python-为什么多线程和不同的函数/作用域共享单个导入过程

自从我多年前使用python以来，这个陷阱一直是最难发现的bug

让我展示一个过于简化的示例，我有以下文件/dir：

[xiaobai@xiaobai import_pitfall]$ tree -F -C -a
.
├── import_all_pitall/
│   ├── hello.py
│   └── __init__.py
└── thread_test.py

1 directory, 3 files
[xiaobai@xiaobai import_pitfall]$

螺纹_test.py的内容：

[xiaobai@xiaobai import_pitfall]$ cat thread_test.py 
import time
import threading

def do_import1():
    print( "do_import 1A" )
    from import_all_pitall import hello
    print( "do_import 1B", id(hello), locals() )

def do_import2():
    print( "do_import 2A" )
    from import_all_pitall import hello as h
    print( "do_import 2B", id(h), locals() )

def do_import3():
    print( "do_import 3A" )
    import import_all_pitall.hello as h2
    #no problem if import different module #import urllib as h2
    print( "do_import 3B", id(h2), locals() )

print( "main 1" )
t = threading.Thread(target=do_import1)
print( "main 2" )
t.start()
print( "main 3" )
t2 = threading.Thread(target=do_import2)
print( "main 4" )
t2.start()
print( "main 5" )
print(globals()) #no such hello
#time.sleep(2) #slightly wait for do_import 1A import finished to test print hello below.
#print( "main 6", id(hello), locals() ) #"name 'hello' not defined" error even do_import1 was success
do_import3()
print( "main -1" )
[xiaobai@xiaobai import_pitfall]$

hello.py的内容：

[xiaobai@xiaobai import_pitfall]$ cat import_all_pitall/hello.py
print( "haha0" )
import time
t = time.time()
print( "haha1" )
def do_task():
    success = 0
    while not success:
        try:
            time.sleep(1)
            undefined_func( "Done haha" )
            success = 1
        except Exception as e:
            print("exception occur", e)
            print( "haha time is ", t )
do_task()
print( "haha -1" )
[xiaobai@xiaobai import_pitfall]$

print( "haha0" )
import time
t = time.time()
print( "haha1" )
def do_task():
    success = 0
    while not success:
        try:
            time.sleep(1)
            #undefined_func( "Done haha" )
            success = 1
        except Exception as e:
            print("exception occur", e)
            print( "haha time is ", t )
do_task()
print( "haha -1" )

而import\u all\u pitall/init.py是一个空文件

让我们运行它：

[xiaobai@xiaobai import_pitfall]$ python thread_test.py 
main 1
main 2
do_import 1A
 main 3
haha0
haha1
main 4
do_import 2A
main 5
{'do_import1': <function do_import1 at 0x7f9d884760c8>, 'do_import3': <function do_import3 at 0x7f9d884a6758>, 'do_import2': <function do_import2 at 0x7f9d884a66e0>, '__builtins__': <module '__builtin__' (built-in)>, '__file__': 'thread_test.py', 't2': <Thread(Thread-2, started 140314429765376)>, '__package__': None, 'threading': <module 'threading' from '/usr/lib64/python2.7/threading.pyc'>, 't': <Thread(Thread-1, started 140314438158080)>, 'time': <module 'time' from '/usr/lib64/python2.7/lib-dynload/timemodule.so'>, '__name__': '__main__', '__doc__': None}
do_import 3A
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
^C('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
^C('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
^C^C('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
... #Forever

