Python OpenCV | cv2.putText () method

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OpenCV Python is a Python link library designed to solve computer vision problems. The cv2.putText () method is used to draw a text string on any image.

cv2 puttext

void cv::putText ( InputOutputArray img,
    const String & text,
    Point org,
    int fontFace,
    double fontScale,
    Scalar color,
    int thickness = 1,
    int lineType = LINE_8,
    bool bottomLeftOrigin = false
  )    

Syntax: cv2.putText(image, text, org, font, fontScale, color[, thickness[, lineType[, bottomLeftOrigin]]])

Parameters:
image: It is the image on which text is to be drawn.

text: Text string to be drawn.

org: It is the coordinates of the bottom-left corner of the text string in the image. The coordinates are represented as tuples of two values i.e. (X coordinate value, Y coordinate value).

font: It denotes the font type. Some of font types are FONT_HERSHEY_SIMPLEX, FONT_HERSHEY_PLAIN, , etc.

fontScale: Font scale factor that is multiplied by the font-specific base size.

color: It is the color of text string to be drawn. For BGR, we pass a tuple. eg: (255, 0, 0) for blue color.

thickness: It is the thickness of the line in px.

lineType: This is an optional parameter.It gives the type of the line to be used.

bottomLeftOrigin: This is an optional parameter. When it is true, the image data origin is at the bottom-left corner. Otherwise, it is at the top-left corner.

Return Value: It returns an image.

How to write text on a image in windows using Python OpenCV2?

StackOverflow question

I want to put some text on an Image.
I am writing the code as:

cv2.putText(image,"Hello World!!!", (x,y), cv2.CV_FONT_HERSHEY_SIMPLEX, 2, 255)

It gives ERROR, saying ’module’ object has no attribute ’CV_FONT_HERSHEY_SIMPLEX’

Query
Can’t I use the font type as above? I searched in internet, but found only the syntax related to to Opencv C++ for initFont.
Then I thought of using putText to pass the font type as parameter.
But it is not working for me.

Any suggestions?

Answer:

This code uses cv2.putText to overlay text on an image. You need NumPy and OpenCV installed.

import numpy as np
import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)

# Write some Text

font                   = cv2.FONT_HERSHEY_SIMPLEX
bottomLeftCornerOfText = (10,500)
fontScale              = 1
fontColor              = (255,255,255)
lineType               = 2

cv2.putText(img,’Hello World!’, 
    bottomLeftCornerOfText, 
    font, 
    fontScale,
    fontColor,
    lineType)

#Display the image
cv2.imshow("img",img)

#Save image
cv2.imwrite("out.jpg", img)

cv2.waitKey(0)

Write Text at the center of the image using Python cv2.puttext

If you know the shape (width, height) of the text you write on the image, you can put it in the center aligned on the image.

The approximate shape of the text in the example above is (268, 36). You may need to find the shape of a specific text using Paint or some other application.

import numpy as np
import cv2

image = cv2.imread(’sample.png’,cv2.IMREAD_UNCHANGED)

position = ((int) (image.shape[1]/2 - 268/2), (int) (image.shape[0]/2 - 36/2))

cv2.putText(
     image, #numpy array on which text is written
     "Python Examples", #text
     position, #position at which writing has to start
     cv2.FONT_HERSHEY_SIMPLEX, #font family
     1, #font size
     (209, 80, 0, 255), #font color
     3) #font stroke
cv2.imwrite(’output.png’, image)

Output image:

cv2 puttext

Python OpenCV puttext

cv2 puttext Example #1

def detect(imgfile):
    origimg = cv2.imread(imgfile)
    img = preprocess(origimg)
    
    img = img.astype(np.float32)
    img = img.transpose((2, 0, 1))

    net.blobs[’data’].data[...] = img
    out = net.forward() 
    box, conf, cls = postprocess(origimg, out)

    for i in range(len(box)):
       p1 = (box[i][0], box[i][1])
       p2 = (box[i][2], box[i][3])
       cv2.rectangle(origimg, p1, p2, (0,255,0))
       p3 = (max(p1[0], 15), max(p1[1], 15))
       title = "%s:%.2f" % (COCO_CLASSES[int(cls[i])], conf[i])
       cv2.putText(origimg, title, p3, cv2.FONT_ITALIC, 0.6, (0, 255, 0), 1)
    cv2.imshow("SSD", origimg)
 
