I can do this in Python with my `rwatch` module.

    >>> from datetime import datetime
    >>> x = lambda: f'{datetime.now()}'
    >>> x
    <function <lambda> at 0x7f9f435e4510>
    >>> x()
    '2017-09-17 02:14:12.834957'
    >>> from sys import setrwatch
    >>> setrwatch({id(x): lambda f, o: o()})

    >>> x # look, ma: no parens!
    '2017-09-17 02:14:16.306198'
    >>> x
    '2017-09-17 02:14:17.734965'