Implement segment truncation

Closes #161 (requires implementation for specific segments though)

docs/source/develop/segments.rst

@@ -102,6 +102,16 @@ powerline:
 
     It must return new value of :ref:`contents <dev-segments-seg-contents>` key.
 
+.. _dev-segments-truncate:
+
+``truncate``
+    Like :ref:`expand function <dev-segments-expand>`, but for truncating 
+    segments. Here ``amount`` means the number of display cells which must be 
+    freed.
+
+    This function is called for all segments before powerline starts purging 
+    them to free space.
+
 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 
@@ -251,10 +261,11 @@ Segment dictionary contains the following keys:
   ``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.
+  ``expand``, ``truncate``
+    Partially applied :ref:`expand <dev-segments-expand>` or :ref:`truncate 
+    <dev-segments-truncate>` function. 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 

powerline/renderer.py

@@ -1,9 +1,12 @@
 # vim:fileencoding=utf-8:noet
 
-from powerline.theme import Theme
-from unicodedata import east_asian_width, combining
 import os
 
+from unicodedata import east_asian_width, combining
+from itertools import chain
+
+from powerline.theme import Theme
+
 try:
 	NBSP = unicode(' ', 'utf-8')
 except NameError:
@@ -262,11 +265,17 @@ class Renderer(object):
 
 		# Create an ordered list of segments that can be dropped
 		segments_priority = sorted((segment for segment in segments if segment['priority'] is not None), key=lambda segment: segment['priority'], reverse=True)
-		for segment in segments_priority:
-			current_width = self._render_length(theme, segments, divider_widths)
-			if current_width <= width:
-				break
-			segments.remove(segment)
+		no_priority_segments = filter(lambda segment: segment['priority'] is None, segments)
+		current_width = self._render_length(theme, segments, divider_widths)
+		if current_width > width:
+			for segment in chain(segments_priority, no_priority_segments):
+				if segment['truncate'] is not None:
+					segment['contents'] = segment['truncate'](self.pl, current_width - width, segment)
+			for segment in segments_priority:
+				if current_width <= width:
+					break
+				segments.remove(segment)
+				current_width = self._render_length(theme, segments, divider_widths)
 
 		# Distribute the remaining space on spacer segments
 		segments_spacers = [segment for segment in segments if segment['expand'] is not None]

powerline/segment.py

@@ -93,7 +93,7 @@ def get_attr_func(contents_func, key, args, is_space_func=False):
 				try:
 					return func(pl=pl, amount=amount, segment=segment, **args)
 				except Exception as e:
-					pl.exception('Exception while computing segment expansion: {0}', str(e))
+					pl.exception('Exception while computing {0} function: {1}', key, str(e))
 					return segment['contents'] + (' ' * amount)
 			return expand_func
 		else:
@@ -292,6 +292,7 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 				'width': None,
 				'align': None,
 				'expand': None,
+				'truncate': None,
 				'startup': None,
 				'shutdown': None,
 				'mode': None,
@@ -305,6 +306,7 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 			startup_func = get_attr_func(_contents_func, 'startup', args)
 			shutdown_func = getattr(_contents_func, 'shutdown', None)
 			expand_func = get_attr_func(_contents_func, 'expand', args, True)
+			truncate_func = get_attr_func(_contents_func, 'truncate', args, True)
 
 			if hasattr(_contents_func, 'powerline_requires_filesystem_watcher'):
 				create_watcher = lambda: create_file_watcher(pl, common_config['watcher'])
@@ -319,6 +321,7 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 			shutdown_func = None
 			contents_func = None
 			expand_func = None
+			truncate_func = None
 
 		return {
 			'name': name or function_name,
@@ -339,6 +342,7 @@ def gen_segment_getter(pl, ext, common_config, theme_configs, default_module, ge
 			'width': segment.get('width'),
 			'align': segment.get('align', 'l'),
 			'expand': expand_func,
+			'truncate': truncate_func,
 			'startup': startup_func,
 			'shutdown': shutdown_func,
 			'mode': None,

tests/test_configuration.py

@@ -494,6 +494,25 @@ class TestSegmentAttributes(TestRender):
 		}
 		self.assertRenderEqual(p, '{56} ----pl,{6-}>>{--}', width=10)
 
+	@add_args
+	def test_truncate(self, p, config):
+		def m1(divider=',', **kwargs):
+			return divider.join(kwargs.keys()) + divider
+
+		def truncate(pl, amount, segment, **kwargs):
+			return segment['contents'][:-amount]
+
+		m1.truncate = truncate
+		sys.modules['bar'] = Args(m1=m1)
+		config['themes/test/default']['segments'] = {
+			'left': [
+				{
+					'function': 'bar.m1'
+				}
+			]
+		}
+		self.assertRenderEqual(p, '{56} p{6-}>>{--}', width=4)
+
 
 class TestSegmentData(TestRender):
 	@add_args