[xiaobai@xiaobai import_pitfall]$ python3 thread_test.py 
main 1
main 2
do_import 1A
main 3
main 4
do_import 2A
main 5
{'t': <Thread(Thread-1, started 140404878976768)>, '__spec__': None, 'time': <module 'time' from '/usr/lib64/python3.4/lib-dynload/time.cpython-34m.so'>, '__cached__': None, '__loader__': <_frozen_importlib.SourceFileLoader object at 0x7fb296b87048>, 'do_import2': <function do_import2 at 0x7fb296ac56a8>, 'do_import1': <function do_import1 at 0x7fb296bb0bf8>, '__doc__': None, '__file__': 'thread_test.py', 'do_import3': <function do_import3 at 0x7fb28ef19f28>, 't2': <Thread(Thread-2, started 140404870584064)>, '__name__': '__main__', '__package__': None, '__builtins__': <module 'builtins' (built-in)>, 'threading': <module 'threading' from '/usr/lib64/python3.4/threading.py'>}
haha0
haha1
haha -1
do_import 1B 140404879178392 {'hello': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
do_import 2B 140404879178392 {'h': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
Traceback (most recent call last):
  File "thread_test.py", line 31, in <module>
    print( "main 6", id(hello), locals() ) #"name 'hello' not defined" error even do_import1 was success
NameError: name 'hello' is not defined
[xiaobai@xiaobai import_pitfall]$ 

然后运行它：

[xiaobai@xiaobai import_pitfall]$ python3 thread_test.py 
main 1
main 2
do_import 1A
main 3
main 4
do_import 2A
main 5
{'do_import3': <function do_import3 at 0x7f31a462c048>, '__package__': None, 't2': <Thread(Thread-2, started 139851179529984)>, '__name__': '__main__', '__cached__': None, 'threading': <module 'threading' from '/usr/lib64/python3.4/threading.py'>, '__doc__': None, 'do_import2': <function do_import2 at 0x7f31ac1d56a8>, 'do_import1': <function do_import1 at 0x7f31ac2c0bf8>, '__spec__': None, 't': <Thread(Thread-1, started 139851187922688)>, '__file__': 'thread_test.py', 'time': <module 'time' from '/usr/lib64/python3.4/lib-dynload/time.cpython-34m.so'>, '__loader__': <_frozen_importlib.SourceFileLoader object at 0x7f31ac297048>, '__builtins__': <module 'builtins' (built-in)>}
do_import 3A
haha0
haha1
haha -1
do_import 1B 139851188124312 {'hello': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
do_import 2B 139851188124312 {'h': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
do_import 3B 139851188124312 {'h2': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
main -1
[xiaobai@xiaobai import_pitfall]$ 

让我们运行它：

[xiaobai@xiaobai import_pitfall]$ python thread_test.py 
main 1
main 2
do_import 1A
 main 3
haha0
haha1
main 4
do_import 2A
main 5
{'do_import1': <function do_import1 at 0x7f9d884760c8>, 'do_import3': <function do_import3 at 0x7f9d884a6758>, 'do_import2': <function do_import2 at 0x7f9d884a66e0>, '__builtins__': <module '__builtin__' (built-in)>, '__file__': 'thread_test.py', 't2': <Thread(Thread-2, started 140314429765376)>, '__package__': None, 'threading': <module 'threading' from '/usr/lib64/python2.7/threading.pyc'>, 't': <Thread(Thread-1, started 140314438158080)>, 'time': <module 'time' from '/usr/lib64/python2.7/lib-dynload/timemodule.so'>, '__name__': '__main__', '__doc__': None}
do_import 3A
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
^C('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
^C('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
^C^C('exception occur', NameError("global name 'undefined_func' is not defined",))
('haha time is ', 1439451183.753475)
... #Forever

[xiaobai@xiaobai import_pitfall]$ python3 thread_test.py 
main 1
main 2
do_import 1A
main 3
main 4
do_import 2A
main 5
{'t': <Thread(Thread-1, started 140404878976768)>, '__spec__': None, 'time': <module 'time' from '/usr/lib64/python3.4/lib-dynload/time.cpython-34m.so'>, '__cached__': None, '__loader__': <_frozen_importlib.SourceFileLoader object at 0x7fb296b87048>, 'do_import2': <function do_import2 at 0x7fb296ac56a8>, 'do_import1': <function do_import1 at 0x7fb296bb0bf8>, '__doc__': None, '__file__': 'thread_test.py', 'do_import3': <function do_import3 at 0x7fb28ef19f28>, 't2': <Thread(Thread-2, started 140404870584064)>, '__name__': '__main__', '__package__': None, '__builtins__': <module 'builtins' (built-in)>, 'threading': <module 'threading' from '/usr/lib64/python3.4/threading.py'>}
haha0
haha1
haha -1
do_import 1B 140404879178392 {'hello': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
do_import 2B 140404879178392 {'h': <module 'import_all_pitall.hello' from '/home/xiaobai/note/python/import_pitfall/import_all_pitall/hello.py'>}
Traceback (most recent call last):
  File "thread_test.py", line 31, in <module>
    print( "main 6", id(hello), locals() ) #"name 'hello' not defined" error even do_import1 was success
NameError: name 'hello' is not defined
[xiaobai@xiaobai import_pitfall]$ 

