ``exclude_modes``, ``include_modes``
+    :ref:`Mode display control lists <config-themes-seg-exclude_modes>`. May be 
+    empty, but may not be ``None``.
+
+  ``width``, ``align``
+    :ref:`Width and align options <config-themes-seg-align>`. May be ``None``.
+
+  ``expand``
+    Partially applied :ref:`expand function <dev-segments-expand>`. Accepts 
+    ``pl``, ``amount`` and ``segment`` positional parameters, keyword parameters 
+    from :ref:`args <config-themes-seg-args>` key were applied.
+
+  ``startup``
+    Partially applied :ref:`startup function <dev-segments-startup>`. Accepts 
+    ``pl`` and ``shutdown_event`` positional parameters, keyword parameters from 
+    :ref:`args <config-themes-seg-args>` key were applied.
+
+  ``shutdown``
+    :ref:`Shutdown function <dev-segments-shutdown>`. Accepts no argument.
+
+  ``mode``
+    :ref:`Segment-specific mode <dev-listers-mode>`. May be ``None``.
+
 Segments layout
 ===============
 

powerline/segment.py

@@ -82,14 +82,20 @@ segment_getters = {
 }
 
 
-def get_attr_func(contents_func, key, args):
+def get_attr_func(contents_func, key, args, is_space_func=False):
 	try:
 		func = getattr(contents_func, key)
 	except AttributeError:
 		return None
 	else:
-		if args is None:
+		if is_space_func:
-			return lambda: func()
+			def expand_func(pl, amount, segment):
+				try:
+					return func(pl=pl, amount=amount, segment=segment, **args)
+				except Exception as e:
+					pl.exception('Exception while computing segment expansion: {0}', str(e))
+					return segment['contents'] + (' ' * amount)
+			return expand_func
 		else:
 			return lambda pl, shutdown_event: func(pl=pl, shutdown_event=shutdown_event, **args)
 
@@ -297,7 +303,8 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 
 		if segment_type == 'function':
 			startup_func = get_attr_func(_contents_func, 'startup', args)
-			shutdown_func = get_attr_func(_contents_func, 'shutdown', None)
+			shutdown_func = getattr(_contents_func, 'shutdown', None)
+			expand_func = get_attr_func(_contents_func, 'expand', args, True)
 
 			if hasattr(_contents_func, 'powerline_requires_filesystem_watcher'):
 				create_watcher = lambda: create_file_watcher(pl, common_config['watcher'])
@@ -311,6 +318,7 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 			startup_func = None
 			shutdown_func = None
 			contents_func = None
+			expand_func = None
 
 		return {
 			'name': name or function_name,
@@ -330,7 +338,7 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 			'include_modes': segment.get('include_modes', []),
 			'width': segment.get('width'),
 			'align': segment.get('align', 'l'),
-			'expand': None,
+			'expand': expand_func,
 			'startup': startup_func,
 			'shutdown': shutdown_func,
 			'mode': None,

powerline/theme.py

@@ -150,7 +150,7 @@ class Theme(object):
 			for segment in parsed_segments:
 				width = segment['width']
 				align = segment['align']
-				if width == 'auto':
+				if width == 'auto' and segment['expand'] is None:
 					segment['expand'] = expand_functions.get(align)
 					if segment['expand'] is None:
 						self.pl.error('Align argument must be “r”, “l” or “c”, not “{0}”', align)

tests/test_configuration.py

@@ -474,6 +474,26 @@ class TestSegmentAttributes(TestRender):
 		}
 		self.assertRenderEqual(p, '{56} pl;{6-}>>{--}')
 
+	@add_args
+	def test_expand(self, p, config):
+		def m1(divider=',', **kwargs):
+			return divider.join(kwargs.keys()) + divider
+
+		def expand(pl, amount, segment, **kwargs):
+			return ('-' * amount) + segment['contents']
+
+		m1.expand = expand
+		sys.modules['bar'] = Args(m1=m1)
+		config['themes/test/default']['segments'] = {
+			'left': [
+				{
+					'function': 'bar.m1',
+					'width': 'auto'
+				}
+			]
+		}
+		self.assertRenderEqual(p, '{56} ----pl,{6-}>>{--}', width=10)
+
 
 class TestSegmentData(TestRender):
 	@add_args