1. 兩個字串可以用 addtion operator 相加連接在一起
2. 字串和數字在 python 語法中 不能相加 (其它語言可以)
3. string 是immutable (不可改變的)

形式語言理論（特別是編程語言），字串串接（Concatenation ），又稱字串相加、連接、序連、串連、相連，指將兩個字串的首尾相接的操作

mystring1 = "Hello, everyone !\\n"
mystring2 = "My name is Albert "
result = mystring1 + mystring2
print(result)

output -------------------------------------------------------------------
[Running] python -u "d:\\2022_python_全攻略\\15. String Concatenation"
Hello everyone!
My name is Albert

#string + number Error 

mystring1 = "Hello team everyone"
myluckynumber = 777
result = mystring1 + myluckynumber
print(result)

--------------------------------------------------
[Running] python -u "d:\\2022_python_全攻略\\15. String Concatenation"
Traceback (most recent call last):
  File "d:\\2022_python_\\u5168\\u653b\\u7565\\15. String Concatenation", line 9, in <module>
    result = mystring1 + myluckynumber
***TypeError: can only concatenate str (not "int") to str***

#string is immutable 

mystring1 = "Hello"
mystring1[0] = "h"
print(mystring1)
---------------------------------------------------------

[Running] python -u "d:\\2022_python_全攻略\\15. String Concatenation"
Traceback (most recent call last):
  File "d:\\2022_python_\\u5168\\u653b\\u7565\\15. String Concatenation", line 14, in <module>
    mystring1[0] = "h"
***TypeError: 'str' object does not support item assignment***