I ran into this while backporting some code from 3.6 to 3.5 and 3.4. The following piece of code prints one line every second, as expected, on 3.6 and 3.5, but it doesn't on 3.4. It looks like the signal is able to wake up the main thread from sleeping:

```
import time
import signal


def handle(signum, mask):
    print("received signal at", time.time())


signal.setitimer(signal.ITIMER_REAL, 1, 1)
signal.signal(signal.SIGALRM, handle)

for _ in range(2):
    now = time.time()
    delta = 1 - (time.time() - int(time.time()))
    time.sleep(delta)
    print(now, delta, time.time())
```

If I remove the timer, then everything works as expected on 3.4.

Here is some sample output from 3.6:

```
1510913832.4726589 0.5273411273956299 1510913833.000339
received signal at 1510913833.477888
1510913833.000449 0.9995510578155518 1510913834.000187
```

And some from 3.4:

```
1510913813.525479 0.4745209217071533 1510913814.004106
received signal at 1510913814.529313
1510913814.004233 0.9957659244537354 1510913814.529413
```

I forgot to mention that I am on macOS Sierra (10.12.6) and I ran the code on Python 3.4.6, 3.5.3 and 3.6.3, respectively.

I'm sorry but Python 3.4 doesn't accept bugfixes anymore. Same for Python 3.5.
https://devguide.python.org/#status-of-python-branches

> The following piece of code prints one line every second, as expected, on 3.6 and 3.5

Good, at least Python behaves as you want :-) Maybe it's an enhancement caused by the PEP 475.

Fair enough. From reading PEP 475, it does look like that's what fixed it. Thanks Victor!