Scratch and Python 2018 Spring - Python Lecture 2

Flow Control

• Flow chart
• Boolean Values
  • True
  • False
  • true and false are legal names for variables.
• Comparison operators
  • ==
    • float is not accurate.
  • !=
  • <
  • <=
  • >
  • >=
  • Compare values of different types
    • string versus int
    • int versus float
• Truthy and falsey values
  • Truthy values
    • Non-zero integers
    • Non-zero float
    • Non-empty string
  • Falsey values
    • 0
    • 0.0
    • ''
  • The are more truthy and falsey values.
• Boolean operators
  • and
  • or
  • not
  • Precedence: not > and > or
  • Note: the results of the above operations might not be one of True and False. They may result in truthy value and falsey value.
• Conditions: a Boolean expression
• Block of code Colored structure
  • Selection
    • if
    • if-else
    • if-elif
    • if-elif-else
    • example
      ​​​​​​​​​​​​x = int(input("請輸入成績(0~100) : "))
      ​​​​​​​​​​​​if x > 100 or x < 0:
      ​​​​​​​​​​​​    print('不要騙喔! OAO')
      ​​​​​​​​​​​​elif x >= 90:
      ​​​​​​​​​​​​    print('Nice!!')
      ​​​​​​​​​​​​elif x >= 60:
      ​​​​​​​​​​​​    print('Pass')
      ​​​​​​​​​​​​else:
      ​​​​​​​​​​​​    print('好課值得一修再修3修重修畢業後繼續修')
      

  • Iteration
    • while loop
      • Similar to if but go back to while for the next iteration after the loop body executed, not for the next statement.
    • break statement
      • Skip the following codes in the same block and end the loop.
    • Sample code 1
      ​​​​​​​​​​​​## Sample 1
      ​​​​​​​​​​​​while True:
      ​​​​​​​​​​​​    x = input('助教帥不帥? (y/n)')
      ​​​​​​​​​​​​    if x != 'y':
      ​​​​​​​​​​​​        print('你確定?再給你一次機會')
      ​​​​​​​​​​​​    else:
      ​​​​​​​​​​​​        break
      ​​​​​​​​​​​​print('謝謝誇獎')
      

    • for loop
      • for item in bag: is to pull the next item out of the bag.
      • bag can be a string, a tuple, a list, and other iterables.
        • 'a','b','c' are items in the bag 'abc'.
        • x,y are items in the bag (x,y).
        • 1,2,987 are items in the bag [1,2,987].
        • 0,1,2, …, n-1 are items in the bag range(n).
        • m,m+1,m+2, …, n-1 are items in the bag range(m,n).
    • continue statement
      • Skip the following codes in the same block and start the next iteration.
    • Sample code 2
      ​​​​​​​​​​​​## Sample 2
      ​​​​​​​​​​​​print('數質數 1 ~ 20')
      ​​​​​​​​​​​​prime = [2, 3, 5, 7, 11, 13, 17, 19]
      
      ​​​​​​​​​​​​for num in range(1, 21):
      ​​​​​​​​​​​​    is_prime = False
      ​​​​​​​​​​​​    for p in prime:
      ​​​​​​​​​​​​        if num == p:
      ​​​​​​​​​​​​            is_prime = True
      ​​​​​​​​​​​​            break
      ​​​​​​​​​​​​    if is_prime == False:
      ​​​​​​​​​​​​        continue
      ​​​​​​​​​​​​    print(num)
      

  • Function definition
    • def: define a custom function
    • return
      • return x: the value of the result is x
      • return: the value of the result is None
        • None 是一個 NoneType 型態的值，而 NoneType 只有 None 一種值，代表沒有。None是個 falsey value。
    • Sample code
      ​​​​​​​​​​​​#========Function========#
      ​​​​​​​​​​​​def findmax(arr):
      ​​​​​​​​​​​​    ans = arr[0]
      ​​​​​​​​​​​​    for tmp in arr:
      ​​​​​​​​​​​​        if ans < tmp:
      ​​​​​​​​​​​​            ans = tmp
      ​​​​​​​​​​​​    return ans
      ​​​​​​​​​​​​#========Function========#	
      
      ​​​​​​​​​​​​array = [0, 2, 9, 3, 5, 6, 7]
      ​​​​​​​​​​​​print(findmax(array))
      

  • Exception handling
    • try-except
    • example
    ​​​​​​​​x = input("做除法, 請輸入2個數字,用空白隔開\n").split()
    ​​​​​​​​try:
    ​​​​​​​​    print(int(x[0]) / int(x[1]))
    ​​​​​​​​except:
    ​​​​​​​​    print('除數不得為0')
    

• Python 3 language reference

Debugger

參考資料：Spyder Debugger Tutorial

作業四

• Draw Rokumonsen
• Draw Ichimonsen (一文銭)
• Draw Sanjurokumonsen (三十六文銭)

pyautogui: open your eyes!

