Alternate implementation of NLTK's concordance() - no dependencies needed!

search.py

def concordance(string, search_term, width=25):
    """
    Alternative implementation of NLTK's concordance() that 
    allows printing to stdout or saving to a variable and 
    does not require NLTK.
    
    Just feed it a raw string, JSON string, etc. with any line
    breaks stripped out.
    """
    # Offset tracks our progress as we parse through the string
    offset = 0
    
    # Indexes lets us store all the positions we find your term in
    indexes = []
    
    # Keep scanning through the string until we reach the end
    while offset < len(string):
        try:
            # From the current position to the end of the string, find
            # the next potential position for your search term
            position = string[offset:].lower().index(search_term.lower())
        except ValueError:
            # Your term wasn't found; exit.
            break
        
        if position:
            # Your term was found. Add it to the list of indexes
            indexes.append(position + offset)
            # Now increase the offset to the position of your term,
            # plus the length of its letters so we resume scanning
            # after the end of it.
            offset += position + len(search_term)
        
    # For each position where the case was found, return the leading and 
    # trailing characters
    return tuple(string[index-width:index+width+len(search_term)] 
                 for index in indexes)