In Python, strings are an immutable sequence of characters.

find() & index()

We can search for the first occurrence of a certain character or substring within a string and return the index using the find() and index() methods.

weekend = 'Saturday Sunday'

print(weekend.find('Sun'))
# 9

print(weekend.index('Sun'))
# 9

The only difference between find() and index() is that, when the search string is not found, find() returns -1, while index() raises a ValueError.

weekend = 'Saturday Sunday'

print(weekend.find('Mon'))
# -1

print(weekend.index('Mon'))
# ValueError: substring not found

rfind() & rindex()

The related rfind() and rindex() methods work similarly, except they search for the first occurrence from the end rather than the beginning of the string:

weekend = 'Saturday Sunday'

print(weekend.rfind('ay'))
# 13

print(weekend.rindex('ay'))
# 13

startswith() & endswith()

We can also check for a certain character or substring at the beginning or end of a string using the startswith() and endswith() methods respectively:

weekend = 'Saturday Sunday'

print(weekend.startswith('T'))
# False

print(weekend.endswith('ay'))
# True

replace()

We can replace a given substring with a new string using the replace() method.

The replace() method replaces all occurrences of the input and returns a new string.

weekend = 'Saturday Sunday'

weekend_mod = weekend.replace('day', 'night')

print(weekend_mod)
# Saturnight Sunnight