Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 26 2018, 23:26:24) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "copyright", "credits" or "license()" for more information.
>>> s = "Hello, how are you?"
>>> s
'Hello, how are you?'
>>> len(s)
19
>>> s[0]
'H'
>>> s[1]
'e'
>>> s[1] = "a"
Traceback (most recent call last):
  File "<pyshell#5>", line 1, in <module>
    s[1] = "a"
TypeError: 'str' object does not support item assignment
>>> s
'Hello, how are you?'
>>> s + s
'Hello, how are you?Hello, how are you?'
>>> s[7:16]
'how are y'
>>> s[15]
'y'
>>> t = ""
>>> t
''
>>> u = " "
>>> u
' '
>>> len(t)
0
>>> len(u)
1
>>> len(s)
19
>>> def capi(s):
	n = len(s)
	t = ""
	for i in range(n):
		c = s[i].upper()
		t = t + c
	return(t)

>>> capi("Hello")
'HELLO'
>>> capi("Hello you!")
'HELLO YOU!'
>>> def lowi(s):
	n = len(s)
	t = ""
	for i in range(n):
		c = s[i].lower()
		t = t + c
	return(t)

>>> lowi("Hello! you!")
'hello! you!'
>>> s[0]
'H'
>>> s[0].isupper()
True
>>> s[0].islower()
False
>>> s[1].isupper()
False
>>> s
'Hello, how are you?'
>>> s[5].isupper()
False
>>> def myfunc(s):
	n = len(s)
	t = ""
	for i in range(n):
		if s[i].isupper():
			c = s[i].lower()
		else:
			c = s[i].upper()
		t = t + c
	return(t)

>>> myfunc("Hello!")
'hELLO!'
>>> v = "Hello, my old friend!"
>>> myfunc(v)
'hELLO, MY OLD FRIEND!'
>>>