cv2.imshow crashing my application (Terminating app due to uncaught exception 'NSInternalInconsistencyException)

Expected behaviour

To display an image

Actual behaviour

Application crashes with

018-10-23 13:54:08.643 python[48930:2909663] *** Assertion failure in +[NSUndoManager _endTopLevelGroupings], /BuildRoot/Library/Caches/com.apple.xbs/Sources/Foundation/Foundation-1451/Foundation/Misc.subproj/NSUndoManager.m:361
2018-10-23 13:54:08.645 python[48930:2909663] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '+[NSUndoManager(NSInternal) _endTopLevelGroupings] is only safe to invoke on the main thread.'
*** First throw call stack:
(
0 CoreFoundation 0x00007fff32116fcb __exceptionPreprocess + 171
1 libobjc.A.dylib 0x00007fff58db0c76 objc_exception_throw + 48
2 CoreFoundation 0x00007fff3211cd62 +[NSException raise:format:arguments:] + 98
3 Foundation 0x00007fff3421b300 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 193
4 Foundation 0x00007fff341a9e54 +[NSUndoManager(NSPrivate) _endTopLevelGroupings] + 469
5 AppKit 0x00007fff2f668e56 -[NSApplication run] + 997
6 QtGui 0x00000001124dcc12 _ZN26QEventDispatcherMacPrivate22ensureNSAppInitializedEv + 128
7 QtGui 0x00000001124dc4ad _ZN19QEventDispatcherMac13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE + 693
8 QtCore 0x0000000113177d9f _ZN16QCoreApplication13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE + 81
9 cv2.cpython-37m-darwin.so 0x000000010d50a7ef cvWaitKey + 207
10 cv2.cpython-37m-darwin.so 0x000000010d50ae37 _ZN11GuiReceiver12createWindowE7QStringi + 263
11 cv2.cpython-37m-darwin.so 0x000000010d50ac6c cvNamedWindow + 540
12 cv2.cpython-37m-darwin.so 0x000000010d50d3a1 _ZN11GuiReceiver9showImageE7QStringPv + 161
13 cv2.cpython-37m-darwin.so 0x000000010d50d24c cvShowImage + 572
14 cv2.cpython-37m-darwin.so 0x000000010d50598b _ZN2cv6imshowERKNS_6StringERKNS_11_InputArrayE + 475
15 cv2.cpython-37m-darwin.so 0x000000010caf6114 ZL18pyopencv_cv_imshowP7_objectS0_S0 + 404
16 Python 0x000000010b0cca4e _PyMethodDef_RawFastCallKeywords + 541
17 Python 0x000000010b0cbf8b _PyCFunction_FastCallKeywords + 44
18 Python 0x000000010b164ad7 call_function + 554
19 Python 0x000000010b15cb57 _PyEval_EvalFrameDefault + 5281
20 Python 0x000000010b0cc379 function_code_fastcall + 117
21 Python 0x000000010b0ccd21 _PyObject_Call_Prepend + 150
22 Python 0x000000010b0cc0a2 PyObject_Call + 137
23 Python 0x000000010b15cebd _PyEval_EvalFrameDefault + 6151
24 Python 0x000000010b1653ff _PyEval_EvalCodeWithName + 1747
25 Python 0x000000010b0cbb8a _PyFunction_FastCallDict + 441
26 Python 0x000000010b0ccd21 _PyObject_Call_Prepend + 150
27 Python 0x000000010b0cc0a2 PyObject_Call + 137
28 Python 0x000000010b15cebd _PyEval_EvalFrameDefault + 6151
29 Python 0x000000010b1653ff _PyEval_EvalCodeWithName + 1747
30 Python 0x000000010b0cbb8a _PyFunction_FastCallDict + 441
31 Python 0x000000010b15cebd _PyEval_EvalFrameDefault + 6151
32 Python 0x000000010b1653ff _PyEval_EvalCodeWithName + 1747
33 Python 0x000000010b0cbb8a _PyFunction_FastCallDict + 441
34 Python 0x000000010b15cebd _PyEval_EvalFrameDefault + 6151
35 Python 0x000000010b0cc379 function_code_fastcall + 117
36 Python 0x000000010b164b42 call_function + 661
37 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
38 Python 0x000000010b0cc379 function_code_fastcall + 117
39 Python 0x000000010b164b42 call_function + 661
40 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
41 Python 0x000000010b0cc379 function_code_fastcall + 117
42 Python 0x000000010b164b42 call_function + 661
43 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
44 Python 0x000000010b0cc379 function_code_fastcall + 117
45 Python 0x000000010b0ccd21 _PyObject_Call_Prepend + 150
46 Python 0x000000010b10af5b slot_tp_call + 71
47 Python 0x000000010b0cbde0 _PyObject_FastCallKeywords + 381
48 Python 0x000000010b164b3b call_function + 654
49 Python 0x000000010b15cb57 _PyEval_EvalFrameDefault + 5281
50 Python 0x000000010b0d7fe4 gen_send_ex + 242
51 Python 0x000000010b15c28e _PyEval_EvalFrameDefault + 3032
52 Python 0x000000010b1653ff _PyEval_EvalCodeWithName + 1747
53 Python 0x000000010b0cbf50 _PyFunction_FastCallKeywords + 225
54 Python 0x000000010b164b42 call_function + 661
55 Python 0x000000010b15cbfd _PyEval_EvalFrameDefault + 5447
56 Python 0x000000010b1653ff _PyEval_EvalCodeWithName + 1747
57 Python 0x000000010b0cbf50 _PyFunction_FastCallKeywords + 225
58 Python 0x000000010b164b42 call_function + 661
59 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
60 Python 0x000000010b0cc379 function_code_fastcall + 117
61 Python 0x000000010b164b42 call_function + 661
62 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
63 Python 0x000000010b0cc379 function_code_fastcall + 117
64 Python 0x000000010b164b42 call_function + 661
65 Python 0x000000010b15cb57 _PyEval_EvalFrameDefault + 5281
66 Python 0x000000010b0cc379 function_code_fastcall + 117
67 Python 0x000000010b164b42 call_function + 661
68 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
69 Python 0x000000010b0cc379 function_code_fastcall + 117
70 Python 0x000000010b0ccd21 _PyObject_Call_Prepend + 150
71 Python 0x000000010b10b96f slot_tp_init + 80
72 Python 0x000000010b108464 type_call + 178
73 Python 0x000000010b0cbde0 _PyObject_FastCallKeywords + 381
74 Python 0x000000010b164b3b call_function + 654
75 Python 0x000000010b15cb57 _PyEval_EvalFrameDefault + 5281
76 Python 0x000000010b0cc379 function_code_fastcall + 117
77 Python 0x000000010b164b42 call_function + 661
78 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
79 Python 0x000000010b0cc379 function_code_fastcall + 117
80 Python 0x000000010b0ccd21 _PyObject_Call_Prepend + 150
81 Python 0x000000010b0cc0a2 PyObject_Call + 137
82 Python 0x000000010b15cebd _PyEval_EvalFrameDefault + 6151
83 Python 0x000000010b0cc379 function_code_fastcall + 117
84 Python 0x000000010b164b42 call_function + 661
85 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
86 Python 0x000000010b0cc379 function_code_fastcall + 117
87 Python 0x000000010b164b42 call_function + 661
88 Python 0x000000010b15cb3e _PyEval_EvalFrameDefault + 5256
89 Python 0x000000010b0cc379 function_code_fastcall + 117
90 Python 0x000000010b0ccd21 _PyObject_Call_Prepend + 150
91 Python 0x000000010b0cc0a2 PyObject_Call + 137
92 Python 0x000000010b1cbcea t_bootstrap + 71
93 libsystem_pthread.dylib 0x00007fff59c2b6c1 _pthread_body + 340
94 libsystem_pthread.dylib 0x00007fff59c2b56d _pthread_body + 0
95 libsystem_pthread.dylib 0x00007fff59c2ac5d thread_start + 13
)
li
bc++abi.dylib: terminating with uncaught exception of type NSException

