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+++ b/genshi/template/interpolation.py
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+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2007 Edgewall Software
+# All rights reserved.
+#
+# This software is licensed as described in the file COPYING, which
+# you should have received as part of this distribution. The terms
+# are also available at http://genshi.edgewall.org/wiki/License.
+#
+# This software consists of voluntary contributions made by many
+# individuals. For the exact contribution history, see the revision
+# history and logs, available at http://genshi.edgewall.org/log/.
+
+"""String interpolation routines, i.e. the splitting up a given text into some
+parts that are literal strings, and others that are Python expressions.
+"""
+
+from itertools import chain
+import os
+from tokenize import tokenprog
+
+from genshi.core import TEXT
+from genshi.template.base import TemplateSyntaxError, EXPR
+from genshi.template.eval import Expression
+
+__all__ = ['interpolate']
+__docformat__ = 'restructuredtext en'
+
+NAMESTART = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_'
+NAMECHARS = NAMESTART + '.0123456789'
+PREFIX = '$'
+
+def interpolate(text, basedir=None, filename=None, lineno=-1, offset=0,
+                lookup='lenient'):
+    """Parse the given string and extract expressions.
+    
+    This function is a generator that yields `TEXT` events for literal strings,
+    and `EXPR` events for expressions, depending on the results of parsing the
+    string.
+    
+    >>> for kind, data, pos in interpolate("hey ${foo}bar"):
+    ...     print kind, `data`
+    TEXT u'hey '
+    EXPR Expression('foo')
+    TEXT u'bar'
+    
+    :param text: the text to parse
+    :param basedir: base directory of the file in which the text was found
+                    (optional)
+    :param filename: basename of the file in which the text was found (optional)
+    :param lineno: the line number at which the text was found (optional)
+    :param offset: the column number at which the text starts in the source
+                   (optional)
+    :param lookup: the variable lookup mechanism; either "lenient" (the
+                   default), "strict", or a custom lookup class
+    :return: a list of `TEXT` and `EXPR` events
+    :raise TemplateSyntaxError: when a syntax error in an expression is
+                                encountered
+    """
+    filepath = filename
+    if filepath and basedir:
+        filepath = os.path.join(basedir, filepath)
+    pos = [filepath, lineno, offset]
+
+    textbuf = []
+    textpos = None
+    for is_expr, chunk in chain(lex(text, pos, filepath), [(True, '')]):
+        if is_expr:
+            if textbuf:
+                yield TEXT, u''.join(textbuf), textpos
+                del textbuf[:]
+                textpos = None
+            if chunk:
+                try:
+                    expr = Expression(chunk.strip(), pos[0], pos[1],
+                                     lookup=lookup)
+                    yield EXPR, expr, tuple(pos)
+                except SyntaxError, err:
+                    raise TemplateSyntaxError(err, filepath, pos[1],
+                                              pos[2] + (err.offset or 0))
+        else:
+            textbuf.append(chunk)
+            if textpos is None:
+                textpos = tuple(pos)
+
+        if '\n' in chunk:
+            lines = chunk.splitlines()
+            pos[1] += len(lines) - 1
+            pos[2] += len(lines[-1])
+        else:
+            pos[2] += len(chunk)
+
+def lex(text, textpos, filepath):
+    offset = pos = 0
+    end = len(text)
+    escaped = False
+
+    while 1:
+        if escaped:
+            offset = text.find(PREFIX, offset + 2)
+            escaped = False
+        else:
+            offset = text.find(PREFIX, pos)
+        if offset < 0 or offset == end - 1:
+            break
+        next = text[offset + 1]
+
+        if next == '{':
+            if offset > pos:
+                yield False, text[pos:offset]
+            pos = offset + 2
+            level = 1
+            while level:
+                match = tokenprog.match(text, pos)
+                if match is None:
+                    raise TemplateSyntaxError('invalid syntax',  filepath,
+                                              *textpos[1:])
+                pos = match.end()
+                tstart, tend = match.regs[3]
+                token = text[tstart:tend]
+                if token == '{':
+                    level += 1
+                elif token == '}':
+                    level -= 1
+            yield True, text[offset + 2:pos - 1]
+
+        elif next in NAMESTART:
+            if offset > pos:
+                yield False, text[pos:offset]
+                pos = offset
+            pos += 1
+            while pos < end:
+                char = text[pos]
+                if char not in NAMECHARS:
+                    break
+                pos += 1
+            yield True, text[offset + 1:pos].strip()
+
+        elif not escaped and next == PREFIX:
+            escaped = True
+            pos = offset + 1
+
+        else:
+            yield False, text[pos:offset + 1]
+            pos = offset + 1
+
+    if pos < end:
+        yield False, text[pos:]