将分配分成两行是否仍然一样有效？

Question

从python的运行时效率角度来看，这些效率是否相等？

x = foo()
x = bar(x)

VS

x = bar(foo())

我有一个更复杂的问题，基本上可以归结为这个问题：显然，从代码长度的角度来看，第二个更为有效，但运行时是否也更好？如果不是，为什么不呢？

Answer 1

比较一下：

第一种情况：

%%timeit
def foo():
    return "foo"

def bar(text):
    return text + "bar"

def test():
    x = foo()
    y = bar(x)
    return y

test()
#Output:
'foobar'
529 ns ± 114 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

第二种情况：

%%timeit

def foo():
    return "foo"

def bar(text):
    return text + "bar"

def test():   
    x = bar(foo())
    return x

test()
#Output:
'foobar'
447 ns ± 34.6 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

但这只是针对每种情况运行一次%% timeit的比较。以下是每种情况下20次迭代的时间（以ns为单位的时间）：

df = pd.DataFrame({'First Case(time in ns)': [623,828,634,668,715,659,703,687,614,623,697,634,686,822,671,894,752,742,721,742], 
               'Second Case(time in ns)': [901,786,686,670,677,683,685,638,628,670,695,657,698,707,726,796,868,703,609,852]})

df.plot(kind='density', figsize=(8,8))

It was observed, with each iteration, the differences were diminishing. This plot shows that the performance difference isn't significant. From a readability perspective, the second case looks better.

In the first case, two expressions are evaluated: the first expression assigns the return value from foo() to x first and then the second expression calls bar() on that value. This adds some overhead. In the second case only one expression is evaluated, calling both functions at once and returning the value.