    k = cv2.waitKey(0) & 0xff
        #Exit if ESC pressed
    if k == 27 : return False
    return True 

cv2.puttext Example #2

def drawBoundingBox(self,imgcv,result):
        for box in result:
            # print(box)
            x1,y1,x2,y2 = (box[’topleft’][’x’],box[’topleft’][’y’],box[’bottomright’][’x’],box[’bottomright’][’y’])
            conf = box[’confidence’]
            # print(conf)
            label = box[’label’]
            if conf < self.predictThresh:
                continue
            # print(x1,y1,x2,y2,conf,label)
            cv2.rectangle(imgcv,(x1,y1),(x2,y2),(0,255,0),6)
            labelSize=cv2.getTextSize(label,cv2.FONT_HERSHEY_COMPLEX,0.5,2)
            # print(’labelSize>>’,labelSize)
            _x1 = x1
            _y1 = y1#+int(labelSize[0][1]/2)
            _x2 = _x1+labelSize[0][0]
            _y2 = y1-int(labelSize[0][1])
            cv2.rectangle(imgcv,(_x1,_y1),(_x2,_y2),(0,255,0),cv2.FILLED)
            cv2.putText(imgcv,label,(x1,y1),cv2.FONT_HERSHEY_COMPLEX,0.5,(0,0,0),1)
        return imgcv 

cv2 puttext Example #3

def draw_labels(x, y, class_names=None):
    img = x.numpy()
    if img.ndim == 2 or img.shape[2] == 1:
        img = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
    boxes, classes = tf.split(y, (4, 1), axis=-1)
    classes = classes[..., 0]
    wh = np.flip(img.shape[0:2])
    min_wh = np.amin(wh)
    if min_wh <= 100:
        font_size = 0.5
    else:
        font_size = 1
    for i in range(len(boxes)):
        x1y1 = tuple((np.array(boxes[i][0:2]) * wh).astype(np.int32))
        x2y2 = tuple((np.array(boxes[i][2:4]) * wh).astype(np.int32))
        img = cv2.rectangle(img, x1y1, x2y2, (255, 0, 0), 1)
        if class_names:
            img = cv2.putText(img, class_names[classes[i]], x1y1, cv2.FONT_HERSHEY_COMPLEX_SMALL, font_size,
                              (0, 0, 255), 1)
        else:
            img = cv2.putText(img, str(classes[i]), x1y1, cv2.FONT_HERSHEY_COMPLEX_SMALL, 1, (0, 0, 255), 1)
    return img 

cv2.puttext Example #4

def draw_outputs(img, outputs, class_names=None):
    boxes, objectness, classes = outputs
    #boxes, objectness, classes = boxes[0], objectness[0], classes[0]
    wh = np.flip(img.shape[0:2])
    if img.ndim == 2 or img.shape[2] == 1:
        img = cv2.cvtColor(img, cv2.COLOR_GRAY2BGR)
    min_wh = np.amin(wh)
    if min_wh <= 100:
        font_size = 0.5
    else:
        font_size = 1
    for i in range(classes.shape[0]):
        x1y1 = tuple((np.array(boxes[i][0:2]) * wh).astype(np.int32))
        x2y2 = tuple((np.array(boxes[i][2:4]) * wh).astype(np.int32))
        img = cv2.rectangle(img, x1y1, x2y2, (255, 0, 0), 1)
        img = cv2.putText(img, ’{}’.format(int(classes[i])), x1y1, cv2.FONT_HERSHEY_COMPLEX_SMALL, font_size,
                          (0, 0, 255), 1)
    return img 

cv2 puttext Example #5

def detect(imgfile):
    origimg = cv2.imread(imgfile)
    img = preprocess(origimg)
    
    img = img.astype(np.float32)
    img = img.transpose((2, 0, 1))

    net.blobs[’data’].data[...] = img
    out = net.forward() 
    box, conf, cls = postprocess(origimg, out)

