altair_png_magic.py

from io import BytesIO

def render_altair(chart):
    b = BytesIO()
    chart.save(b, scale_factor=2.0, format='png', webdriver='firefox')
    b.seek(0)
    return b.read()


import ast

def exec_then_eval(code, shell):
    block = ast.parse(code, mode='exec')

    # assumes last node is an expression
    last = ast.Expression(block.body.pop().value)

    _globals, _locals = {}, {}
    shell.ex(compile(block, '<string>', mode='exec'))
    return shell.ev(compile(last, '<string>', mode='eval'))

@magics_class
class AltairMagic(Magics):
    @cell_magic
    def altair(self, line, cell):
        return Image(render_altair(exec_then_eval(cell, self.shell)))


ip = get_ipython()
ip.register_magics(AltairMagic)