Analysis of grammar elements in turtle Library

Python Of turtle Library is an intuitive and interesting graphics rendering function library .turtle（ Sea turtle ） The concept of graphic drawing was born in 1969 year , And successfully applied to LOGO programing language . because turtle The concept of graphic drawing is very valuable and popular ,Python Accepted the concept , Formed a Python Of turtle library , And become one of the standard libraries .

One 、 Drawing coordinate system

turtle The graphics drawn by the library have a basic framework ： A little turtle crawls in the coordinate system , Its crawling track forms a drawing pattern . For little turtles , Yes “ Forward “、” back off ”、“ rotate ” Wait for crawling behavior , The exploration of the coordinate system is also through “ The way forward ”、“ Backward direction ”、“ Left direction ” and “ Right side direction ” Wait for the little turtle to complete its own angle and orientation . At the beginning of drawing , The little turtle is in the middle of the canvas , The coordinates here are （0,0）, The direction of travel is horizontal to the right . for example , Use the following code to draw the coordinate system shown in the figure .

turtle.setup(650,350,200,200)

This code uses turtle.setup（） function , This function is used as follows ：

turtle.setup(width,height,startx,starty)

effect ： Set the size and position of the main form .

The parameters are as follows ：

width： Window width , If the value is an integer , Represents the pixel value ; If the value is decimal , Represents the ratio of window width to screen .

height： Window height , If the value is an integer , Represents the pixel value ; If the value is decimal , Represents the ratio of window height to screen .

startx： The pixel distance between the left side of the window and the left side of the screen , If the value is None, The window is located in the horizontal center of the screen .

starty： The pixel distance between the top of the window and the top of the screen , If the value is None, The window is located in the vertical center of the screen .

Two 、 Brush control function

1、turtle.penup() and turtle.pendown() function

        turtle The paintbrush in （ Little Turtle ） Can be controlled by a set of functions ,turtle.penup() and turtle.pendown() It's a group. , They said to lift the brush and drop it respectively , The function is defined as follows ：

turtle.penup()

Alias turtle.pu(),turtle.up()

effect ： Lift up the brush. , After that, moving the brush will not draw graphics .

Parameters ： nothing .

turtle.pendown()

Alias turtle.pd(),turtle.down()

effect ： Drop the brush , Then moving the brush will draw the figure .

Parameters ： nothing .

2、turtle.pensize() function

turtle.pensize() Function to set the brush size , The function is defined as follows ：

turtle.pensize(width)

Alias turtle.width()

effect ： Set brush width , Returns the current brush width when there is no parameter input .

The parameters are as follows ：

width： Set the brush line width , If the value is None Or empty , The function returns the current brush width .

3、turtle.pencolor() function

turtle.pencolor() Function to set the color of the brush . The function is defined as follows ：

turtle.pencolor(colorstring) or turtle.pencolor(（r,g,b）)

effect ： Set brush color , Returns the current brush color when there is no parameter input .

The parameters are as follows ：

colorstring： A string representing a color , for example ,“purple”、“red”、“blue” etc. .

(r,g,b)： Color for RGB The number , for example （51,204,140）.

        quite a lot RGB Colors have fixed English names , These English names can be used as colorstring Input to turtle.pencolor() Function , You can also use (r,g,b) Enter the color value directly in the form . Here are some typical RGB Color ：

English name	RGB	Hexadecimal	Chinese name
white	255 255 255	#FFFFFF	white
black	0 0 0	#000000	black
grey	190 190 190	#BEBEBE	gray
darkgreen	0 100 0	#006400	Dark green
gold	255 215 0	#FFD700	golden
violet	238 130 238	#EE82EE	Violet
purple	160 32 240	#A020F0	violet

3、 ... and 、 Shape drawing function

1、turtle.fd() function

        turtle Through a set of functions to control the travel action of the brush , Then draw the shape .turtle.fd() Function is most commonly used to control the brush to advance a distance in the current direction of travel , The function is defined as follows ：

turtle.fd(distance)

Alias

turtle.forwardd(distance)

effect ： Advance one in the current direction of the little turtle distance distance .

The parameters are as follows ：

distance： Pixel value of travel distance , When the value is negative , It means moving in the opposite direction .

example 2.1 in 5、14、16 Line code ：
 

#e2.1DrawPython.py
import turtle
turtle.setup(650,350,200,200)
turtle.penup()
turtle.fd(-250)
turtle.pendown()
turtle.pensize(25)
turtle.pencolor("purple")
turtle.seth(-40)
for i in range(4):
    turtle.circle(40,80)
    turtle.circle(-40,80)
turtle.circle(40, 80/2)
turtle.fd(40)
turtle.circle(16,180)
turtle.fd(40 * 2/3)

respectively ： Travel a distance in the current direction or the opposite direction of the brush , Then draw a straight line . The first 5 Line code matches the 4 Line lift brush function , therefore , It will not draw a straight line , Instead, move the brush to a certain position .

2、turtle.seth() function

turtle.seth() Function to change the drawing direction of the brush , The function is defined as follows ：

turtle.seth(to_angle)

Alias

turtle.setheading(to_angle)

effect ： Set the current travel direction of the little turtle to to_angle, This angle is the absolute direction angle value .

The parameters are as follows ：

to_angle： The integer value of the angle .

As shown in the figure turtle The angular coordinate system of the Library , for turtle.seth() And so on .

         It should be noted that ,turtle  The angular coordinate system of the library takes the east direction as the absolute 0 degree , This is also the initial crawling direction of the little turtle , Due west is absolute  180  degree , This directional coordinate system is the absolute directional system of the direction , It has nothing to do with the little turtle crawling in the current direction . therefore , This absolute coordinate system can be used to change the direction of the little turtle at any time .
         The sample code  2.1 pass the civil examinations 9 Set the direction of the little turtle to -40 degree , namely 320 degree , Then the little turtle moved in this direction .

3、for  Loop statements and  turtle.circle（） function

for i in range(4):
    turtle.circle(40,80)
    turtle.circle(-40,80)
turtle.circle(40, 80/2)
turtle.circle(16,180)

         Due to indentation , The sample code  2.1  pass the civil examinations 10、11、12 Line is a reserved word  for  The whole of guidance , This is another circular structure , be called “ Traversal cycle ”.for  The circulation format of language direction is as follows ：
for i in range(< cycles >）：
< Sentence block  1>
         The sample code 2.1  pass the civil examinations 10 Yes for The loop represents the second 11、12 Lines of code execute continuously  4 Time .
turtle.circle（） Function is used to draw an arc , The function is defined as follows ：
turtle.circle (radius, extent=None)
effect ： According to radius radius  draw extent  Arc of angle .

The parameters are as follows .
radius： Arc radius , When the value is positive , The radius is to the left of the little turtle , When the value is negative , The radius is to the right of the little turtle .
extent：  The angle at which the arc is drawn , When the parameter is not set or the parameter is set to  None  when , Draw the entire circle .
The sample code  2.1  Each western number in the middle includes the angle value 、 The radius value and other parameters are determined according to the style adjustment of the drawing content .