发生这种情况是因为首先评估函数参数，并且在定义 p1 和 p2 时，您实际上是在调用 foo1('this')，它在 p1 和 progress_bar() 的定义处执行函数p2 的实例化。
有关演示这一点的简单示例，请参见下文：
def fn():
print 'called'
return 1
target1 = fn()
target = fn
print target1
print target
打印出来：
>>> called # Got called as soon as you called fn via fn()
>>> 1 # Assigned the return value of fn to target1
>>> <function fn at 0x12DA77F0> # Didn't get called, assigned the fn definition to target
我让您的示例与下面的 Threads 一起使用（编辑：在查看了一些 Process examples 之后，它们似乎应该使用与以下代码相同的语法（只需更改导入并使用Process 而不是Thread)，但由于某种原因，即使在复制示例之后，我也无法打印Process 方法。可能是由于我的自定义python 设置，但是不完全确定。）：
from threading import Thread
import time
def foo(thread):
print(time.ctime())
time.sleep(10)
print(time.ctime())
def progress_bar(timer = 10):
digits = 4
delete = '\b' * 6
time_slot = float(timer) / 100
for i in range(1, 101):
delete_bar = '\b' * 52
if i == 1:
bar = '|' + ' ' * 50 + '|'
else:
bar = '|' + '=' * (i / 2 - 1) + '>' + ' ' * (50 - i / 2) + '|'
print("{0}{1:{2}}{3}{4}".format(delete, str(i) + '%', digits, bar, delete_bar),)
time.sleep(time_slot)
print('')
def main():
t1 = Thread(target=foo, args=('this',)) # Notice, not foo('this') <- this executes foo('this') at definition
t1.start()
t2 = Thread(target=progress_bar) # Again, notice, no parens - target is just the function definition
t2.start()
t1.join()
t2.join()
打印出来
Tue Apr 05 15:09:34 2016
('1% | |',)
('2% |> |',)
('3% |> |',)
('4% |=> |',)
('5% |=> |',)
('6% |==> |',)
('7% |==> |',)
('8% |===> |',)
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('10% |====> |',)
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Tue Apr 05 15:09:44 2016
('99% |================================================> |',)
('100%|=================================================>|',)