python - 为什么对数字调用 float() 比在 Python 中添加 0.0 慢？

Question

在 Python 中将整数转换为 float 比将 0.0 加到该整数上慢的原因是什么？

import timeit


def add_simple():
    for i in range(1000):
        a = 1 + 0.0


def cast_simple():
    for i in range(1000):
        a = float(1)


def add_total():
    total = 0
    for i in range(1000):
        total += 1 + 0.0


def cast_total():
    total = 0
    for i in range(1000):
        total += float(1)


print "Add simple timing: %s" % timeit.timeit(add_simple, number=1)
print "Cast simple timing: %s" % timeit.timeit(cast_simple, number=1)
print "Add total timing: %s" % timeit.timeit(add_total, number=1)
print "Cast total timing: %s" % timeit.timeit(cast_total, number=1)

其输出为:

Add simple timing: 0.0001220703125

Cast simple timing: 0.000469923019409

Add total timing: 0.000164985656738

Cast total timing: 0.00040078163147

Answer 1

如果你使用 dis 模块，你可以开始了解原因:

In [11]: dis.dis(add_simple)
  2           0 SETUP_LOOP              26 (to 29)
              3 LOAD_GLOBAL              0 (range)
              6 LOAD_CONST               1 (1000)
              9 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             12 GET_ITER
        >>   13 FOR_ITER                12 (to 28)
             16 STORE_FAST               0 (i)

  3          19 LOAD_CONST               4 (1.0)
             22 STORE_FAST               1 (a)
             25 JUMP_ABSOLUTE           13
        >>   28 POP_BLOCK
        >>   29 LOAD_CONST               0 (None)
             32 RETURN_VALUE

In [12]: dis.dis(cast_simple)
  2           0 SETUP_LOOP              32 (to 35)
              3 LOAD_GLOBAL              0 (range)
              6 LOAD_CONST               1 (1000)
              9 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             12 GET_ITER
        >>   13 FOR_ITER                18 (to 34)
             16 STORE_FAST               0 (i)

  3          19 LOAD_GLOBAL              1 (float)
             22 LOAD_CONST               2 (1)
             25 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             28 STORE_FAST               1 (a)
             31 JUMP_ABSOLUTE           13
        >>   34 POP_BLOCK
        >>   35 LOAD_CONST               0 (None)
             38 RETURN_VALUE

注意 CALL_FUNCTION

Python 中的函数调用(相对)很慢。 . 查找也是如此。因为转换为 float 需要一个函数调用——这就是它变慢的原因。