Recently at my workplace our IT team finally upgraded our distributed Python versions to 3.5.0. While this is a huge upgrade from 2.6, this still came with some growing pains. Unfortunately, Python 3.5.0 doesn’t meet some of the minimum requirements of some popular libraries, including . aiohttp With these restrictions I still needed to write a script that could pull hundreds of .csv files from our APIs and manipulate the data. Python by itself isn’t event-driven and natively asynchronous (like NodeJS), but the same effect can still be achieved. This article will help detail what I learned while also showing the benefits of asynchronous operations. If you have a higher version of Python available (3.5.2+), I highly recommend using aiohttp instead. It’s an incredibly robust library and a great solution for this kind of problem. There are many tutorials online detailing how best to use the library. Disclaimer: Assumptions This article makes the following assumptions: You already have familiarity with Python and most of its syntaxYou already have familiarity with basic web requestsYou have a lose concept of asynchronous operations If you’re just looking for the solution, scroll down to the bottom and the full code is posted. Enjoy! Setup Before getting started, ensure that you have installed on your machine. The easiest way to install is by typing the following command into your terminal: requests $ python -m pip install requests Alternatively, if you don’t have administrative permissions you can install the library with this command: $ python -m pip install requests --user The wrong approach: synchronous requests To demonstrate the benefits of our parallel approach, let’s first look at approaching the problem in a synchronous manner. I’ll also give an overview of what’s going on in the code. Ultimately, we want to do able to perform a GET request to the URL containing the .csv file and measure the time it takes to read the text inside. We’ll be downloading multiple .csv files of varying sizes from /, which provides plenty of data for our example. https://people.sc.fsu.edu/~jburkardt/data/csv As a disclaimer, we’ll be using the Session object from the requests library to perform our GET request. First, we’ll need a function that executes the web request:c base_url = session.get(base_url + csv) response:
        data = response.text response.status_code != :
            print( .format(url)) data : def fetch (session, csv) "https://people.sc.fsu.edu/~jburkardt/data/csv/" with as if 200 "FAILURE::{0}" # Return .csv data for future consumption return View code gist here This function takes in a object and the name of the .csv file desired, performs the web request, then returns the text inside the response. Session Next, we need a function that can efficiently loop a list of our desired files and measure the time it takes to perform the request: timeit default_timer() csvs_to_fetch = [ , , , , , , , , , , , ] requests.Session() session:
        print( .format( , )) total_start_time = default_timer() csv csvs_to_fetch:
            fetch(session, csv)
            elapsed = default_timer() - total_start_time
            time_completed_at = .format(elapsed)
            print( .format(csv, time_completed_at)) from import : def get_data_synchronous () "ford_escort.csv" "cities.csv" "hw_25000.csv" "mlb_teams_2012.csv" "nile.csv" "homes.csv" "hooke.csv" "lead_shot.csv" "news_decline.csv" "snakes_count_10000.csv" "trees.csv" "zillow.csv" with as "{0:<30} {1:>20}" "File" "Completed at" # Set any session parameters here before calling `fetch` # For instance, if you needed to set Headers or Authentication # this can be done before starting the loop for in "{:5.2f}s" "{0:<30} {1:>20}" View code gist here This function creates our object and then loops through each .csv file in the list. Once the operation is completed, the measured time is calculated and displayed in an easy-to-read format. Session csvs_to_fetch fetch Finally, our function will be simple (for now) and call our function: main get_data_synchronous()

main() : def main () # Simple for now View code gist here Once we put it all together, here is what the code looks like for our synchronous example: requests timeit default_timer base_url = session.get(base_url + csv) response:
        data = response.text response.status_code != :
            print( .format(url)) data csvs_to_fetch = [ , , , , , , , , , , , ] requests.Session() session:
        print( .format( , )) total_start_time = default_timer() csv csvs_to_fetch:
            fetch(session, csv)
            elapsed = default_timer() - total_start_time
            time_completed_at = .format(elapsed)
            print( .format(csv, time_completed_at)) get_data_synchronous()

