这几天开始学Python了，为加深印象，我把学过的怕忘的知识整理记录下来。
学习Python用的教材是《Python语言程序设计（英文版）》。
练习代码用的编译器是Python3.5（64-bit）。
第一条语句：
print("Welcome to Python")
Python的句尾都是没有标点符号(punctuation)的。
此外，Python是一门格外注意缩进的语言。在体内的句子可以用tab和space来进行缩进，但要按照一定的规则，否则就会报错。
目前我遇到的有关缩进的报错有这几种：
1. TabError: inconsistent use of tabs and spaces in indentation
错误原因：tab和space混用不当，造成缩进问题。我试了一下，貌似用体内用tab缩进后再用space缩进不会报错，但若先用space再用tab则可能造成该错误。
2. SyntaxError: invalid syntax
错误原因：语法错误，一行的最前方如果是...表明还在体内，此时若要写体外语句，则需要按下回车，最前方为>>>时则可以另起一体。
3. IndentationError: unexpected indent
错误原因：不需要缩进时缩进
4. IndentationError: expected an indented block
错误原因：该缩进时没缩进
turtle()
turtle.write("turtle is very interesting") #turtle writes down "turtle is very interesting" from the present position
turtle.forward(100) #turtle goes forward 100 pixels in its present direction
turtle.color("red") #the line turtle draws becomes red
turtle.right(90)
turtle.left(45) #turtle turns right 90 degrees or turns left 45 degrees
turtle.penup() # turtle moves without track
turtle.goto(50,100) #the primitive position of turtle is (0,0), and 50 is the xcor of turtle and 100 is the ycor of turtle
turtle.pendown() #turtle returns to the paper and will move with track
turtle.done() #
turtle.pensize(3) # assign the size of line
turtle.begin_fill()
...
turtle.end_fill() # the closed graph between begin_fill() and end_fill() will be filled with the color of turtle
turtle.circle(50) #turtle draws a circle with radius 50.
turtle.circle(50,steps=3) # draws a closed graph with 3 equivalent edges. When the steps become bigger, the graph resembles circle more.
turtle.speed(10) # the drawing speed ranging 1-10, 10 is the fastest, and 1 is the slowest
turtle.undo()
turtle.screensize(w,h) # w is width and h is height of the screen
turtle.clear() # clear the screen content but don't change the position and status of turtle
turtle.reset() # clear the screen content and reset turtle to the initiative status
input & output:
variable = input("please input a number") #variable is a string
variable1 = eval(input("please input a number")) # variable1 is a number
variable2 = eval(variable)
>>>print(input("please input a number")) #assume input number 8
'8'
>>>print(eval(input("please input a number"))) #assume input number 8
8
print("The float average of ",variable1," and ",variable2," is ",(variable1+variable2)/2)
print("The integer average of ",variable1," and ",variable2," is ",(variable1+variable2)//2)
>>>print("AAA",end='&&&')
AAA&&&
Exchange:
x,y=y,x # Fantastic! Thanks to the highly encapsulation of Python we can easily exchange the value of two variables.
Time:
import time
currentTime=time.time()
ord and chr function:
ch='a'
>>>ord(ch)
97
>>>chr(98)
'b'
Random()
import random
random.random() # generate a random float ranging [0,1)
random.randint(a,b) # generate an integer ranging [a,b]
random.randrange(a,b) # generate an integer ranging [a,b)
If:
if boolean-expression:
statement
elif boolean-expression:
statement
else:
statement
猫和老鼠游戏：
import time
import turtle
boxsize=200
caught=False
score=0
window=turtle.Screen()
mouse=turtle.Turtle()
cat=turtle.Turtle()
def checkbound():
global boxsize
if mouse.xcor()>boxsize:
mouse.goto(boxsize,mouse.ycor())
if mouse.xcor()<-boxsize:
mouse.goto(-boxsize,mouse.ycor())
if mouse.ycor()>boxsize:
mouse.goto(mouse.xcor(),boxsize)
if mouse.ycor<-boxsize:
mouse.goto(mouse.xcor(),-boxsize)
def up():
mouse.forward(10)
checkbound()
def left():
mouse.left(45)
def right():
mouse.right(45)
def back():
mouse.backward(10)
checkbound()
def quitTurtle():
window.bye()
mouse.penup()
mouse.goto(100,100)
window.onkeypress(up,"Up")
window.onkeypress(left,"Left")
window.onkeypress(right,"Right")
window.onkeypress(back,"Down")
window.onkeypress(quitTurtle,"Escape")
difficulty=window.numinput("难度","请输入游戏的难度（1~5）")
window.listen()
while not caught:
cat.setheading(cat.towards(mouse))
cat.forward(8+difficulty)
score+=1
if cat.distance(mouse)<5:
caught=True
time.sleep(0.2-(0.01*difficulty))
window.textinput("Game Over","游戏得分："+str(score*difficulty))
window.bye()