TL;DR

With BeautifulSoup 4 use element.encode_contents() if you want a UTF-8 encoded bytestring or use element.decode_contents() if you want a Python Unicode string. For example the DOM’s innerHTML method might look something like this:

def innerHTML(element):
    """Returns the inner HTML of an element as a UTF-8 encoded bytestring"""
    return element.encode_contents()

---

These functions aren’t currently in the online documentation so I’ll quote the current function definitions and the doc string from the code.

encode_contents – since 4.0.4

def encode_contents(
    self, indent_level=None, encoding=DEFAULT_OUTPUT_ENCODING,
    formatter="minimal"):
    """Renders the contents of this tag as a bytestring.

    :param indent_level: Each line of the rendering will be
       indented this many spaces.

    :param encoding: The bytestring will be in this encoding.

    :param formatter: The output formatter responsible for converting
       entities to Unicode characters.
    """

See also the documentation on formatters; you’ll most likely either use formatter="minimal" (the default) or formatter="html" (for html entities) unless you want to manually process the text in some way.

encode_contents returns an encoded bytestring. If you want a Python Unicode string then use decode_contents instead.

---

decode_contents – since 4.0.1

decode_contents does the same thing as encode_contents but returns a Python Unicode string instead of an encoded bytestring.

def decode_contents(self, indent_level=None,
                   eventual_encoding=DEFAULT_OUTPUT_ENCODING,
                   formatter="minimal"):
    """Renders the contents of this tag as a Unicode string.

    :param indent_level: Each line of the rendering will be
       indented this many spaces.

    :param eventual_encoding: The tag is destined to be
       encoded into this encoding. This method is _not_
       responsible for performing that encoding. This information
       is passed in so that it can be substituted in if the
       document contains a <META> tag that mentions the document's
       encoding.

    :param formatter: The output formatter responsible for converting
       entities to Unicode characters.
    """

---

BeautifulSoup 3

BeautifulSoup 3 doesn’t have the above functions, instead it has renderContents

def renderContents(self, encoding=DEFAULT_OUTPUT_ENCODING,
                   prettyPrint=False, indentLevel=0):
    """Renders the contents of this tag as a string in the given
    encoding. If encoding is None, returns a Unicode string.."""

This function was added back to BeautifulSoup 4 (in 4.0.4) for compatibility with BS3.