main() import from import : def fetch (session, csv) "https://people.sc.fsu.edu/~jburkardt/data/csv/" with as if 200 "FAILURE::{0}" # Return .csv data for future consumption return : def get_data_synchronous () "ford_escort.csv" "cities.csv" "hw_25000.csv" "mlb_teams_2012.csv" "nile.csv" "homes.csv" "hooke.csv" "lead_shot.csv" "news_decline.csv" "snakes_count_10000.csv" "trees.csv" "zillow.csv" with as "{0:<30} {1:>20}" "File" "Completed at" # Set any session parameters here before calling `fetch` # For instance, if you needed to set Headers or Authentication # this can be done before starting the loop for in "{:5.2f}s" "{0:<30} {1:>20}" : def main () # Simple for now View code gist here Let’s take a look at the results when we run this script: Synchronous example. Notice how each operation doesn’t start until the last one is completed Thankfully, we can vastly improve this performance with Python 3’s built-in library! asyncio The right approach: performing multiple requests at once asynchronously In order to get this to work, we’ll have to rework some of our existing functions. Beginning with : fetch requests timeit default_timer START_TIME = default_timer() base_url = session.get(base_url + csv) response:
        data = response.text response.status_code != :
            print( .format(url)) elapsed = default_timer() - START_TIME
        time_completed_at = .format(elapsed)
        print( .format(csv, time_completed_at)) data import from import # We'll need access to this variable later : def fetch (session, csv) "https://people.sc.fsu.edu/~jburkardt/data/csv/" with as if 200 "FAILURE::{0}" # Now we will print how long it took to complete the operation from the # `fetch` function itself "{:5.2f}s" "{0:<30} {1:>20}" return View code gist here Next, we need to make our function asynchronous: get_data asyncio timeit default_timer concurrent.futures ThreadPoolExecutor csvs_to_fetch = [ , , , , , , , , , , , ]
    print( .format( , )) ThreadPoolExecutor(max_workers= ) executor: requests.Session() session: loop = asyncio.get_event_loop() START_TIME = default_timer() tasks = [
                loop.run_in_executor(
                    executor,
                    fetch,
                    *(session, csv) ) csv csvs_to_fetch
            ] response asyncio.gather(*tasks): import from import from import async : def get_data_asynchronous () "ford_escort.csv" "cities.csv" "hw_25000.csv" "mlb_teams_2012.csv" "nile.csv" "homes.csv" "hooke.csv" "lead_shot.csv" "news_decline.csv" "snakes_count_10000.csv" "trees.csv" "zillow.csv" "{0:<30} {1:>20}" "File" "Completed at" # max_workers is set to 10 simply for this example, Note: # you'll have to tweak with this number for your own projects # as you see fit with 10 as with as # Set any session parameters here before calling `fetch` # Initialize the event loop # Set the START_TIME for the `fetch` function # Use list comprehension to create a list of # tasks to complete. The executor will run the `fetch` # function for each csv in the csvs_to_fetch list # Allows us to pass in multiple arguments to `fetch` for in # Initializes the tasks to run and awaits their results for in await pass View code gist here This code will now create multiple threads for each .csv file and execute the function for each that needs to be downloaded. fetch Finally, our function needs a small tweak to properly initialize our async function: main loop = asyncio.get_event_loop()
    future = asyncio.ensure_future(get_data_asynchronous())
    loop.run_until_complete(future)

main() : def main () View code gist here Now, let’s run the new code and see the results: Asynchronous example. Notice how the files are not being obtained in order. With this small change, all 12 of these .csv files were able to be downloaded in 3.43s vs 10.84s. That is a nearly 70% decrease in the time it took to download! The Asynchronous Code requests asyncio concurrent.futures ThreadPoolExecutor timeit default_timer

START_TIME = default_timer() base_url = session.get(base_url + csv) response:
        data = response.text response.status_code != :
            print( .format(url))

        elapsed = default_timer() - START_TIME
        time_completed_at = .format(elapsed)
        print( .format(csv, time_completed_at)) data csvs_to_fetch = [ , , , , , , , , , , , ]
    print( .format( , )) ThreadPoolExecutor(max_workers= ) executor: requests.Session() session: loop = asyncio.get_event_loop()
            START_TIME = default_timer()
            tasks = [
                loop.run_in_executor(
                    executor,
                    fetch,
                    *(session, csv) ) csv csvs_to_fetch
            ] response asyncio.gather(*tasks): loop = asyncio.get_event_loop()
    future = asyncio.ensure_future(get_data_asynchronous())
    loop.run_until_complete(future)

main() import import from import from import : def fetch (session, csv) "https://people.sc.fsu.edu/~jburkardt/data/csv/" with as if 200 "FAILURE::{0}" "{:5.2f}s" "{0:<30} {1:>20}" return async : def get_data_asynchronous () "ford_escort.csv" "cities.csv" "hw_25000.csv" "mlb_teams_2012.csv" "nile.csv" "homes.csv" "hooke.csv" "lead_shot.csv" "news_decline.csv" "snakes_count_10000.csv" "trees.csv" "zillow.csv" "{0:<30} {1:>20}" "File" "Completed at" with 10 as with as # Set any session parameters here before calling `fetch` # Allows us to pass in multiple arguments to `fetch` for in for in await pass : def main () View code gist here I hope you enjoyed this article and can use these skills for any projects that require an older Python version (or maybe without as many dependencies). Although Python may not have a straightforward to an async / await pattern, it isn’t difficult at all to achieve fantastic results. Enjoy!

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How to run asynchronous web requests in parallel with Python 3.5 (without aiohttp)

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