Python Study (7) – Statements

Introduction

Programming statements are used to expresse action or assign a variable in program.Each statement  has own syntax and purpose. We can combine these statements to achieve specific purpose.

Assignment Statements

Assingment statement is used to set reference of object or value stored in monery address by a variable name.

(1) normal assignment : fruit = “apple”

(2) sequence assignment:  a,b = 'Jack', 'John'

(3) List assignment :

(4) Augmented assignment:
     >>>a =1
     >>>a+=1

(5) Sequence assignment

• Sequence one to one

>>>seq = [1, 2, 3, 4] >>> a,b,c,d = seq >>> a,b = seq  #seq have 4 elements, but assignemnt elements just have 2 Traceback (most recent call last):   File "<pyshell#9>", line 1, in <module>     a,b = seq ValueError: too many values to unpack (expected 2) >>> a,b,c,d = seq >>>a 1

• Sequence one and rest

>>>seq = [1, 2, 3, 4] >>> e, *f = seq[0:3]    #e get first element, other element assign f >>> e 1 >>> f [2, 3] >>> *g, h = seq[0:3]  # >>> g [1, 2] >>> h 3 >>> i, *j, k = seq[0:4] >>> i 1 >>> j [2, 3] >>> k 4

• Boundary cases

>>>seq = [1, 2, 3, 4] >>>

(6) Mulitple-target assignment
     There has many assignment variable.

>>> a = b =0 >>> b = b + 1 >>> a,b (0, 1) >>> a =b =[] >>> b.append(10) ([10], [10]) >>> a=[] >>> b= [] >>> a.append(21) ([],[42])

 

Control Statements

1.if/elif/else statment

• Syntax

if expression :       statement1 elif:        statement2 else:       statement3

• Example
  (1) There are a fruit list , we check apple if it is in the list.

fruit = ["apple","banana"] if "apple" in fruit:     print("found it!) else:     print("don’t found it!") -------------------------------------- found it !

       (2) Jack

grade = {"Jack":90, "Joe":60, "Bryant":50,  "Hacker":30} if grade["Hacker"]>=90:     print("perfect!") elif grade["Hacker"]<=90 and grade["Hacker"] >=60 :     print("great") elif grade["Hacker"]<60:     print("hard") -------------------------------------- hard

2. for/else

• Syntax

for var inm sequence:     statment1

       (1) 

list1 = {1,2,3,4,5} for a in list1 :     print(a) -------------------------------------- 1 2 3 4 5

3.while/else

• Syntax

while( expression):      statment1

• Exception

while True:     inputNumebr = input("Enter a numebr:")     if inputNumebr== "stop":break         print(int(inputNumebr)) -----------------------------------------------------------------------------------------Enter a numebr:1234 1234 Enter a numebr:stop

Note : you input a number 


Handling Errors

In python when the programe is executed how to handle it and throw message to notify operator.

• Syntax

try:     statement01 except:     statement02

• Exception

while True:     inputNumebr = input("Enter a numebr:")     if inputNumebr== "stop":break     try:         print(int(inputNumebr))     except:         print("Please input a number!") -----------------------------------------------------------------------------------------Enter a numebr:1234 1234 Enter a numebr:stop

Note : When  input is not a  number , it will enter into except section and    show “Please input a number”