The substr() method extracts a part of a string. The substr() method begins at a specified position, and returns a specified number of characters. The substr() method does not change the original string. To extract characters from the end of the string, use a negative start position.

The easiest and most effective way to see if a string contains a substring is by using if ... in statements, which return True if the substring is detected. Alternatively, by using the find() function, it's possible to get the index that a substring starts at, or -1 if Python can't find the substring.

A string's substr() method extracts length characters from the string, counting from the start index. If start >= str.length , an empty string is returned. If start < 0 , the index starts counting from the end of the string. More formally, in this case the substring starts at max(start + str.length, 0) .