String searching

Determine if a given substring is contained in a target string.

Validating user input for a required substring Parsing text to see if a marker appears before processing Filtering lists of strings based on the presence of a keyword

Text processing scripts Data cleaning pipelines Web scraping utilities

if text.rfind(substr) != -1:

if ... .rfind(...) != -1:

text: the string to be searched; substr: the substring to look for

• .find()
• .index()
• startswith()
• endswith()
• 'in' operator

• if substr in text:
• if text.find(substr) != -1:
• using re.search

Confusing rfind with find (which gives first occurrence) Using == -1 to test for presence Assuming rfind returns a boolean

str.find() returns first occurrence; str.index() raises ValueError if not found; 'in' operator offers a more readable presence check

Coming from Java/JavaScript: lastIndexOf returns -1 just like rfind, so the pattern translates directly — but remember rfind searches from the right. Coming from languages with boolean contains() methods: may expect rfind to return True/False — it returns an integer index, requiring the explicit != -1 comparison.

• string search idioms
• substring search
• find/rfind patterns

rfind scans from the end; if only existence is needed, the 'in' operator is faster and clearer. Use rfind when you also need the position of the last occurrence for further slicing or manipulation.

Enables efficient substring presence checks using rfind, allowing conditional logic based on substring presence.

Like scanning a book from the back cover to see if a word appears; if you find a match, the word is somewhere in the text.

Note that rfind scans from the end and returns -1 when not found; using != -1 is necessary because -1 is falsy in Python but an explicit check avoids confusion with index 0.

Prefer the `in` operator for readability: `if sub in s:`