source: python/vendor/Python-2.7.6/Lib/idlelib/AutoExpand.py

import string
import re

###$ event <<expand-word>>
###$ win <Alt-slash>
###$ unix <Alt-slash>

class AutoExpand:

    menudefs = [
        ('edit', [
            ('E_xpand Word', '<<expand-word>>'),
         ]),
    ]

    wordchars = string.ascii_letters + string.digits + "_"

    def __init__(self, editwin):
        self.text = editwin.text
        self.state = None

    def expand_word_event(self, event):
        curinsert = self.text.index("insert")
        curline = self.text.get("insert linestart", "insert lineend")
        if not self.state:
            words = self.getwords()
            index = 0
        else:
            words, index, insert, line = self.state
            if insert != curinsert or line != curline:
                words = self.getwords()
                index = 0
        if not words:
            self.text.bell()
            return "break"
        word = self.getprevword()
        self.text.delete("insert - %d chars" % len(word), "insert")
        newword = words[index]
        index = (index + 1) % len(words)
        if index == 0:
            self.text.bell()            # Warn we cycled around
        self.text.insert("insert", newword)
        curinsert = self.text.index("insert")
        curline = self.text.get("insert linestart", "insert lineend")
        self.state = words, index, curinsert, curline
        return "break"

    def getwords(self):
        word = self.getprevword()
        if not word:
            return []
        before = self.text.get("1.0", "insert wordstart")
        wbefore = re.findall(r"\b" + word + r"\w+\b", before)
        del before
        after = self.text.get("insert wordend", "end")
        wafter = re.findall(r"\b" + word + r"\w+\b", after)
        del after
        if not wbefore and not wafter:
            return []
        words = []
        dict = {}
        # search backwards through words before
        wbefore.reverse()
        for w in wbefore:
            if dict.get(w):
                continue
            words.append(w)
            dict[w] = w
        # search onwards through words after
        for w in wafter:
            if dict.get(w):
                continue
            words.append(w)
            dict[w] = w
        words.append(word)
        return words

    def getprevword(self):
        line = self.text.get("insert linestart", "insert")
        i = len(line)
        while i > 0 and line[i-1] in self.wordchars:
            i = i-1
        return line[i:]