Don't compute length of ZeroWidthTokens.

This simplify subclassing of prompt that inject escape sequences for
prompt toolkit.

IPython/terminal/prompts.py

@@ -1,73 +1,74 b''
 """Terminal input and output prompts."""
 from __future__ import print_function
 
 from pygments.token import Token
 import sys
 
 from IPython.core.displayhook import DisplayHook
 
+from prompt_toolkit.layout.utils import token_list_width
+
 class Prompts(object):
     def __init__(self, shell):
         self.shell = shell
 
     def in_prompt_tokens(self, cli=None):
         return [
             (Token.Prompt, 'In ['),
             (Token.PromptNum, str(self.shell.execution_count)),
             (Token.Prompt, ']: '),
         ]
 
     def _width(self):
-        in_tokens = self.in_prompt_tokens()
-        return sum(len(s) for (t, s) in in_tokens)
+        return token_list_width(self.in_prompt_tokens())
 
     def continuation_prompt_tokens(self, cli=None, width=None):
         if width is None:
             width = self._width()
         return [
             (Token.Prompt, (' ' * (width - 5)) + '...: '),
         ]
 
     def rewrite_prompt_tokens(self):
         width = self._width()
         return [
             (Token.Prompt, ('-' * (width - 2)) + '> '),
         ]
 
     def out_prompt_tokens(self):
         return [
             (Token.OutPrompt, 'Out['),
             (Token.OutPromptNum, str(self.shell.execution_count)),
             (Token.OutPrompt, ']: '),
         ]
 
 class ClassicPrompts(Prompts):
     def in_prompt_tokens(self, cli=None):
         return [
             (Token.Prompt, '>>> '),
         ]
 
     def continuation_prompt_tokens(self, cli=None, width=None):
         return [
             (Token.Prompt, '... ')
         ]
 
     def rewrite_prompt_tokens(self):
         return []
 
     def out_prompt_tokens(self):
         return []
 
 class RichPromptDisplayHook(DisplayHook):
     """Subclass of base display hook using coloured prompt"""
     def write_output_prompt(self):
         sys.stdout.write(self.shell.separate_out)
         self.prompt_end_newline = False
         if self.do_full_cache:
             tokens = self.shell.prompts.out_prompt_tokens()
             if tokens and tokens[-1][1].endswith('\n'):
                 self.prompt_end_newline = True
             if self.shell.pt_cli:
                 self.shell.pt_cli.print_tokens(tokens)
             else:
                 print(*(s for t, s in tokens), sep='')