polling

Polling is a powerful python utility used to wait for a function to return a certain expected condition. Some possible uses cases include:

• Wait for API response to return with code 200
• Wait for a file to exist (or not exist)
• Wait for a thread lock on a resource to expire

Installation

pip install polling

Examples

Example: Poll every minute until a url returns 200 status code

import requests
polling.poll(
    lambda: requests.get('http://google.com').status_code == 200,
    step=60,
    poll_forever=True)

If you are creating a new cloud provider instance (e.g. waiting for an EC2 instance to come online), you can continue to poll despite getting ConnectionErrors:

import requests
polling.poll(
    lambda: requests.get('your.instance.ip').status_code == 200,
    step=60,
    ignore_exceptions=(requests.exceptions.ConnectionError,),
    poll_forever=True)

Example: Poll for a file to exist

# This call will wait until the file exists, checking every 0.1 seconds and stopping after 3 seconds have elapsed
file_handle = polling.poll(
    lambda: open('/tmp/myfile.txt'),
    ignore_exceptions=(IOError,),
    timeout=3,
    step=0.1)

# Polling will return the value of your polling function, so you can now interact with it
file_handle.close()

Example: Polling for Selenium WebDriver elements

from selenium import webdriver
driver = webdriver.Firefox()

driver.get('http://google.com')
search_box = polling.poll(
    lambda: driver.find_element_by_id('search'),
    step=0.5,
    timeout=7)

search_box.send_keys('python polling')

Example: Using the polling timeout exception

# An exception will be raised by the polling function on timeout (or the maximum number of calls is exceeded).
# This exception will have a 'values' attribute. This is a queue with all values that did not meet the condition.
# You can access them in the except block.

import random
try:
    polling.poll(lambda: random.choice([0, (), False]), step=0.5, timeout=1)
except polling.TimeoutException, te:
    while not te.values.empty():
        # Print all of the values that did not meet the exception
        print te.values.get()

Example: Using a custom condition callback function

import requests

def is_correct_response(response):
    """Check that the response returned 'success'"""
    return response == 'success'

polling.poll(
    lambda: requests.put('http://mysite.com/api/user', data={'username': 'Jill'},
    check_success=is_correct_response,
    step=1,
    timeout=10)

Release notes

0.3.0

• Support Python 3.4+

0.2.0

• Allow users to access a "last" attribute on the exceptions. This should hold the last evaluated value, which is the more common use case than getting the first value.
• Fix a bug that actually ran 1 more time than value specified by max_tries

0.1.0

• First version