python – Page 15

Terminating a Python script

February 19, 2019

import sys
sys.exit()

os._exit(errorcode)

References
https://stackoverflow.com/questions/73663/terminating-a-python-script

Detect Ctrl+C in Python

February 6, 2019

#!/usr/bin/env python
import signal
import sys
def signal_handler(sig, frame):
        print('You pressed Ctrl+C!')
        sys.exit(0)
signal.signal(signal.SIGINT, signal_handler)
print('Press Ctrl+C')
signal.pause()

References
https://stackoverflow.com/questions/1112343/how-do-i-capture-sigint-in-python

Preventing Windows OS from sleeping while your python code runs

February 6, 2019

class WindowsInhibitor:
    '''Prevent OS sleep/hibernate in windows; code from:
    https://github.com/h3llrais3r/Deluge-PreventSuspendPlus/blob/master/preventsuspendplus/core.py
    API documentation:
    https://msdn.microsoft.com/en-us/library/windows/desktop/aa373208(v=vs.85).aspx'''
    ES_CONTINUOUS = 0x80000000
    ES_SYSTEM_REQUIRED = 0x00000001

    def __init__(self):
        pass

    def inhibit(self):
        import ctypes
        print("Preventing Windows from going to sleep")
        ctypes.windll.kernel32.SetThreadExecutionState(
            WindowsInhibitor.ES_CONTINUOUS | \
            WindowsInhibitor.ES_SYSTEM_REQUIRED)

    def uninhibit(self):
        import ctypes
        print("Allowing Windows to go to sleep")
        ctypes.windll.kernel32.SetThreadExecutionState(
            WindowsInhibitor.ES_CONTINUOUS)

import os

osSleep = None
# in Windows, prevent the OS from sleeping while we run
if os.name == 'nt':
    osSleep = WindowsInhibitor()
    osSleep.inhibit()

# do slow stuff

if osSleep:
    osSleep.uninhibit()

References
https://trialstravails.blogspot.com/2017/03/preventing-windows-os-from-sleeping.html

Create an Enum in Python

February 3, 2019

from enum import Enum
class Color(Enum):
     RED = 1
     GREEN = 2
     BLUE = 3

References
https://docs.python.org/3/library/enum.html

Calculate the difference between two datetime objects in Python

February 2, 2019

dateTimeDifference = dateTimeA - dateTimeB
# Divide difference in seconds by number of seconds in hour (3600)  
dateTimeDifferenceInHours = dateTimeDifference.total_seconds() / 3600

References
https://stackoverflow.com/questions/43305577/python-calculate-the-difference-between-two-datetime-time-objects/43308104

Python String Formatting

February 2, 2019

1 –

'Hello, {}'.format(name)

Output

'Hello, Bob'

2 –

'Hey {name}, there is a 0x{errno:x} error!'.format(name=name, errno=errno)

Output

'Hey Bob, there is a 0xbadc0ffee error!'

3 –

f'Hello, {name}!'

Output

'Hello, Bob!'

References
https://realpython.com/python-string-formatting/

Convert a PIL Image into a numpy array

February 1, 2019

pix = numpy.array(pic)

References
https://stackoverflow.com/questions/384759/how-to-convert-a-pil-image-into-a-numpy-array

Basic NumPy for working with OpenCV

January 29, 2019

Create an array from a list

my_list = [1, 2, 3]
my_array = np.array(my_list)

Create an array with arrange

my_array = np.arange(0, 10)
my_array2 = np.arange(0, 10, 2) # with step 2

Create an array with zeros

my_array = np.zeros((2, 3)) # default type if float
my_array2 = np.zeros((2, 3), np.int32) # with type int

Create an array with ones

my_array = np.ones((10, 8), np.int32)

Create an array from random

np.random.seed(101)
array1 = np.random.randint(0, 100, 10)
array2=np.random.randint(0,100,10)

Max, Min, Mean

np.random.seed(101)
array1 = np.random.randint(0, 100, 10)
print(array1.max())
print(array1.argmax()) # index of max value
print(array1.min())
print(array1.argmin()) # index of min value
print(array1.mean())

