Odd Python / C Library interaction

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Odd Python / C Library interaction - embedka - Oct-09-2019

I'm running a script from a Linux terminal with Python 3.6.8 and the script started failing when I tried to expand it with a function definition. I whittled it down to the basics and found that the device fails to connect when there is a function definition followed by a print statement in the code, but not when there's a print statement followed by a function definition. It doesn't even matter what's in the print and whether or not the function definition is used.

This code successfully connects to (and disconnects from) the device:

import DeviceInterface

device_class = DeviceInterface.Device()

print()
def dummy_function_that_does_nothing():
    pass

with device_class:
    pass

This code, which swaps the function definition and print statement, gives a device connection error:

import DeviceInterface

device_class = DeviceInterface.Device()

def dummy_function_that_does_nothing():
    pass
print()

with device_class:
    pass

These examples are the exact file contents of the script being run (nothing added or omitted for this post). The DeviceInterface module is a ctypes wrapper around a C-based .so library. That library is not mine and I don't have rights to distribute it, but it's using Aravis v0.6.4. The connection failure is caused by a null pointer being returned from a call to arv_camera_new().

I would expect no difference between the 2 versions of code above. There seems to be something deeper going on in Python or Linux libraries that I don't understand. I suspect a memory issue in the library, but I do not understand how and why Python is exciting the error by changing what I thought was a meaningless part of the code.

Other things I've tried

I've inserted delays in various places, but none had an effect on whether the device successfully connected, so it doesn't seem to be a timing issue as I originally suspected.
I tried running both versions a number of times, and the problem has been very repeatably linked to the order of the function definition and the print statement (as opposed to being able to randomly connect).
If I remove the print statement entirely, it succeeds regardless of where I put the function definition.
I thought it might have to do with garbage collection killing a socket. I tried disabling the garbage collection with gc.disable() at the start of the script, but it didn't change the behavior.

This code, which adds an additional function definition, successfully connects:

import DeviceInterface

device_class = DeviceInterface.Device()

def dummy_function_that_does_nothing():
    pass
print()
def dummy_function_that_does_nothing_again():
    pass

with device_class:
    pass

This code, which adds an additional function definition and another print statement, fails to connect:

import DeviceInterface

device_class = DeviceInterface.Device()

def dummy_function_that_does_nothing():
    pass
print()
def dummy_function_that_does_nothing_again():
    pass
print()

with device_class:
    pass

Changing the print statement to print(flush=True) or print(sys.stderr) did not change the functionality. However, print(end="") caused the problem to go away.
Running python with unbuffered stdin/stdout/stderr (python3 -u odd_behavior_test.py) caused the failure to go away.