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

If you don't mind using numpy, you can simply use the concatenate function from there, see the following code. Attention: I use OpenCV 4.0.0 where the order of the return values of findContours is different.

import cv2
import numpy as np

# Input image
input = cv2.imread('images/kchZb.png', cv2.IMREAD_GRAYSCALE)

# Modify input image to extract "original" image
_, input = cv2.threshold(input[10:400, 40:580], 224, 255, cv2.THRESH_BINARY)

# Find contours
cnts, _ = cv2.findContours(input, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

# Concatenate all contours
cnts = np.concatenate(cnts)

# Determine and draw bounding rectangle
x, y, w, h = cv2.boundingRect(cnts)
cv2.rectangle(input, (x, y), (x + w - 1, y + h - 1), 255, 2)

# Output image
cv2.imwrite('images/output.png', input)
cv2.imshow('Input', input)
cv2.waitKey(0)

Disclaimer: I'm new to Python in general, and specially to the Python API of OpenCV (C++ for the win). Comments, improvements, highlighting Python no-gos are highly welcome!