拿一下代码来解释:
# Threading example
import time, thread
def myfunction(string, sleeptime, lock, *args):
while True:
lock.acquire()
time.sleep(sleeptime)
lock.release()
time.sleep(sleeptime)
if __name__ == "__main__":
lock = thread.allocate_lock()
thread.start_new_thread(myfunction, ("Thread #: 1", 2, lock))
thread.start_new_thread(myfunction, ("Thread #: 2", 2, lock))
当python编译器执行python源文件时,它会执行里边所有的代码。
在执行代码之前。会定义 一些特别的变量。比如,如果编译器把当前模块(源文件)作为主程序,它会设置一个特别的变量__name__的值为__main__,如果这个文件是从别的模块导入,__name__将会被设置成模块的名字。
如果我们执行(假设文件名为threading_example.py):
python threading_example.py
在设置完specia variable时,会先执行import ,然后产生一个函数对象和变量myfunction,这个变量指向def定义的函数,然后看if语句,如果__name__的值为__main__,会接着 执行下边的语句。
下面再举一个更详细的例子。
如果你的脚本被import到了别的模块,它的函数和 类定义会被导入,然后先执行top-level code(在函数和类定义之外的代码),if语句下的代码会在条件满足时才执行。
# file one.py
def func():
print("func() in one.py")
print("top-level in one.py")
if __name__ == "__main__":
print("one.py is being run directly")
else:
print("one.py is being imported into another module")
# file two.py
import one
print("top-level in two.py")
one.func()
if __name__ == "__main__":
print("two.py is being run directly")
else:
print("two.py is being imported into another module")
现在,如果先执行以下命令:
python one.py
输出:
top-level in one.py
one.py is being run directly
但是如果执行:
python two.py
输出将会是:
top-level in one.py
one.py is being imported into another module
top-level in two.py
func() in one.py
two.py is being run directly
因此,当one被导入时,one.py的__name__会等于one,而不是__main__。也就是说文件的名字__name__后边的值代表的是我想运行这个文件本身还是想它被别的文件导入。