---

hello不是全局的，但为什么它可以由不同函数中的不同线程共享？为什么python不允许唯一的本地导入？为什么python会共享导入过程，而它会让所有其他线程毫无理由地等待，仅仅因为导入过程中有一个线程挂起？

我建议您在所有打印中打印threading.current\u thread.name和name您的线程。要理解是谁干的这件事就容易多了

仔细看，你从哪里进口2B和3B

Python当前正在加载模块，Python导入processus是线程安全的。这意味着两个线程不能同时加载模块。这不是关于处理时间。时间，而是关于锁定文件

我打印了id，发现他们都共享相同的id 140589697897480

是的，因为Python只加载一次模块。将Python模块视为单例

Hello不是全局的，但为什么它可以由不同函数中的不同线程共享

这是因为hello是指向共享模块的局部变量。如前所述，如果将模块视为单线程，然后将同一进程中线程之间的所有内存共享，那么单线程将与所有线程共享

正如很多人所说，这不是一个bug，而是一个特性：

这里是另一个例子。让我们考虑两个文件：main.py是执行的文件，而other.py是导入的文件

下面是main.py：

这是另一个

我使用日志记录是为了避免当两个线程试图同时在标准输出中写入时出现问题。下面是我使用python 2.7得到的结果：

Thread loading the module :  Thread import 1
INFO:root:I am Thread import 1 and who did the import job ? Thread import 1
INFO:root:Thread import 1
INFO:root:I am Thread import 2 and who did the import job ? Thread import 1

---

如您所见，该模块只导入一次。

回答其中一个问题-

我打印了这个id，发现它们都共享同一个id 140589697897480。因此，3个函数共享相同的导入对象/进程

是的，当您导入模块时，python导入模块对象并将其缓存在sys.modules中。然后，对于该模块的任何后续导入，python从sys.modules获取模块对象并返回该对象，它不会再次导入

关于同一问题的第二部分-

但这对我来说没有意义，我认为对象是函数的本地对象，因为如果我尝试在全局范围上打印导入的hello对象，它将抛出错误

好的，sys.modules不是本地的，但是是的，hello是函数的本地名称。如上所述，如果您再次尝试导入模块，python将首先查找sys.modules以查看它是否已被导入，并返回它是否包含模块，否则导入它并添加到sys.modules

对于第一个程序，当导入python模块时，它从顶层运行，在hello.py中有一个inifite循环，而1:，因为1始终为真。因此，进口永远不会结束

如果不希望无限循环运行，则应将导入模块时不希望运行的代码放入其中-

if __name__ == '__main__':

上面if语句中的代码只会运行，如果直接运行脚本，则在导入模块时不会运行

我猜你说-

在我注释掉hello.py中的“undefined_func Done haha”之后

---

实际上，您注释掉了完整的无限循环，因此导入成功。

当您说-“undefined_func Done haha”-之后hello代码到底是什么样子的？是的，那么infinte循环被删除，因此导入工作正常。我的回答应该能回答你的其他问题。有任何文件对此提出警告吗？i、 在一个线程中未完成的导入会使其他线程挂起。我找不到任何文档，但您可以查看问题的答案--它们清楚地说明python导入是线程安全的，因此这意味着在导入模块时，它首先获得锁，如果任何其他线程同时尝试导入模块，它将等待该锁。是否有任何文档对此发出警告？i、 一个线程中未完成的导入会使其他线程挂起。是的：谢谢。但文件却非常模糊。它没有指出sys.modules，导入相同的模块，导入过程是共享的。甚至不存在于

if __name__ == '__main__':