The Python package lambdex [1] provides a way to write multi-line anonymous functions in Python, with a syntax that's close enough to the "normal" Python.
Interesting. This looks weird though, because now there is `def_` (not `def`) and `lambda`, instead of only needing `lambda` or only `def`. I am guessing you would also need to `def_` from somewhere. And you need to change a lot of parens inside to brackets, making it even more unnatural in Python sense.