Steps to reproduce

• example code

  img = cv2.imread('front.jpg')
  cv2.imshow('front.jpg', img)
  cv2.waitKey(0)

• operating system
  macOS High Sierra version 10.13.3

• architecture (e.g. x86)
  x64

• opencv-python version
  latest *

[skvark]

Are you trying to call imshow outside of the main thread (is only safe to invoke on the main thread)?

@skvark Nope, i don't think i am. I am using Flask framework this is done in a post request.

[skvark]

Well, Flask spawns a new thread when a request is received (https://stackoverflow.com/a/38876915/5795428). You can't use imshow in threads.

[WolfgangFahl]

Please reopen the bug and let opencv's python adapter behave properly by throwing an exception. Crashing is not really an option ... if you don't do this every user with this usecase will have to wrap the imshow call anyway to protect against this uglyness.

[skvark]

This repository is just a build toolchain. There is no other actual Python code in this repository than some import related logic. The bindings (a static library) are generated from C++ code automatically. No custom behaviour has been added to the binary.

This same issue often pops up in the OpenCV repository, for example opencv/opencv#13590. It's quite common that GUI frameworks allow GUI interactions only from the main thread.

Some other links:

• imshow and waitKey combination in multi-thread environment on Linux opencv#8407 (comment)
• https://stackoverflow.com/questions/49096804/cv2-image-show-doesnt-work-when-multithreading

Edit: as a side note, OpenCV's algorithms / other features are not thread safe.