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I have a pandas dataframe and on each row, I would like to execute a function. However, the function includes I/O call to a remote server and thus it is very slow if I call it simply using .apply() to the dataframe.

Here is an example:

def f(data):
    r = requests.get(data["url"])
    x = process(r.content)
    y = process_2(r.content)
    z = process_3(r.content)
    print("Done")

    return [x, y, z]

df.apply(lambda x: f(x), axis=1)

In this code, the problem is requests.get(data["url"]) takes a while and thus the entire apply() function is very slow at finishing. The print() is printed on the console at a few seconds of interval.

Is it possible to execute the apply() function asynchronously and get the result faster? My dataframe has 5,000+ rows and the function call to each raw should take a few seconds.

Answers

Asynchronous I/O approach with well-known asyncio + aiohttp libraries:

Demonstrated on sample Dataframe and simple webpage content processing routines (to show the mechanics of the approach).
Let's say we need to count all header, link(<a>) and span tags through all urls and store the resulting counters in the source dataframe.

import pandas as pd
import asyncio
import aiohttp
from bs4 import BeautifulSoup


def count_headers(html):
    return len(list(html.select('h1,h2,h3,h4,h5,h6')))

def count_links(html):
    return len(list(html.find_all('a')))

def count_spans(html):
    return len(list(html.find_all('spans')))


df = pd.DataFrame({'id': [1, 2, 3], 'url': ['https://stackoverflow.com/questions',
                                            'https://facebook.com',
                                            'https://wiki.archlinux.org']})
df['head_c'], df['link_c'], df['span_c'] = [None, None, None]
# print(df)

async def process_url(df, url):
    async with aiohttp.ClientSession() as session:
        resp = await session.get(url)
        content = await resp.text()
        soup = BeautifulSoup(content, 'html.parser')
        headers_count = count_headers(soup)
        links_count = count_links(soup)
        spans_count = count_spans(soup)
        print("Done")

        df.loc[df['url'] == url, ['head_c', 'link_c', 'span_c']] = \
            [[headers_count, links_count, spans_count]]


async def main(df):
    await asyncio.gather(*[process_url(df, url) for url in df['url']])
    print(df)


loop = asyncio.get_event_loop()
loop.run_until_complete(main(df))
loop.close()

The output:

Done
Done
Done
   id                                  url  head_c  link_c  span_c
0   1  https://stackoverflow.com/questions      25     306       0
1   2                 https://facebook.com       3      55       0
2   3           https://wiki.archlinux.org      15      91       0

Enjoy the performance difference.