[Python] How to group contours and draw a single bounding rectangle

I need to group contours and draw a single bounding rectangle that encloses all the contours, something like this



from matplotlib import pyplot as plt
import cv2 as cv

img = cv.imread('shapes1.png', 0)
imgRGB = cv.cvtColor(img.copy(), cv.COLOR_GRAY2RGB)

_, ctrs, _ = cv.findContours(img, cv.RETR_EXTERNAL, cv.CHAIN_APPROX_SIMPLE)

boxes = []
for ctr in ctrs:
x, y, w, h = cv.boundingRect(ctr)
boxes.append([x, y, w, h])

for box in boxes:
top_left = (box[0], box[1])
bottom_right = (box[0] + box[2], box[1] + box[3])
cv.rectangle(imgRGB, top_left, bottom_right, (0,255,0), 2)

fig = plt.figure(figsize = (10, 10))
ax = fig.add_subplot(111)
ax.imshow(imgRGB, cmap='gray')




Is there any straight forward way to do it rather than merging all bounding rectangles programmatically

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