    for i in range(len(box)):
       p1 = (box[i][0], box[i][1])
       p2 = (box[i][2], box[i][3])
       cv2.rectangle(origimg, p1, p2, (0,255,0))
       p3 = (max(p1[0], 15), max(p1[1], 15))
       title = "%s:%.2f" % (CLASSES[int(cls[i])], conf[i])
       cv2.putText(origimg, title, p3, cv2.FONT_ITALIC, 0.6, (0, 255, 0), 1)
    cv2.imshow("SSD", origimg)
 
    k = cv2.waitKey(0) & 0xff
        #Exit if ESC pressed
    if k == 27 : return False
    return True 

cv2.puttext Example #6

def draw_boxes_frame(frame, frame_size, boxes_dicts, class_names, input_size):
  """Draws detected boxes in a video frame"""
  boxes_dict = boxes_dicts[0]
  resize_factor = (frame_size[0] / input_size[1], frame_size[1] / input_size[0])
  for cls in range(len(class_names)):
    boxes = boxes_dict[cls]
    color = (0, 0, 255)
    if np.size(boxes) != 0:
      for box in boxes:
        xy = box[:4]
        xy = [int(xy[i] * resize_factor[i % 2]) for i in range(4)]
        cv2.rectangle(frame, (xy[0], xy[1]), (xy[2], xy[3]), color[::-1], 2)
        (test_width, text_height), baseline = cv2.getTextSize(class_names[cls],
                                                              cv2.FONT_HERSHEY_SIMPLEX,
                                                              0.75, 1)
        cv2.rectangle(frame,
                      (xy[0], xy[1]),
                      (xy[0] + test_width, xy[1] - text_height - baseline),
                      color[::-1],
                      thickness=cv2.FILLED)
        cv2.putText(frame, class_names[cls], (xy[0], xy[1] - baseline), cv2.FONT_HERSHEY_SIMPLEX, 0.75, (0, 0, 0), 1) 

cv2 puttext Example #7

def ProcessFrame(self, frame):
        # segment arm region
        segment = self.SegmentArm(frame)

        # make a copy of the segmented image to draw on
        draw = cv2.cvtColor(segment, cv2.COLOR_GRAY2RGB)

        # draw some helpers for correctly placing hand
        cv2.circle(draw,(self.imgWidth/2,self.imgHeight/2),3,[255,102,0],2)       
        cv2.rectangle(draw, (self.imgWidth/3,self.imgHeight/3), (self.imgWidth*2/3, self.imgHeight*2/3), [255,102,0],2)

        # find the hull of the segmented area, and based on that find the
        # convexity defects
        [contours,defects] = self.FindHullDefects(segment)

        # detect the number of fingers depending on the contours and convexity defects
        # draw defects that belong to fingers green, others red
        [nofingers,draw] = self.DetectNumberFingers(contours, defects, draw)

        # print number of fingers on image
        cv2.putText(draw, str(nofingers), (30,30), cv2.FONT_HERSHEY_SIMPLEX, 1, (255,255,255))
        return draw 

cv2.puttext Example #8

def vis_class(img, pos, class_str, bg_color):
    """Visualizes the class."""
    font_color = cfg.VIS.SHOW_CLASS.COLOR
    font_scale = cfg.VIS.SHOW_CLASS.FONT_SCALE

    x0, y0 = int(pos[0]), int(pos[1])
    # Compute text size.
    txt = class_str
    font = cv2.FONT_HERSHEY_SIMPLEX
    ((txt_w, txt_h), _) = cv2.getTextSize(txt, font, font_scale, 1)
    # Place text background.
    back_tl = x0, y0 - int(1.3 * txt_h)
    back_br = x0 + txt_w, y0
    cv2.rectangle(img, back_tl, back_br, bg_color, -1)
    # Show text.
    txt_tl = x0, y0 - int(0.3 * txt_h)
    cv2.putText(img, txt, txt_tl, font, font_scale, font_color, lineType=cv2.LINE_AA)

