OpenCV Python: cv2.findContours - ValueError: too many values to unpack

Question

I'm writing an opencv program and I found a script on another stackoverflow question: Computer Vision: Masking a human hand

When I run the scripted answer, I get the following error:

Traceback (most recent call last):
    File "skinimagecontour.py", line 13, in <module>
    contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
ValueError: too many values to unpack

The code:

import sys
import numpy
import cv2

im = cv2.imread('Photos/test.jpg')
im_ycrcb = cv2.cvtColor(im, cv2.COLOR_BGR2YCR_CB)

skin_ycrcb_mint = numpy.array((0, 133, 77))
skin_ycrcb_maxt = numpy.array((255, 173, 127))
skin_ycrcb = cv2.inRange(im_ycrcb, skin_ycrcb_mint, skin_ycrcb_maxt)
cv2.imwrite('Photos/output2.jpg', skin_ycrcb) # Second image

contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
for i, c in enumerate(contours):
    area = cv2.contourArea(c)
    if area > 1000:
        cv2.drawContours(im, contours, i, (255, 0, 0), 3)
cv2.imwrite('Photos/output3.jpg', im)

Any help is appreciated!

Community · Accepted Answer · 2020-06-20 09:12:55Z

160

I got the answer from the OpenCV Stack Exchange site. Answer

THE ANSWER:

I bet you are using the current OpenCV's master branch: here the return statements have changed, see http://docs.opencv.org/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=findcontours.

Thus, change the corresponding line to read:
_, contours, _= cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
Or: since the current trunk is still not stable and you probably will run in some more problems, you may want to use OpenCV's current stable version 2.4.9.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Aug 26, 2014 at 15:50

ahmadux

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2 Comments

grep Over a year ago

I don't understand how to fix it?

eden Over a year ago

@grep there is that 3rd return statement. compare the one in question and the answer lines

Safwan · Accepted Answer · 2021-12-16 06:44:25Z

50

This works in all cv2 versions:

contours, hierarchy = cv2.findContours(
    skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)[-2:]

Explanation: By using [-2:], we are basically taking the last two values from the tuple returned by cv2.findContours. Since in some versions, it returns (image, contours, hierarchy) and in other versions, it returns (contours, hierarchy), contours, hierarchy are always the last two values.

edited Dec 16, 2021 at 6:44

answered May 15, 2019 at 6:23

Safwan

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3 Comments

Benten Over a year ago

ahh-mazinnnnnnn

Ahmed J. Over a year ago

Worked for me as well.

Apoorva Jain Over a year ago

It's amazing hack

miken32 · Accepted Answer · 2017-02-06 21:01:16Z

15

You have to change this line;

image, contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

edited Feb 6, 2017 at 21:01

miken32

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answered Feb 6, 2017 at 20:54

Volkan Altıntaş

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1 Comment

Obsidian Age Over a year ago

Welcome to StackOverflow! While answers are always appreciated, this question was asked 2 years ago, and already had an accepted solution. And at that, you don't tell him what he has to change the line to. Please try to avoid 'bumping' questions to the top by providing answers to them, unless the question was not already marked as resolved, or you found a dramatically better alternative approach to the problem :)

Priyansh gupta · Accepted Answer · 2019-09-03 22:23:24Z

6

I'm using python3.x and opencv 4.1.0 i was getting error in the following code :

cnts, _ = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

ERROR : too many values to Unpack

then i came to know that above code is used in python2.x SO i just replaced above code with below one(IN python3.x) by adding one more '_' in the left most side have a look

_,cnts, _ = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

answered Sep 3, 2019 at 22:23

Priyansh gupta

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1 Comment

user11840960 Over a year ago

The simplest solution

Edwardabk · Accepted Answer · 2018-06-06 04:49:50Z

5

The thing you need to do is just add '_' where you are not using the required var , originally given by:

im2, contours, hierarchy = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

to

_ , contours, _ = cv2.findContours(thresh,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

Here the original doc is given: https://docs.opencv.org/3.1.0/d4/d73/tutorial_py_contours_begin.html

answered Jun 6, 2018 at 4:49

Edwardabk

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Comments

kishea · Accepted Answer · 2018-03-16 14:11:56Z

2

python is right.
you cannot unpack 3 values from the turple and place them in a turple of two contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

use

img, contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

answered Mar 16, 2018 at 14:11

kishea

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Comments

Christoph Rackwitz · Accepted Answer · 2025-03-30 11:33:35Z

@Safwan has provided a simple hack by choosing the last two tuples returned by cv2.findContours() function. But what if you want all the values returned from every tuple of the function independent of the version of cv2?

The following snippet will work irrespective of the OpenCV version (2, 3 or 4) installed in your system/environment and will also store all the tuples in individual variables.

OpenCV version 2 & 4, return 2 variables: contours and its hierarchy
OpenCV version 3, returns 3 variables: input image, contours and its hierarchy

First, get the version of OpenCV installed (we don't want the entire version just the major number either 2, 3 or 4) :

import cv2
major_version = cv2.__version__[0]

Based on the version either of the following two statements will be executed and the corresponding variables will be populated:

if major_version == '4' or major_version == '2':
    contours, hierarchy = cv2.findContours(img_binary, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

elif major_version == '3':
    im2, contours, hierarchy = cv2.findContours(img_binary, cv2.RETR_TREE, cv.CHAIN_APPROX_SIMPLE)

The contours can be used for further processing.

Greg McMullen · Accepted Answer · 2020-07-23 00:17:45Z

0

No big deal, just you might be using open-cv 3.something, which returns 3 values at the point of error and you must be catching only 2, just add any random variable before the contours variable -

_,contours,_ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)

edited Jul 23, 2020 at 0:17

Greg McMullen

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answered Jul 22, 2020 at 16:51

Rushil Goomer

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Comments

Stephen Rauch · Accepted Answer · 2021-05-21 00:17:14Z

0

If the error is:

not enough values to unpack (expected 3, got 2)

then use:

ctrs,hier=cv2.findContours(im_th.copy(),cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

instead of

_,ctrs,hier=cv2.findContours(im_th.copy(),cv2.RETR_EXTERNAL,cv2.CHAIN_APPROX_SIMPLE)

edited May 21, 2021 at 0:17

Stephen Rauch♦

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answered May 20, 2021 at 19:44

Sweety Hissariya

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