OpenCV: Resolving NoneType errors

nonetype_output

Each week I receive and respond to at least 2-3 emails and 3-4 blog post comments regarding NoneType  errors in OpenCV and Python.

For beginners, these errors can be hard to diagnose — by definition they aren’t very informative.

Since this question is getting asked so often I decided to dedicate an entire blog post to the topic.

While NoneType  errors can be caused for a nearly unlimited number of reasons, in my experience, both as a computer vision developer and chatting with other programmers here on PyImageSearch, in over 95% of the cases, NoneType  errors in OpenCV are caused by either:

  1. An invalid image path passed to cv2.imread .
  2. A problem reading a frame from a video stream/video file via cv2.VideoCapture  and the associated .read  method.

To learn more about NoneType  errors in OpenCV (and how to avoid them), just keep reading.

Looking for the source code to this post?
Jump right to the downloads section.

OpenCV: Resolving NoneType errors

In the first part of this blog post I’ll discuss exactly what NoneType  errors are in the Python programming language.

I’ll then discuss the two primary reasons you’ll run into NoneType  errors when using OpenCV and Python together.

Finally, I’ll put together an actual example that not only causes a NoneType  error, but also resolves it as well.

What is a NoneType error?

When using the Python programming language you’ll inevitably run into an error that looks like this:

Where something  can be replaced by whatever the name of the actual attribute is.

We see these errors when we think we are working with an instance of a particular Class or Object, but in reality we have the Python built-in type None .

As the name suggests, None  represents the absence of a value, such as when a function call returns an unexpected result or fails entirely.

Here is an example of generating a NoneType  error from the Python shell:

Here I create a variable named foo  and set it to None .

I then try to set the bar  attribute of foo  to True , but since foo  is a NoneType  object, Python will not allow this — hence the error message.

Two reasons for 95% of OpenCV NoneType errors

When using OpenCV and Python bindings, you’re bound to come across NoneType  errors at some point.

In my experience, over 95% of the time these NoneType  errors can be traced back to either an issue with cv2.imread  or cv2.VideoCapture .

I have provided details for each of the cases below.

Case #1: cv2.imread

If you are receiving a NoneType  error and your code is calling cv2.imread , then the likely cause of the error is an invalid file path supplied to cv2.imread .

The cv2.imread  function does not explicitly throw an error message if you give it an invalid file path (i.e., a path to a nonexistent file). Instead, cv2.imread  will simply return None .

Anytime you try to access an attribute of a None  image loaded from disk via cv2.imread  you’ll get a NoneType  error.

Here is an example of trying to load a nonexistent image from disk:

As you can see, cv2.imread  gladly accepts the image path (even though it doesn’t exist), realizes the image path is invalid, and returns None . This is especially confusing for Python programmers who are used to these types of functions throwing exceptions.

As an added bonus, I’ll also mention the AssertionFailed  exception.

If you try to pass an invalid image (i.e., NoneType  image) into another OpenCV function, Python + OpenCV will complain that the image doesn’t have any width, height, or depth information — and how could it, the “image” is a None  object after all!

Here is an example of an error message you might see when loading a nonexistent image from disk and followed by immediately calling an OpenCV function on it:

These types of errors can be harder to debug since there are many reasons why an AssertionError  could be thrown. But in most cases, your first step should be be ensuring that your image was correctly loaded from disk.

A final, more rare problem you may encounter with cv2.imread  is that your image does exist on disk, but you didn’t compile OpenCV with the given image I/O libraries installed.

For example, let’s say you have a .JPEG file on disk and you knew you had the correct path to it.

You then try to load the JPEG file via cv2.imread  and notice a NoneType  or AssertionError .

How can this be?

The file exists!

In this case, you likely forgot to compile OpenCV with JPEG file support enabled.

In Debian/Ubuntu systems, this is caused by a lack of libjpeg  being installed.

For macOS systems, you likely forgot to install the jpeg  library via Homebrew.

To resolve this problem, regardless of operating system, you’ll need to re-compile and re-install OpenCV. Please see this page for more details on how to compile and install OpenCV on your particular system.

Case #2: cv2.VideoCapture and .read

Just like we see NoneType  errors and AssertionError  exceptions when using cv2.imread , you’ll also see these errors when working with video streams/video files as well.

To access a video stream, OpenCV uses the cv2.VideoCapture  which accepts a single argument, either:

  1. string representing the path to a video file on disk.
  2. An integer representing the index of a webcam on your computer.

Working with video streams and video files with OpenCV is more complex than simply loading an image via cv2.imread , but the same rules apply.

If you try to call the .read  method of an instantiated cv2.VideoCapture  (regardless if it’s a video file or webcam stream) and notice a NoneType  error or  AssertionError , then you likely have a problem with either:

  1. The path to your input video file (it’s probably incorrect).
  2. Not having the proper video codecs installed, in which case you’ll need to install the codecs, followed by re-compiling and re-installing OpenCV (see this page for a complete list of tutorials).
  3. Your webcam not being accessible via OpenCV. This could be for any number of reasons, including missing drivers, an incorrect index passed to cv2.VideoCapture , or simply your webcam is not properly attached to your system.

