imread Should Raise If Path Doesn't Exist

[zhanwenchen]

System information (version)

• OpenCV => 4.0.1
• Operating System / Platform => MacOS 10.14 Mojave
• Compiler => Xcode 10.1

Detailed description

cv2.imread doesn't check file existence and reads nonexistent files into a NoneType object. In comparison, matplotlib.pyplot.imread and PIL both throw.

Steps to reproduce

import cv2

nonetype_thingy = cv2.imread('nonexistent_path.jpg') # this is somehow legal
cv2.imshow('lel', nonetype_thingy) # this will throw a "cv2.error: OpenCV(4.0.1) /Users/username/opencv/modules/highgui/src/window.cpp:352: error: (-215:Assertion failed) size.width>0 && size.height>0 in function 'imshow'"

[alalek]

Mentioned API assumes that empty (None in Python) result is returned.
Changing this convention will break existed OpenCV applications.

[LaurentBerger]

@alalek What's about to add :
static const size_t CV_IO_IMREAD_EXCEPTION = utils::getConfigurationParameterSizeT("OPENCVIO_IMREAD_EXCEPTION", 0);

and new user can set to 1 if they want exception ?

[LJXLJXLJX]

Yes, without raising exceptions here, bugs can't be located in time.
It may takes a lot of time to debug manually to detect this tiny mistake.
I think this design does not meet the user-friendly concept.

[zhanwenchen]

@alalek I much prefer breaking changes to mismatched expectations. Why would this "break existed OpenCV applications?" Also why are those applications using this special fake "NoneType" thing anyway?

[mpolicki]

Current imread documentation says:

If the image cannot be read (because of missing file, improper permissions, unsupported or invalid format), the function returns an empty matrix

From a software-development-best-practices standpoint, this is completely unacceptable. Nowhere will you find advice like the following:

If an error occurs, don't report it to the caller, just fail silently.

Instead, the advice you typically find is something like (emphasis added):

Check input and fail on nonsensical input or invalid state as early as possible, preferably with an exception or error response that will make the exact problem clear to your caller.

So, at the very least, imread needs to report to the caller which error it has encountered, if any. The mechanism of this error reporting should be appropriate to the language. In C++, that mechanism is exceptions. Therefore, imread should throw the appropriate exception if it encounters an error.

[sturkmen72]

i am working on #17753 and planning to work also on error handling. now waiting a review to continue

[alalek]

imread() comes from legacy C API which don't have C++ exception or similar error handling.
Adding exceptions into imread() directly break many applications.
So API itself should be evolved.