Is class variable initialization from an imported module thread safe in Python?

user_8485028 0
Viewed 169

I am aware of the fact that class variables initialization happens at the time of the import. If that is the case, in the code below, does the class variable initialization of class A result in race conditions?

a.py:

class A:
    # Static code here...
    pass

b.py:

class B:
    def func(self):
        from a import A

c.py:

import threading
from b import B

b1 = B()
b2 = B()

t1 = threading.Thread(target=b1.func)
t2 = threading.Thread(target=b2.func)

t1.start()
t2.start()

t1.join()
t2.join()

Terminal:

$ python c.py
class-variables thread-safety python-3.x multithreading python
2 Answers

there's a thread lock on the C side in Cpython implementation of importlib, but just to be sure, I ran the following script:

imported file:

import time
print('importing, in imported file')
time.sleep(2) # drop the GIL
print('finished importing, in imported file')

importing file:

import threading

def import_other_file():
    print('going to import')
    import ex12
    print('finished importing')

t1 = threading.Thread(target=import_other_file)
t2 = threading.Thread(target=import_other_file)

t1.start()
t2.start()

t1.join()
t2.join()

output:

going to import
going to import
importing, in imported file
finished importing, in imported file
finished importing
finished importing

does the class variable initialization of class A result in race conditions?

so NO, there will be no race condition due to the lock on the C side.

user_156492300

I believe there is no data race here (assuming you are using CPython or a similar implementation) due to the Global Interpreter Lock. Only one thread can execute Python bytecode at any given time.

However which of the two threads actually ends up importing A first cannot (or at least should not) be relied upon.

user_32824360
Related