Indexing an array

matrix = np.arange(0, 100).reshape(10, 10)
print(matrix)

print(matrix[2,3])

row = 1
col = 4
print(matrix[row,col])

Manipulating an array

matrix = np.arange(0, 100).reshape(10, 10)
print(matrix)

matrix[0:3, 0:3] = 0
print(matrix)

Copying an array

matrix: np.ndarray = np.arange(0, 100).reshape(10, 10)
print(matrix)

matrix2 = matrix.copy()
matrix2[0:3, 0:3] = 0
print(matrix2)

Slicing an array

matrix = np.arange(0, 100).reshape(10, 10)

print(matrix[1, :])  # whole row 1
print(matrix[:, 1])  # whole col 1
print(matrix[1, 1:4])  # row 1 from col 1 to 4
print(matrix[1:4, 1])  # col 1 from row 1 to 4

new_matrix: np.ndarray = matrix[1, :]
print(new_matrix.reshape(5, 2))

Reshaping an array

np.random.seed(101)
array1 = np.random.randint(0, 100, 10)
print(array1)
print(array1.shape)

array2 = array1.reshape((2, 5))
print(array2)
print(array2.shape)

References
https://www.tutorialspoint.com/numpy/index.htm

Image Operations with OpenCV Python

January 28, 2019

Read Pixel

import cv2
import numpy as np
import pprint

image = cv2.imread('cow.jpg',cv2.IMREAD_COLOR)
pixel= image[55,55]
pprint.pprint(pixel)

Change Pixel

import cv2
import numpy as np
import pprint

image = cv2.imread('cow.jpg', cv2.IMREAD_COLOR)
image[55, 55] = [255, 255, 255]
pixel = image[55, 55]
pprint.pprint(pixel)

Change the color of Region of Image

import cv2
import numpy as np
import pprint

image = cv2.imread('cow.jpg', cv2.IMREAD_COLOR)
image[150:200, 100:150] = [255, 255, 255]
cv2.imshow("image", image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Image Characteristics

import cv2
import numpy as np
import pprint

image = cv2.imread('cow.jpg', cv2.IMREAD_COLOR)
print(image.shape)
print(image.size)
print(image.dtype)

Copy Paste Region

import cv2
import numpy as np
import pprint

image = cv2.imread('cow.jpg', cv2.IMREAD_COLOR)

watch_face = image[200:250,107:194]
image[0:50,0:87] = watch_face

cv2.imshow('image',image)
cv2.waitKey(0)
cv2.destroyAllWindows()

References
https://pythonprogramming.net/image-operations-python-opencv-tutorial/
https://www.youtube.com/watch?v=1pzk_DIL_wo&list=PLQVvvaa0QuDdttJXlLtAJxJetJcqmqlQq&index=4
https://github.com/mhdr/OpenCV/tree/master/004

Drawing and Writing on Image with OpenCV Python

January 28, 2019

Drawing Line

# Draw a diagonal blue line with thickness of 5 px
cv.line(img,(0,0),(511,511),(255,0,0),5)

Drawing Rectangle

cv.rectangle(img,(384,0),(510,128),(0,255,0),3)

Drawing Circle

cv.circle(img,(447,63), 63, (0,0,255), -1)

Drawing Ellipse

cv.ellipse(img,(256,256),(100,50),0,0,180,255,-1)

Drawing Polygon

pts = np.array([[10,5],[20,30],[70,20],[50,10]], np.int32)
pts = pts.reshape((-1,1,2))
cv.polylines(img,[pts],True,(0,255,255))

Adding Text to Images

font = cv.FONT_HERSHEY_SIMPLEX
cv.putText(img,'OpenCV',(10,500), font, 4,(255,255,255),2,cv.LINE_AA)

References
https://docs.opencv.org/4.0.1/dc/da5/tutorial_py_drawing_functions.html
https://www.youtube.com/watch?v=U6uIrq2eh_o&list=PLQVvvaa0QuDdttJXlLtAJxJetJcqmqlQq&index=3
https://pythonprogramming.net/drawing-writing-python-opencv-tutorial/
https://github.com/mhdr/OpenCV/tree/master/003