    return img 

cv2 puttext Example #9

def plot_one_box(x, img, color=None, label=None, line_thickness=None):
    # Plots one bounding box on image img
    tl = line_thickness or round(0.002 * (img.shape[0] + img.shape[1]) / 2) + 1  # line thickness
    color = color or [random.randint(0, 255) for _ in range(3)]
    c1, c2 = (int(x[0]), int(x[1])), (int(x[2]), int(x[3]))
    cv2.rectangle(img, c1, c2, color, thickness=tl)
    if label:
        tf = max(tl - 1, 1)  # font thickness
        t_size = cv2.getTextSize(label, 0, fontScale=tl / 3, thickness=tf)[0]
        c2 = c1[0] + t_size[0], c1[1] - t_size[1] - 3
        cv2.rectangle(img, c1, c2, color, -1)  # filled
        cv2.putText(img, label, (c1[0], c1[1] - 2), 0, tl / 3, [225, 255, 255], thickness=tf, lineType=cv2.LINE_AA) 

cv2.puttext Example #10

def vis_det_and_mask(im, class_name, dets, masks, thresh=0.8):
    """Visual debugging of detections."""
    num_dets = np.minimum(10, dets.shape[0])
    colors_mask = random_colors(num_dets)
    colors_bbox = np.round(np.random.rand(num_dets, 3) * 255)
    # sort rois according to the coordinates, draw upper bbox first
    draw_mask = np.zeros(im.shape[:2], dtype=np.uint8)

    for i in range(1):
        bbox = tuple(int(np.round(x)) for x in dets[i, :4])
        mask = masks[i, :, :]
        full_mask = unmold_mask(mask, bbox, im.shape)

        score = dets[i, -1]
        if score > thresh:
            word_width = len(class_name)
            cv2.rectangle(im, bbox[0:2], bbox[2:4], colors_bbox[i], 2)
            cv2.rectangle(im, bbox[0:2], (bbox[0] + 18 + word_width*8, bbox[1]+15), colors_bbox[i], thickness=cv2.FILLED)
            apply_mask(im, full_mask, draw_mask, colors_mask[i], 0.5)
            draw_mask += full_mask
            cv2.putText(im, ’%s’ % (class_name), (bbox[0]+5, bbox[1] + 12), cv2.FONT_HERSHEY_PLAIN,
								1.0, (255,255,255), thickness=1)
    return im 

Displaying text over an image in OpenCV puttext

To overlay text on a frame, use the function:

  • putText( frame, text, coordinates,
  • font type, font scale,
  • color [, pen thickness [, linetype [, origin]]])
  • frame - the image on which we impose the text;
  • text - of course, the text that we are going to display in the frame;
  • coordinates - a tuple of two coordinates of the lower left corner of the text, for example (5,10);
  • font type - one of the constants listed below;
  • font scale - the font has a certain standard size, which is quite large. This option allows you to reduce or increase the font relative to the standard. For example, to increase by two times - we write 2, to decrease by 2 times - 0.5;
  • color - a tuple of three numbers from 0 to 255 that define the color in the RGB model. You need to remember that in this tuple, the colors go back to front: BGR. Blue color - (255,0,0);
  • pen thickness is an optional parameter;
  • line type — optional parameter, one of three values: LINE_8 small dotted line, LINE_4 large dotted line, LINE_AA smoothed line;
  • coordinate center is an optional parameter. By default, text coordinates are measured from the top left corner. If this parameter is True, then they will be from the bottom left corner.
mport cv2
import video

if __name__ == ’__main__’:
    cv2.namedWindow( "result" )

cap = video.create_capture(0)

color_yellow = (0,255,255)

while True:
    flag, img = cap.read()
    try:
        cv2.putText(img, "Hello world!", (20,20), cv2.FONT_HERSHEY_SIMPLEX, 1, color_yellow, 2)
        cv2.imshow(’result’, img)
    except:
        cap.release()
        raise
 
    ch = cv2.waitKey(5)
    if ch == 27:
        break

cap.release()
cv2.destroyAllWindows()

Python OpenCV | cv2.putText () method __del__: Questions

How can I make a time delay in Python?

5 answers

I would like to know how to put a time delay in a Python script.

