Lazy-load zoneinfo to improve startup time

[angusholder]

Motivation

In Django, ORM model classes are instantiated immediately at startup, so any heavy operations in their init path slow down your manage.py CLI startup time. Currently, instantiating a TimeZoneField indirectly calls zoneinfo.available_timezones() twice (via ZoneInfoBackend), which performs a lot of IO and parsing. On my machines this adds 25-35ms to startup time. Not huge on its own, but Django startup times can easily build up with a few libraries being a little bit slow. I measured this with the following script:

import subprocess
import timeit


PYTHON = 'python3.12'

def baseline():
    subprocess.run([PYTHON, '-c', 'import zoneinfo'])

def slow():
    subprocess.run([PYTHON, '-c', 'import zoneinfo; _ = zoneinfo.available_timezones(); _ = zoneinfo.available_timezones()'])

if __name__ == '__main__':
    N = 20

    # Take the fastest reading of all N runs
    baseline_time = min(timeit.repeat(stmt=baseline, repeat=N, number=1))
    slow_time = min(timeit.repeat(stmt=slow, repeat=N, number=1))

    diff = slow_time - baseline_time

    print(f'baseline: {baseline_time * 1000:.3f}ms')
    print(f'slow: {slow_time * 1000:.3f}ms')
    print(f'Difference: {diff * 1000:.3f}ms')

Timing on Heztner CCX53

baseline: 34.565ms
slow: 69.756ms
Difference: 35.191ms

Timing on Apple M4 Pro

baseline: 36.593ms
slow: 62.454ms
Difference: 25.861ms

Solution

I fixed this by making the zoneinfo reads happen lazily. This required changing TimeZoneField's choices field to be a lazy object itself, which unfortunately did make the code a bit more complex.

I only touched the ZoneInfoBackend, and left PYTZBackend alone because I don't think even has this issue - a cursory look at its source shows it lists timezones directly in the Python file, so isn't performing IO.