Constrained list insertion in Python.

constrained_insert.py

def constrained_insert(list_, item, before=None, after=None, strict=True):
    """ Given a list and an item to be inserted into that list, attempts
        to insert the item into the list in such a way that it is before
        any items listed in the 'before' argument and after any of those
        listed in the 'after' argument. If this is not possible, raises
        a RuntimeError.
        
        If any of the values in before or after do not appear in the
        list, ValueError will be raised, unless 'strict' is set to
        False.
    """
    
    if before is not None and isinstance(before, basestring):
        before = (before, )
    elif before is None:
        before = ()
    if after is not None and isinstance(after, basestring):
        after = (after, )
    elif after is None:
        after = ()
    
    for i in before + after:
        if i not in list_:
            if strict:
                raise ValueError('%s does not appear in the list' % (i,))
            else:
                if i in before:
                    before.remove(i)
                if i in after:
                    after.remove(i)
    
    beforepos = min([len(list_)] + [list_.index(s)
                                    for s in before
                                    if s in list_])
    
    afterpos = max([0] + [list_.index(s)
                          for s in after
                          if s in list_]) + 1
    
    if before and after and afterpos > beforepos:
        raise RuntimeError(('Unabled to resolve a position in %s to satisfy ' \
                           'the constraints before: %s; after: %s.') % \
                           (list_, before, after))
    
    if before and not after:
        list_.insert(beforepos, item)
    else:
        # if we don't want it directly before something, insert it as
        # early in the list as the 'after' constraint will allow.
        list_.insert(afterpos, item)
    
    return list_