I would like to implement something similar to this OCaml in Python: let example = fun v opt_n -> let fltr = fun i -> i mod 2 = 0 let fltr = match opt_n with | None -> fltr | Some n -> fun i -> i mod n = 0 && fltr(n) fltr v This is easily composable/extendable, I can add as many predicates as I want at runtime. This is of course a simplified example, in real life I have many optional inclusion/exclusion sets, and predicate checks for membership. Doing this the naive way in Python fails: def example(v: int, opt_n=None): """ doesn't work! """ # doesn't need to be a lambda, an explicitely defined function fails too fltr = lambda i: i % 2 == 0 if opt_n is not None: # fails miserably -> maximum recursion depth exceeded fltr = lambda i: fltr(i) and i % opt_n == 0 return fltr(v) example(10, 5) This is annoying because it seems that since fltr can only appear once on the left side of the assignment, I have to inline the initial fltr in every case afterward: def example(v: int, opt_n=None, opt_m=None): """annoying but works""" fltr = None # some inital filters pred_0 = lambda _: True # do some real checks ... pred_1 = lambda _: True # do some real checks ... if opt_n is not None: # fltr is inlined, only appears on left side, now it works fltr = lambda i: pred_0(i) and pred_1(i) and opt_n % 2 == 0 if opt_m is not None: # much repetition fltr = lambda i: pred_0(i) and pred_1(i) and opt_n % 3 == 0 if fltr is None: # inlined again fltr = lambda i: pred_0(i) and pred_1(i) return fltr(v) Is there any way to fix my mess, maybe I am missing something, and/or what is the recommended way to compose predicates in Python? Continue reading...
