Find index of last occurrence of a substring in a string

You can use rfind() or rindex()
^{Python2 links: rfind() rindex()}

>>> s = 'Hello StackOverflow Hi everybody'

>>> print( s.rfind('H') )
20

>>> print( s.rindex('H') )
20

>>> print( s.rfind('other') )
-1

>>> print( s.rindex('other') )
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: substring not found

The difference is when the substring is not found, rfind() returns -1 while rindex() raises an exception ValueError (Python2 link: ValueError).

If you do not want to check the rfind() return code -1, you may prefer rindex() that will provide an understandable error message. Else you may search for minutes where the unexpected value -1 is coming from within your code...

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Example: Search of last newline character

>>> txt = '''first line
... second line
... third line'''

>>> txt.rfind('\n')
22

>>> txt.rindex('\n')
22

Use .rfind():

>>> s = 'hello'
>>> s.rfind('l')
3

Also don't use str as variable name or you'll shadow the built-in str().