Testing for positive infinity, or negative infinity, individually in Python

有些话、适合烂在心里 提交于 2021-02-05 20:25:09

问题


math.isinf() tests for positive or negative infinity lumped together. What's the pythonic way to test for them distinctly?

Ways to test for positive infinity:

  1. x == float('+inf')
  2. math.isinf(x) and x > 0

Ways to test for negative infinity:

  1. x == float('-inf')
  2. math.isinf(x) and x < 0

Disassembly Way 1:

>>> def ispinf1(x): return x == float("inf")
...
>>> dis.dis(ispinf1)
  1           0 LOAD_FAST                0 (x)
              3 LOAD_GLOBAL              0 (float)
              6 LOAD_CONST               1 ('inf')
              9 CALL_FUNCTION            1
             12 COMPARE_OP               2 (==)
             15 RETURN_VALUE

Disassembly Way 2:

>>> def ispinf2(x): return isinf(x) and x > 0
...
>>> dis.dis(ispinfs)
  1           0 LOAD_GLOBAL              0 (isinf)
              3 LOAD_FAST                0 (x)
              6 CALL_FUNCTION            1
              9 JUMP_IF_FALSE_OR_POP    21
             12 LOAD_FAST                0 (x)
             15 LOAD_CONST               1 (0)
             18 COMPARE_OP               4 (>)
        >>   21 RETURN_VALUE

This answer seems to favor Way 2 except for the x>0.


回答1:


The "pythonic" way is to go with what's readable and maintainable.

That said, x == float("inf") and x == float("-inf") are slightly more readable to me, and I'd prefer them. math.isinf(x) and x > 0 is faster, but only on the order of about 40 nanoseconds per call.

So unless you're checking a whole lot of numbers, it isn't going to make much of a difference in running time.




回答2:


there is also numpy

>>> import numpy as np
>>> np.isneginf([np.inf, 0, -np.inf])
array([False, False,  True], dtype=bool)
>>> np.isposinf([np.inf, 0, -np.inf])
array([ True, False, False], dtype=bool)

and then there is general isinf

>>> np.isinf([np.inf, 0, -np.inf])
array([ True, False,  True], dtype=bool)


来源:https://stackoverflow.com/questions/28102302/testing-for-positive-infinity-or-negative-infinity-individually-in-python

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