Document the use of max_delay and somehow make it possible to use it without inspecting the library source

[nicholasd-ff]

$ python --version 
Python 3.10.12
$ $ pip3 show pyrate_limiter
Name: pyrate-limiter
Version: 3.4.1
Summary: Python Rate-Limiter using Leaky-Bucket Algorithm
Home-page: https://github.com/vutran1710/PyrateLimiter
Author: vutr
Author-email: [email protected]
License: MIT
Location: /home/nicholasd-ff/.local/share/virtualenvs/sbert-ingest-lambda-AE7HPjkv/lib/python3.10/site-packages
Requires: 
Required-by: 

Hi, thanks for all your work on pyrate limiter. I've been testing my code after upgrading from the 2 series to 3.4.1 and have run into some issues with max delay.

In the previous major version of the library it was easy to create a requester that just blocked until it was safe to make a new request, now it seems like max_delay must be set in order to block and setting max_delay requires a formula based on the maximum wait period and knowledge of the internals of the engine. (specifically that 50ms is added to the max_delay so you can't just set it to 1000ms if you want 1 request a second.

For example:

#!/usr/bin/env python3
from __future__ import annotations
from pyrate_limiter import Duration, Limiter, Rate
import time
def make_reqs(l: Limiter):
  start = time.time()
  for i in range(3):
    print(f"\tAttempt: {i} Success: {l.try_acquire('some-bucket')} at T {time.time() - start:.2f}s")

print("With raise_when_fail=False, no max_delay")
make_reqs(Limiter(Rate(1, Duration.SECOND), raise_when_fail=False))
print("With raise_when_fail=False, max_delay=1000")
make_reqs(Limiter(Rate(1, Duration.SECOND), raise_when_fail=False, max_delay=1000))
print("With raise_when_fail=False, max_delay=1500")
make_reqs(Limiter(Rate(1, Duration.SECOND), raise_when_fail=False, max_delay=1500))

Produces the following output:

With raise_when_fail=False, no max_delay
        Attempt: 0 Success: True at T 0.00s
        Attempt: 1 Success: False at T 0.00s
        Attempt: 2 Success: False at T 0.00s
With raise_when_fail=False, max_delay=1000
        Attempt: 0 Success: True at T 0.00s
Required delay too large: actual=1050, expected=1000
        Attempt: 1 Success: False at T 0.00s
Required delay too large: actual=1050, expected=1000
        Attempt: 2 Success: False at T 0.00s
With raise_when_fail=False, max_delay=1500
        Attempt: 0 Success: True at T 0.00s
        Attempt: 1 Success: True at T 1.05s
        Attempt: 2 Success: True at T 2.10s

With a set of rates (e..g 1 request /second 500/hour) it becomes quite complicated to calculate max_delay such that the API actually blocks, otherwise it's just immediately going to return false and you have to implement your own loop to retry.

[vutran1710]

Agree, I will work on it soon

[nicholasd-ff]

I'm happy to contribute a PR and tests if you can explain how you think it should work, the options that make sense to me are:

• sleep for bucket.waiting(item) if raise_when_fail is False and max_wait is unset.
• downgrade the logger.error "Required delay too large" to a warning and sleep for bucket.waiting(item) but only if raise_on_fail is False.

But I admit I haven't thought through all the corner cases this behaviour might produce.

[vutran1710]

Sound good to me.
Testing the delay & wait should be easy using in-memory-bucket i think