replace_with and unwrap fail to check the parent is None

[Zhaogui]

It gives an attributeerror traceback when self.parent is None, which confuses me.

[simple test]

builder = builder.TreeBuilder()
parent_tag = Tag(name='div', attrs={'id': 'parent'}, builder=builder)
replace_tag = Tag(name='div', attrs={'id': 'replace'}, builder=builder)

parent_tag.replace_with(replace_tag)

Traceback
...
    parent_tag.replace_with(replace_tag)
  File "/usr/lib/python2.7/dist-packages/bs4/element.py", line 211, in replace_with
    my_index = self.parent.index(self)
AttributeError: 'NoneType' object has no attribute 'index'

It is similar in method "unwarp".

[Leonard Richardson (leonardr)]

The exception message is bad, but raising an exception is the correct behavior. unwrap() and replace_with() don't alter an element in place: they change the way that element shows up in a tree. If the element has no .parent it's not part of a tree, and .unwrap() and .replace_with() don't mean anything.

Here's an analogy. If you have a list [1, 2, 3] you can replace the 2 with a 5 to get [1, 5, 3]. But if you just have the number 2, you can't replace it with a 5. It's not in a context where 'replacement' makes sense.

I think you probably want to do something different, but I'm not sure what.

I've improved the exception messages to hopefully make it more clear why the exception is happening.