How do I check if a string contains a substring in Python?

The in operator. It reads like English and it's the fastest way.

line = "2026-10-07 09:02 ERROR disk full on /var"

print("ERROR" in line)
print("WARNING" in line)

True
False

Ignoring upper and lower case

in is case-sensitive. Lower both sides first; casefold() is the stricter version of lower() for text in other languages, such as German ß:

subject = "URGENT: Invoice overdue"

print("urgent" in subject)
print("urgent" in subject.casefold())

False
True

Where is it?

find() returns the position, or -1 if it isn't there. (index() does the same but raises ValueError instead of returning -1.)

path = "/home/ada/projects/site/app.py"

print(path.find("projects"))
print(path.find("downloads"))
print(path.count("/"))

10
-1
5

Don't write if path.find("x"): as a yes/no check. A match at the very start returns 0, which is false, and a miss returns -1, which is true: exactly backwards. Use in for yes or no.

Any of several words

message = "Your parcel is out for delivery today"
keywords = ["refund", "delivery", "cancel"]

print(any(word in message for word in keywords))
print([word for word in keywords if word in message])

True
['delivery']

Whole words only

in matches inside words too: "cat" in "concatenate" is True. When that's wrong, use a regular expression with \b, which means "word boundary":

import re

for text in ["the cat sat", "concatenate", "Cat!"]:
    print(text, "->", bool(re.search(r"\bcat\b", text, re.IGNORECASE)))

the cat sat -> True
concatenate -> False
Cat! -> True

Starting or ending with something has its own methods, clearer than slicing: filename.endswith((".jpg", ".png")) and url.startswith("https://"). Both accept a tuple of options.