2973

Answer #1

import time
time.sleep(5)   # Delays for 5 seconds. You can also use a float value.

Here is another example where something is run approximately once a minute:

import time
while True:
    print("This prints once a minute.")
    time.sleep(60) # Delay for 1 minute (60 seconds).

2973

Answer #2

You can use the sleep() function in the time module. It can take a float argument for sub-second resolution.

from time import sleep
sleep(0.1) # Time in seconds

How to delete a file or folder in Python?

5 answers

How do I delete a file or folder in Python?

2639

Answer #1


Path objects from the Python 3.4+ pathlib module also expose these instance methods:

Python OpenCV | cv2.putText () method __dict__: Questions

How do I merge two dictionaries in a single expression (taking union of dictionaries)?

5 answers

Carl Meyer By Carl Meyer

I have two Python dictionaries, and I want to write a single expression that returns these two dictionaries, merged (i.e. taking the union). The update() method would be what I need, if it returned its result instead of modifying a dictionary in-place.

>>> x = {"a": 1, "b": 2}
>>> y = {"b": 10, "c": 11}
>>> z = x.update(y)
>>> print(z)
None
>>> x
{"a": 1, "b": 10, "c": 11}

How can I get that final merged dictionary in z, not x?

(To be extra-clear, the last-one-wins conflict-handling of dict.update() is what I"m looking for as well.)

5839

Answer #1

How can I merge two Python dictionaries in a single expression?

For dictionaries x and y, z becomes a shallowly-merged dictionary with values from y replacing those from x.

  • In Python 3.9.0 or greater (released 17 October 2020): PEP-584, discussed here, was implemented and provides the simplest method:

    z = x | y          # NOTE: 3.9+ ONLY
    
  • In Python 3.5 or greater:

    z = {**x, **y}
    
  • In Python 2, (or 3.4 or lower) write a function:

    def merge_two_dicts(x, y):
        z = x.copy()   # start with keys and values of x
        z.update(y)    # modifies z with keys and values of y
        return z
    

    and now:

    z = merge_two_dicts(x, y)
    

Explanation

Say you have two dictionaries and you want to merge them into a new dictionary without altering the original dictionaries:

x = {"a": 1, "b": 2}
y = {"b": 3, "c": 4}

The desired result is to get a new dictionary (z) with the values merged, and the second dictionary"s values overwriting those from the first.

>>> z
{"a": 1, "b": 3, "c": 4}

A new syntax for this, proposed in PEP 448 and available as of Python 3.5, is

z = {**x, **y}

And it is indeed a single expression.

Note that we can merge in with literal notation as well:

z = {**x, "foo": 1, "bar": 2, **y}

and now:

>>> z
{"a": 1, "b": 3, "foo": 1, "bar": 2, "c": 4}

It is now showing as implemented in the release schedule for 3.5, PEP 478, and it has now made its way into the What"s New in Python 3.5 document.

However, since many organizations are still on Python 2, you may wish to do this in a backward-compatible way. The classically Pythonic way, available in Python 2 and Python 3.0-3.4, is to do this as a two-step process:

z = x.copy()
z.update(y) # which returns None since it mutates z

In both approaches, y will come second and its values will replace x"s values, thus b will point to 3 in our final result.

Not yet on Python 3.5, but want a single expression

If you are not yet on Python 3.5 or need to write backward-compatible code, and you want this in a single expression, the most performant while the correct approach is to put it in a function:

def merge_two_dicts(x, y):
    """Given two dictionaries, merge them into a new dict as a shallow copy."""
    z = x.copy()
    z.update(y)
    return z

and then you have a single expression:

z = merge_two_dicts(x, y)

You can also make a function to merge an arbitrary number of dictionaries, from zero to a very large number:

def merge_dicts(*dict_args):
    """
    Given any number of dictionaries, shallow copy and merge into a new dict,
    precedence goes to key-value pairs in latter dictionaries.
    """
    result = {}
    for dictionary in dict_args:
        result.update(dictionary)
    return result

This function will work in Python 2 and 3 for all dictionaries. e.g. given dictionaries a to g:

z = merge_dicts(a, b, c, d, e, f, g) 

and key-value pairs in g will take precedence over dictionaries a to f, and so on.