• Sample code 1

  ​​​​import pyautogui, time
  
  ​​​​def report():
  ​​​​    pos = pyautogui.position()
  ​​​​    color = pyautogui.screenshot().getpixel(pos)
  ​​​​    print('Mouse position:',pos,'Color:',color)
  
  ​​​​while True:
  ​​​​    report()
  ​​​​    time.sleep(0.1)
  

• screenshot()

  • Pixel
    • screenshot().getpixel()
    • (R,G,B)
  • screenshot().im is iterable
    • Pixels are balls, and screenshot() is a bag containing balls.
  • screenshot().save('pic.png') 存檔到 pic.png。

• How to take a screen shot

  • Windows: print screen key
    • Hold alt, then press print screen
      • This gives you a picture of window on focus
    • Use Microsoft Paint to save the screenshot into png file

• Sample code

  ​​​​import pyautogui, time
  
  ​​​​pyautogui.FAILSAFE = True
  
  ​​​​loc = pyautogui.locateOnScreen('box.png')       # 偵測螢幕上是否有與 box.png 相同的圖片，若有 loc 將會被指派相同於圖片的地方的位置，若沒有 loc 會被指派為 None
  ​​​​
  ​​​​while loc == None:                              # 如果 loc 是 None (剛剛螢幕上沒找到相同的圖片) / 如果有找到，會直接跳過這個迴圈
  ​​​​    time.sleep(0.1)                             # 停 0.1 秒
  ​​​​    loc = pyautogui.locateOnScreen('box.png')   # 再找一次
  
  ​​​​for loc_i in pyautogui.locateAllOnScreen('box.png'):
  ​​​​    print(loc_i)
  ​​​​    center = pyautogui.center(loc_i)
  ​​​​    pyautogui.click(center)
  

• locateOnScreen(pic)

  • 在螢幕上尋找與 pic 相同的圖片，若找到一個，則回傳位置(4-element tuple (left,top,width,height))並停止尋找。
  • 若都沒有找到，則回傳 None。

• locateAllOnScreen(pic)

  • 在螢幕上尋找 全部 與 pic 相同的圖片，吐出一個generator。

作業五 Check all boxes!

Practice here

Issues on screenshot

• screenshot is slow
• locateOnScreen is very slow
• Hard to recognize objects

Fast Screenshot

Save the following code as fastscreenshot.py at your desktop.

import time
from PIL import Image
from mss import mss

# Fast screenshot
# mss 3.0 ver
# Usage just like pyautogui
# Parameter:
#     region: optional, four-integer tuple (left, top, width, height)
def screenshot(region=None, **kwargs):
    im = None
    monitors = None
    if region == None:
        region = kwargs.get('region')
    
    with mss() as sct:

        # Region to capture
        monitor = sct.monitors[1]
        if region != None:
            monitor['left'] = int(region[0])
            monitor['top'] = int(region[1])
            monitor['width'] = int(region[2])
            monitor['height'] = int(region[3])

        # Get pixels on image
        sct_img = sct.grab(monitor)
        im = Image.frombytes('RGBA', sct_img.size, bytes(sct_img.raw), 'raw', 'BGRA')
        im = im.convert('RGB')

    return im

# Evaluate screenshot time in seconds
def evaluate_screenshot_time(region=None):
    t = time.time()
    im = screenshot(region)
    return time.time() - t

Then, save the following as test.py at your desktop.

import time
import pyautogui
from pyautogui import screenshot
# from fastscreenshot import screenshot

def report_time():
    t = time.time()
    left, top, width, height = 5, 66, 77, 8
    screenshot(region=(left,top,width,height))
    print(time.time()-t)

for i in range(100):
    report_time()

If fastscreenshot.py does not work, use the following.

import time
from PIL import Image
from mss import mss

# Fast screenshot
# Usage just like pyautogui
# Parameter:
#     region: optional, four-integer tuple (left, top, width, height)
def screenshot(region=None,**kwargs):
    im = None
    monitors = None
    if region == None:
        region = kwargs.get('region')
    
    with mss() as sct:
        # Retrieve monitors informations:
        monitors = sct.enum_display_monitors()

        # Region to capture
        monitor = dict(monitors[1])
        if region != None:
            monitor['left'] = int(region[0])
            monitor['top'] = int(region[1])
            monitor['width'] = (int(region[2]) // 80 + int(region[2] % 80 != 0)) * 80
            monitor['height'] = int(region[3])

        # Get pixels on image
        sct.get_pixels(monitor)
        im = Image.frombytes('RGB', (sct.width, sct.height), sct.image)

    if region != None:
        if monitor.get('width') != region[2]:
            im = im.crop((0, 0, region[2], region[3]))

    return im


# Evaluate screenshot time in seconds
def evaluate_screenshot_time(region=None):
    t = time.time()
    im = screenshot(region)
    return time.time() - t

Type

• All values have their own types.
• Use type(v) to check the type of value v.
• All defined name (identifier) have their values.
• Use type(name) to check the type of name.

