What does if
if __name__ == "__main__":做什么？
# 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))
When the Python interpreter reads a source file, it executes all of the code found in it.
Before executing the code, it will define a few special variables. For example, if the python interpreter is running that module (the source file) as the main program, it sets the special __name__ variable to have a value "__main__" . If this file is being imported from another module, __name__ will be set to the module's name.
In the case of your script, let's assume that it's executing as the main function, eg you said something like
python threading_example.py
on the command line. After setting up the special variables, it will execute the import statement and load those modules. It will then evaluate the def block, creating a function object and creating a variable called myfunction that points to the function object. It will then read the if statement and see that __name__ does equal "__main__" , so it will execute the block shown there.
One reason for doing this is that sometimes you write a module (a .py file) where it can be executed directly. Alternatively, it can also be imported and used in another module. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves.
See this page for some extra details.
When your script is run by passing it as a command to the Python interpreter,
python myscript.py
all of the code that is at indentation level 0 gets executed. Functions and classes that are defined are, well, defined, but none of their code gets run. Unlike other languages, there's no main() function that gets run automatically - the main() function is implicitly all the code at the top level.
In this case, the top-level code is an if block. __name__ is a built-in variable which evaluates to the name of the current module. However, if a module is being run directly (as in myscript.py above), then __name__ instead is set to the string "__main__" . Thus, you can test whether your script is being run directly or being imported by something else by testing
if __name__ == "__main__":
...
If your script is being imported into another module, its various function and class definitions will be imported and its top-level code will be executed, but the code in the then-body of the if clause above won't get run as the condition is not met. As a basic example, consider the following two scripts:
# 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")
Now, if you invoke the interpreter as
python one.py
The output will be
top-level in one.py
one.py is being run directly
If you run two.py instead:
python two.py
You get
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
Thus, when module one gets loaded, its __name__ equals "one" instead of "__main__" .
The simplest explanation for the __name__ variable (imho) is the following:
Create the following files.
# a.py
import b
and
# b.py
print "Hello World from %s!" % __name__
if __name__ == '__main__':
print "Hello World again from %s!" % __name__
Running them will get you this output:
$ python a.py
Hello World from b!
As you can see, when a module is imported, Python sets globals()['__name__'] in this module to the module's name.
$ python b.py
Hello World from __main__!
Hello World again from __main__!
As you can see, when a file is executed, Python sets globals()['__name__'] in this file to "__main__" .
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