BeautifulSoup4, How to get an HTML tag with specific class.

[Broadsworde]

I have HTML code like the following from a URL:
<img class="this" alt="this" src="this_source1.gif">
<img class="this" alt="this" src="this_source2.gif">
<img class="this" alt="this" src="this_source3.gif">
<img class="this and that" alt="not this" src="this__and_that_source1.gif">
<img class="this and that" alt="not this" src="this__and_that_source2.gif">
<img class="this and that" alt="not this" src="this__and_that_source3.gif">

I'm trying to get the alt value of just the img tags with only class="this"

import requests
from bs4 import BeautifulSoup
url = "https://someurl.com"
resp = requests.get(url)
txt = resp.text
soup = BeautifulSoup(txt, 'lxml')
imgThis = soup.find_all('img', class_='this')
for i in (imgThis):
	imgThis[i]['alt']

The find_all method returns alts for both class_="this" and class_="this and that"

How do I specify only to return class_="this"?