Critiques of Other Answers

Don"t use what you see in the formerly accepted answer:

z = dict(x.items() + y.items())

In Python 2, you create two lists in memory for each dict, create a third list in memory with length equal to the length of the first two put together, and then discard all three lists to create the dict. In Python 3, this will fail because you"re adding two dict_items objects together, not two lists -

>>> c = dict(a.items() + b.items())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: "dict_items" and "dict_items"

and you would have to explicitly create them as lists, e.g. z = dict(list(x.items()) + list(y.items())). This is a waste of resources and computation power.

Similarly, taking the union of items() in Python 3 (viewitems() in Python 2.7) will also fail when values are unhashable objects (like lists, for example). Even if your values are hashable, since sets are semantically unordered, the behavior is undefined in regards to precedence. So don"t do this:

>>> c = dict(a.items() | b.items())

This example demonstrates what happens when values are unhashable:

>>> x = {"a": []}
>>> y = {"b": []}
>>> dict(x.items() | y.items())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: "list"

Here"s an example where y should have precedence, but instead the value from x is retained due to the arbitrary order of sets:

>>> x = {"a": 2}
>>> y = {"a": 1}
>>> dict(x.items() | y.items())
{"a": 2}

Another hack you should not use:

z = dict(x, **y)

This uses the dict constructor and is very fast and memory-efficient (even slightly more so than our two-step process) but unless you know precisely what is happening here (that is, the second dict is being passed as keyword arguments to the dict constructor), it"s difficult to read, it"s not the intended usage, and so it is not Pythonic.

Here"s an example of the usage being remediated in django.

Dictionaries are intended to take hashable keys (e.g. frozensets or tuples), but this method fails in Python 3 when keys are not strings.

>>> c = dict(a, **b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: keyword arguments must be strings

From the mailing list, Guido van Rossum, the creator of the language, wrote:

I am fine with declaring dict({}, **{1:3}) illegal, since after all it is abuse of the ** mechanism.

and

Apparently dict(x, **y) is going around as "cool hack" for "call x.update(y) and return x". Personally, I find it more despicable than cool.

It is my understanding (as well as the understanding of the creator of the language) that the intended usage for dict(**y) is for creating dictionaries for readability purposes, e.g.:

dict(a=1, b=10, c=11)

instead of

{"a": 1, "b": 10, "c": 11}

Response to comments

Despite what Guido says, dict(x, **y) is in line with the dict specification, which btw. works for both Python 2 and 3. The fact that this only works for string keys is a direct consequence of how keyword parameters work and not a short-coming of dict. Nor is using the ** operator in this place an abuse of the mechanism, in fact, ** was designed precisely to pass dictionaries as keywords.

Again, it doesn"t work for 3 when keys are not strings. The implicit calling contract is that namespaces take ordinary dictionaries, while users must only pass keyword arguments that are strings. All other callables enforced it. dict broke this consistency in Python 2:

>>> foo(**{("a", "b"): None})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: foo() keywords must be strings
>>> dict(**{("a", "b"): None})
{("a", "b"): None}

This inconsistency was bad given other implementations of Python (PyPy, Jython, IronPython). Thus it was fixed in Python 3, as this usage could be a breaking change.

I submit to you that it is malicious incompetence to intentionally write code that only works in one version of a language or that only works given certain arbitrary constraints.

More comments:

dict(x.items() + y.items()) is still the most readable solution for Python 2. Readability counts.

My response: merge_two_dicts(x, y) actually seems much clearer to me, if we"re actually concerned about readability. And it is not forward compatible, as Python 2 is increasingly deprecated.

{**x, **y} does not seem to handle nested dictionaries. the contents of nested keys are simply overwritten, not merged [...] I ended up being burnt by these answers that do not merge recursively and I was surprised no one mentioned it. In my interpretation of the word "merging" these answers describe "updating one dict with another", and not merging.

Yes. I must refer you back to the question, which is asking for a shallow merge of two dictionaries, with the first"s values being overwritten by the second"s - in a single expression.

