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

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:

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

Ways to test for negative infinity:

x == float('-inf')
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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python

infinity