isupper ()	Returns True if the string is in uppercase.	>>> str1='welcome' >>>print(str1.isupper())       False
upper()	Returns the exact copy of the string with all letters in uppercase.	>>>str1='welcome' >>>print(str.upper())       WELCOME

lower()	Returns the exact copy of the string with all the letters in lowercase.	>>> str1='SAVE EARTH' >>> print(str1.lower())            save eath
islower()	Returns True if the string is in lowercase.	>>> str1='welcome' >>> print (str1.islower())            True

find(sub[,start[, end]])

The function is used to search the first occurrence of the sub string in the given string. It returns the index at which the substring starts. It returns -1 if the substring does not occur the string.

>>> str1='mammals' >>> str1.find('ma')                0 On omitting the start parameters, the function starts the search from the beginning. >>> str1.find('ma',2)                3 >>> str1.find('ma',2,4)               -1 Displays -1 because the substring could no be found between the index 2 and 4-1. >>> str1.find('ma', 2,5)               3

3, 1,4, 2

When the slicing operation, you can specify a third argument as the stride, which refers to the number of characters to move forward after the first character is retrieved from the string. The default value of stride is 1.

strl ="COMPUTER"
index = 0
for i in strl:
print (strl [:index+ 1])
index+=I

General format of slice operation:
str[start:end]
Where start is the beginning index and end is the last index value of a character in the string. Python takes the end value less than one from the actual index specified.

(i) T
(ii) THIRU
(iii) THIRU
(iv) KURAL

strl = input ("Enter your name")
print (strl * 10)

(i) Once you define a string, python allocate an index value for its each character.
(ii) These index values are otherwise called as subscript which are used to access and manipulate the strings. The subscript can be positive or negative integer numbers.

(i) Slice is a substring of a main string. A substring can be taken from the original string by using [ ]. operator and index or subscript values. 
(ii) Thus, [ ] is also known as slicing operator.Using slice operator, you have to slice one or more substrings from a main string.
General format of slice operation :
Str [start : end]

Output : School School School

Python will not allow deleting a particular character in a string. Whereas you can remove entire string variable using del command.
Example :
>>> strl="How about you"
>>> print (str1)
        How about you
>>> del str1
>>> print (str1)
Traceback (most recent call last):
         File "", line 1, in
                 print (strl)
NameError : name 'str1' is not defined

Strings in python are immutable. That means, once you define a string moditications or deletion is not allowed. However, we can replace the existing string entirely with the new string.

(i) String is a data type in python, which is used to handle array of characters.
(ii) String is a sequence of Unicode characters that may be a combination of letters, numbers, or special symbols enclosed within single, double or even triple quotes.
(iii) Example :
Welcome to learning Python"
"Welcome to learning Python"
""Welcome to learning Python""