class Customer(Base):
    __tablename__ = 'customer'
    id = Column(Integer, primary_key=True)
    name = Column(String)

    billing_address_id = Column(Integer, ForeignKey("address.id"))
    shipping_address_id = Column(Integer, ForeignKey("address.id"))

    billing_address = relationship("Address", foreign_keys=[billing_address_id])
    shipping_address = relationship("Address", foreign_keys=[shipping_address_id])

render_proc = Popen(['node', exec_path], stdout=PIPE, stdin=PIPE)

def render_markdown(string, id=None, force=False):
    """
    Renders markdown.

    Parameters:
     - string(str): Markdown string to render.
     - id(number): If provided, this will enable caching by identifying the
        markdown string with the Id.
     - force(bool): If provided with an id, this will force a new rendering and
        reset the cache

    Returns: rendered string
    """
    render_proc.stdin.write(string.encode('utf-8'))

    read_len, = unpack('<i', render_proc.stdout.read(4))
struct.error: unpack requires a bytes object of length 4

>>> from app.helpers.markdown import render_markdown
>>> render_markdown("# test")
b'<h1>test</h1>\n'