Function

• def your_function():
• return
  • The function is terminated either by return or by the end of the block.
  • None: if there is no return or just return nothing
• def your_function_with_parameter(parameter)
  • Argument
    ​​​​​​​​def square(x):
    ​​​​​​​​    return x**2
    ​​​​​​​​    
    ​​​​​​​​def square_root(x):
    ​​​​​​​​    return x**0.5
    

• def your_function_with_parameters(parameter1, parameter2)
  • More arguments
    ​​​​​​​​def distance(x1,y1,x2,y2):
    ​​​​​​​​    delta_x = x1-x2
    ​​​​​​​​    delta_y = y1-y2
    ​​​​​​​​    dx2 = square(delta_x)
    ​​​​​​​​    dy2 = square(delta_y)
    ​​​​​​​​    return square_root(dx2+dy2)
    

• def your_function_with_parameters(*args)
  • Variable length list of arguments can be passed by args which is a tuple.
  • More flexible
    ​​​​​​​​def square_var_len(*x):
    ​​​​​​​​    print(type(x))
    ​​​​​​​​    print(len(x))
    ​​​​​​​​    ret = tuple()
    ​​​​​​​​    for v in x:
    ​​​​​​​​        ret = ret + (v**2,)
    ​​​​​​​​    return ret
    

  • Access data via index i: args[i]
• def your_function_with_parameters(**kwargs)
  • Keyword arguments
  • print(some_str, end='')
  • print(some_str_1, some_str_2, sep=',')
  • Access data via key string
    • kwargs['end']
    • kwargs['sep']
  • ​​​​​​​​def square_kw(x,**kwargs):
    ​​​​​​​​    if kwargs.get('test'):
    ​​​​​​​​        return 'test'
    ​​​​​​​​    return x**2
    

• Please do not overwrite the arguments in your function now. We will discuss what will happen if you do so later.
• Scope
  • Sample code
    ​​​​​​​​a_global_var = 'A global variable'
    
    ​​​​​​​​def f(arg,*args,**kwargs):
    ​​​​​​​​    print(arg)
    ​​​​​​​​    print('args:',args)
    ​​​​​​​​    print('kwargs:',kwargs)
    ​​​​​​​​    local = 'A local variable'
    ​​​​​​​​    print('local:',local)
    ​​​​​​​​    print('a_global_var:',a_global_var)
    
    ​​​​​​​​f('hi','hi','ni',hihi='hihihi')
    

  • Variables are defined by assignment statements
  • Global
    • Defined in global scope
    • global
      • global some_var: some_var in this function is the global some_var
      • You must do this if you're going to write global variables
  • Local
    • Defined in local scope
  • Principles
    • Local variables cannot be used globally.
    • A local scope cannot access local variables in other scopes.
    • A local variable and a global variable may have the same name, but only local variable can be accessed.
    • You may still read the global variable correctly if no local variable is using the same name.

Task - Debug it!

MZ wants to write a program which will report the color under the mouse pointer until a special color (139,0,0) is touched. But the following does not work, please fix it.

import pyautogui, time

color = None

def report():
    loc = pyautogui.position()
    color = pyautogui.screenshot().getpixel(loc)
    print("position is:",loc,"color is",color)

while True:
    report()
    time.sleep(0.5)
    if color == (139, 0, 0):
        break

There are several approaches to do this. Each of the following hints will leads to an answer.

• Return value might be useful.
• global

Generator

• Defined by a def block containing yield
• Override return
• Sample code
  ​​​​​def squares(n):
  ​​​​​    for i in range(n+1):
  ​​​​​        yield i**2
  
  ​​​​​print('squares is',type(squares))
  ​​​​​print('squares() is',type(squares(5)))
  ​​​​​for i in squares(5):
  ​​​​​    print(i)
  

• Sample code 2
  ​​​​​def f():
  ​​​​​    yield 1
  ​​​​​    yield 2
  ​​​​​    yield 3
  ​​​​​    return 5
  
  ​​​​​for i in f():
  ​​​​​    print(i)
  

• Sample code 3
  ​​​​​def primes(n):
  ​​​​​    if n < 2:
  ​​​​​        return
  ​​​​​    n_is_prime = True
  ​​​​​    for i in primes(n-1):
  ​​​​​        yield i
  ​​​​​        if n % i == 0:
  ​​​​​            n_is_prime = False
  ​​​​​    if n_is_prime:
  ​​​​​        yield n
  
  ​​​​​for i in primes(100):
  ​​​​​    print(i)