Assuming two dictionaries of dictionaries, one might recursively merge them in a single function, but you should be careful not to modify the dictionaries from either source, and the surest way to avoid that is to make a copy when assigning values. As keys must be hashable and are usually therefore immutable, it is pointless to copy them:

from copy import deepcopy

def dict_of_dicts_merge(x, y):
    z = {}
    overlapping_keys = x.keys() & y.keys()
    for key in overlapping_keys:
        z[key] = dict_of_dicts_merge(x[key], y[key])
    for key in x.keys() - overlapping_keys:
        z[key] = deepcopy(x[key])
    for key in y.keys() - overlapping_keys:
        z[key] = deepcopy(y[key])
    return z

Usage:

>>> x = {"a":{1:{}}, "b": {2:{}}}
>>> y = {"b":{10:{}}, "c": {11:{}}}
>>> dict_of_dicts_merge(x, y)
{"b": {2: {}, 10: {}}, "a": {1: {}}, "c": {11: {}}}

Coming up with contingencies for other value types is far beyond the scope of this question, so I will point you at my answer to the canonical question on a "Dictionaries of dictionaries merge".

Less Performant But Correct Ad-hocs

These approaches are less performant, but they will provide correct behavior. They will be much less performant than copy and update or the new unpacking because they iterate through each key-value pair at a higher level of abstraction, but they do respect the order of precedence (latter dictionaries have precedence)

You can also chain the dictionaries manually inside a dict comprehension:

{k: v for d in dicts for k, v in d.items()} # iteritems in Python 2.7

or in Python 2.6 (and perhaps as early as 2.4 when generator expressions were introduced):

dict((k, v) for d in dicts for k, v in d.items()) # iteritems in Python 2

itertools.chain will chain the iterators over the key-value pairs in the correct order:

from itertools import chain
z = dict(chain(x.items(), y.items())) # iteritems in Python 2

Performance Analysis

I"m only going to do the performance analysis of the usages known to behave correctly. (Self-contained so you can copy and paste yourself.)

from timeit import repeat
from itertools import chain

x = dict.fromkeys("abcdefg")
y = dict.fromkeys("efghijk")

def merge_two_dicts(x, y):
    z = x.copy()
    z.update(y)
    return z

min(repeat(lambda: {**x, **y}))
min(repeat(lambda: merge_two_dicts(x, y)))
min(repeat(lambda: {k: v for d in (x, y) for k, v in d.items()}))
min(repeat(lambda: dict(chain(x.items(), y.items()))))
min(repeat(lambda: dict(item for d in (x, y) for item in d.items())))

In Python 3.8.1, NixOS:

>>> min(repeat(lambda: {**x, **y}))
1.0804965235292912
>>> min(repeat(lambda: merge_two_dicts(x, y)))
1.636518670246005
>>> min(repeat(lambda: {k: v for d in (x, y) for k, v in d.items()}))
3.1779992282390594
>>> min(repeat(lambda: dict(chain(x.items(), y.items()))))
2.740647904574871
>>> min(repeat(lambda: dict(item for d in (x, y) for item in d.items())))
4.266070580109954
$ uname -a
Linux nixos 4.19.113 #1-NixOS SMP Wed Mar 25 07:06:15 UTC 2020 x86_64 GNU/Linux

Resources on Dictionaries

5839

Answer #2

In your case, what you can do is:

z = dict(list(x.items()) + list(y.items()))

This will, as you want it, put the final dict in z, and make the value for key b be properly overridden by the second (y) dict"s value:

>>> x = {"a":1, "b": 2}
>>> y = {"b":10, "c": 11}
>>> z = dict(list(x.items()) + list(y.items()))
>>> z
{"a": 1, "c": 11, "b": 10}

If you use Python 2, you can even remove the list() calls. To create z:

>>> z = dict(x.items() + y.items())
>>> z
{"a": 1, "c": 11, "b": 10}

If you use Python version 3.9.0a4 or greater, then you can directly use:

x = {"a":1, "b": 2}
y = {"b":10, "c": 11}
z = x | y
print(z)
{"a": 1, "c": 11, "b": 10}

5839

Answer #3

An alternative:

z = x.copy()
z.update(y)

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