Just for the record, an implementation without using generators, somewhat like in Sect. 3.5 of the Wizard book, and without recursion limit problems. . . def stream_hd(s): # the head of the stream . return s[0] . . def stream_tl(s): # the tail of the stream . return s[1]() . . ## . # The low argument is required: the first element of the stream. . # You either use high= or stop_f= to define the end of the . # stream. Omit both for an infinite stream. stop_f is a function . # that takes low as argument and returns True or False. . # The next_f function takes one argument (an element of the stream, . # same type as the low argument has) . # The stop_f function takes one argument (same type as low) . # @param low object . # @param high object . # @param next_f function . # @param stop_f function . # @return list, a stream . def make_stream(low, high=None, next_f=None, stop_f=None): . if next_f is None: next_f = lambda x: x + 1 . if high is not None: # using high . if low > high: . return None . else: . return [low, lambda: make_stream( . next_f(low), high=high, next_f=next_f)] . elif stop_f is not None: # using stop_f . if low > stop_f(low): . return None . else: . return [low, lambda: make_stream( . next_f(low), next_f=next_f, stop_f=stop_f)] . else: # infinite stream . return [low, lambda: make_stream( . next_f(low), next_f=next_f)] . . ## . # iterative version of filter . # @param pred function, (stream-element) -> bool . # @param s list, a stream . # @return list, a stream . def stream_filter(pred, s): . if s is None: return [] . while True: . elem = stream_hd(s) . if pred(elem): . return [elem, lambda: stream_filter(pred, stream_tl(s))] . else: . s = stream_tl(s) . if s is None: return [] . . . if __name__ == '__main__': . . def is100some(x): return x % 100 == 0 . assert(stream_hd(stream_tl(stream_tl(stream_filter( . is100some, . make_stream(1, 11111111111111))))) == 300) . . def add_33(x): return x + 33 . assert(stream_hd(stream_tl(stream_filter( . is100some, . # stream 1,34,67,100,... . make_stream(1,999999999, next_f = add_33)))) == 3400) . . assert(stream_hd(stream_tl(stream_filter( . is100some, . # infinite stream 1,2,... . make_stream(1)))) == 200) . . def mod_20000(x): return x % 20000 == 0 . # this will need more evaluations than the recursion limit count . infinite_filter = stream_filter(mod_20000, make_stream(1)) . assert( stream_hd(stream_tl(infinite_filter)) == 40000 ) .
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