Again, working with video files is more complex than working with simple image files, so make sure you’re systematic in resolving the issue.

First, try to access your webcam via a separate piece of software than OpenCV.

Or, try to load your video file in a movie player.

If both of those work, you likely have a problem with your OpenCV install.

Otherwise, it’s most likely a codec or driver issue.

An example of creating and resolving an OpenCV NoneType error

To demonstrate a NoneType  error in action I decided to create a highly simplified Python + OpenCV script that represents what you might see elsewhere on the PyImageSearch blog.

Open up a new file, name it display_image.py , and insert the following code:

All this script does is:

  • Parse command line arguments.
  • (Attempts to) load an image from disk.
  • Prints the width, height, and depth of the image to the terminal.
  • Displays the image to our screen.

For most Python developers who are familiar with the command line, this script won’t give you any trouble.

But if you’re new to the command line and are unfamiliar/uncomfortable with command line arguments, you can easily run into a NoneType  error if you’re not careful.

How, you might say?

The answer lies in not properly using/understanding command line arguments.

Over the past few years of running this blog, I’ve seen many emails and blog post comments from readers who are trying to modify the .add_argument  function to supply the path to their image file.

DON’T DO THIS — you don’t have to change a single line of argument parsing code.

Instead, what you should do is spend the next 10 minutes reading through this excellent article that explains what command line arguments are and how to use them in Python:

https://www.cyberciti.biz/faq/python-command-line-arguments-argv-example/

This is required reading if you expect to follow tutorials here on the PyImageSearch blog.

Working with the command line, and therefore command line arguments, are a big part of what it means to be a computer scientist — a lack of command line skills is only going to harm youYou’ll thank me later.

Going back to the example, let’s check the contents of my local directory:

As we can see, I have two files:

  1. display_image.py : My Python script that I’ll be executing shortly.
  2. jemma.png : The photo I’ll be loading from disk.

If I execute the following command I’ll see the jemma.png  image displayed to my screen, along with information on the dimensions of the image:

Figure 1: Loading and displaying an image to my screen with OpenCV and Python.

Figure 1: Loading and displaying an image to my screen with OpenCV and Python.

However, let’s try to load an image path that does not exist:

Sure enough, there is our NoneType  error.

In this case, it was caused because I did not supply a valid image path to cv2.imread .

Summary

In this blog post I discussed NoneType  errors and AssertionError  exceptions in OpenCV and Python.

In the vast majority of these situations, these errors can be attributed to either the cv2.imread  or cv2.VideoCapture  methods.

Whenever you encounter one of these errors, make sure you can load your image/read your frame before continuing. In over 95% of circumstances, your image/frame was not properly read.

Otherwise, if you are using command line arguments and are unfamiliar with them, there is a chance that you aren’t using them properly. In that case, make sure you educate yourself by reading this tutorial on command line arguments — you’ll thank me later.

Anyway, I hope this tutorial has helped you in your journey to OpenCV mastery!

If you’re just getting started studying computer vision and OpenCV, I would highly encourage you to take a look at my book, Practical Python and OpenCV, which will help you grasp the fundamentals.

Otherwise, make sure you enter your email address in the form below to be notified when future blog posts and tutorials are published!

Downloads:

If you would like to download the code and images used in this post, please enter your email address in the form below. Not only will you get a .zip of the code, I’ll also send you a FREE 11-page Resource Guide on Computer Vision and Image Search Engines, including exclusive techniques that I don’t post on this blog! Sound good? If so, enter your email address and I’ll send you the code immediately!

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6 Responses to OpenCV: Resolving NoneType errors

  1. Mika December 26, 2016 at 12:17 pm #

    Great article. If writing production code, one should be kind enough to have some basic tests for inputs. I was kind of puzzled before I tested, if the image file could be opened. OpenCV has many very not descriptive errors

    • Adrian Rosebrock December 27, 2016 at 6:40 am #

      Agreed — writing tests, especially when developing production level OpenCV + Python applications is hugely important.

  2. muhammad tamrin December 26, 2016 at 6:56 pm #

    thanks mr for imformation

  3. Hugh December 27, 2016 at 3:04 pm #

    A simple test can also help in some cases where a multi-threaded frame from a camera is not collected correctly, or an image is not ready.

    image = cv2.imread(path) # from file
    # Check that it was a valid image, otherwise try again
    if (image == None):
    continue

    frame = cthread.read() # from multi-thread USB camera
    # Check that it was a valid frame, otherwise try again
    if frame == None):
    continue

  4. Sinjon March 24, 2017 at 8:28 am #

    Hello Adrian,

    If I still get the NoneType errors when running that downloadable code, what does that mean?

    I’ve run it in terminal with and without the full address of the image, as well as adding in the, h, w and d but am still getting it.

    Any ideas?

    Thanks

    • Adrian Rosebrock March 25, 2017 at 9:24 am #

      If you are still getting NoneType errors after validating that the paths to your images are correct then your version of OpenCV was likely compiled without support to load the image file type.

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