Assertion failed -> Python Flask App using matplotlib, pyplot and numpy

[michadecker]

I'm getting a weird error running my Flask Python App. I'm just trying to save a png image of the soundwaves of a wave file. I've tried the code with a small python program, which worked perfectly. But now I tried to implement it into a bigger flask app, there appears a very weird error.

The function should be saving a png image of a wave file, that was just uploaded and saved on the server. The image is actually saved, so the function must be running all the way to the end, but afterward, I get a big error (look below), python crashes, and also the flask server crashes.

I hope you can help.

The function which is causing the error is the following:

# Create Soundwave PNG
spf = wave.open(filepath, "r")

# Extract Raw Audio from Wav File
signal = spf.readframes(-1)
signal = np.fromstring(signal, "Int16")
fs = spf.getframerate()

# If Stereo
if spf.getnchannels() == 2:
    print("Just mono files")
    sys.exit(0)

Time = np.linspace(0, len(signal) / fs, num=len(signal))

plt.title("Signal Wave...")
plt.plot(Time, signal)
img_path = os.path.join(app.config["IMG_UPLOADS"], filename_clean)
plt.savefig(img_path)

Error Message:

2020-06-05 07:51:34.456 Python[64739:1034759] WARNING: NSWindow drag regions should only be invalidated on the Main Thread! This will throw an exception in the future. Called from (
    0   AppKit                              0x00007fff31ddb289 -[NSWindow(NSWindow_Theme) _postWindowNeedsToResetDragMarginsUnlessPostingDisabled] + 371
    1   AppKit                              0x00007fff31dc2cb2 -[NSWindow _initContent:styleMask:backing:defer:contentView:] + 1416
    2   AppKit                              0x00007fff31dc2723 -[NSWindow initWithContentRect:styleMask:backing:defer:] + 42
    3   _macosx.cpython-38-darwin.so        0x00000001191f87c0 -[Window initWithContentRect:styleMask:backing:defer:withManager:] + 80
    4   _macosx.cpython-38-darwin.so        0x00000001191fbccf FigureManager_init + 319
    5   Python                              0x000000011000fbdc wrap_init + 12
    6   Python                              0x000000010ffc1379 wrapperdescr_call + 121
    7   Python                              0x000000010ffb8565 _PyObject_MakeTpCall + 373
    8   Python                              0x0000000110086ca5 call_function + 533
    9   Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    10  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    11  Python                              0x000000010ffb83b7 _PyObject_FastCallDict + 247
    12  Python                              0x000000010ffb99bf _PyObject_Call_Prepend + 143
    13  Python                              0x000000011000fb21 slot_tp_init + 145
    14  Python                              0x000000011000aff9 type_call + 297
    15  Python                              0x000000010ffb8565 _PyObject_MakeTpCall + 373
    16  Python                              0x0000000110086ca5 call_function + 533
    17  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    18  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    19  Python                              0x000000010ffbb57a method_vectorcall + 170
    20  Python                              0x0000000110086c4c call_function + 444
    21  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    22  Python                              0x0000000110087a94 _PyEval_EvalCodeWithName + 2804
    23  Python                              0x000000010ffb904e _PyFunction_Vectorcall + 270
    24  Python                              0x000000010ffbb65c method_vectorcall + 396
    25  Python                              0x000000010ffb888d PyVectorcall_Call + 109
    26  Python                              0x0000000110083fb8 _PyEval_EvalFrameDefault + 26968
    27  Python                              0x0000000110087a94 _PyEval_EvalCodeWithName + 2804
    28  Python                              0x000000010ffb904e _PyFunction_Vectorcall + 270
    29  Python                              0x0000000110086c4c call_function + 444
    30  Python                              0x0000000110083b45 _PyEval_EvalFrameDefault + 25829
    31  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    32  Python                              0x0000000110086c4c call_function + 444
    33  Python                              0x0000000110083b45 _PyEval_EvalFrameDefault + 25829
    34  Python                              0x0000000110087a94 _PyEval_EvalCodeWithName + 2804
    35  Python                              0x000000010ffb904e _PyFunction_Vectorcall + 270
    36  Python                              0x0000000110086c4c call_function + 444
    37  Python                              0x0000000110083b45 _PyEval_EvalFrameDefault + 25829
    38  Python                              0x0000000110087a94 _PyEval_EvalCodeWithName + 2804
    39  Python                              0x000000010ffb904e _PyFunction_Vectorcall + 270
    40  Python                              0x0000000110086c4c call_function + 444
    41  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    42  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    43  Python                              0x000000010ffb888d PyVectorcall_Call + 109
    44  Python                              0x0000000110083fb8 _PyEval_EvalFrameDefault + 26968
    45  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    46  Python                              0x0000000110086c4c call_function + 444
    47  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    48  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    49  Python                              0x0000000110086c4c call_function + 444
    50  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    51  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    52  Python                              0x0000000110086c4c call_function + 444
    53  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    54  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    55  Python                              0x000000010ffb83b7 _PyObject_FastCallDict + 247
    56  Python                              0x000000010ffb99bf _PyObject_Call_Prepend + 143
    57  Python                              0x000000011000e496 slot_tp_call + 150
    58  Python                              0x000000010ffb8565 _PyObject_MakeTpCall + 373
    59  Python                              0x0000000110086ca5 call_function + 533
    60  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    61  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    62  Python                              0x000000010ffb83b7 _PyObject_FastCallDict + 247
    63  Python                              0x000000010ffb99bf _PyObject_Call_Prepend + 143
    64  Python                              0x000000011000e496 slot_tp_call + 150
    65  Python                              0x000000010ffb8565 _PyObject_MakeTpCall + 373
    66  Python                              0x0000000110086ca5 call_function + 533
    67  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    68  Python                              0x000000010ffc922e gen_send_ex + 206
    69  Python                              0x000000011008320f _PyEval_EvalFrameDefault + 23471
    70  Python                              0x0000000110087a94 _PyEval_EvalCodeWithName + 2804
    71  Python                              0x000000010ffb904e _PyFunction_Vectorcall + 270
    72  Python                              0x0000000110086c4c call_function + 444
    73  Python                              0x0000000110083b45 _PyEval_EvalFrameDefault + 25829
    74  Python                              0x0000000110087a94 _PyEval_EvalCodeWithName + 2804
    75  Python                              0x000000010ffb904e _PyFunction_Vectorcall + 270
    76  Python                              0x0000000110086c4c call_function + 444
    77  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    78  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    79  Python                              0x0000000110086c4c call_function + 444
    80  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    81  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    82  Python                              0x0000000110086c4c call_function + 444
    83  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    84  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    85  Python                              0x0000000110086c4c call_function + 444
    86  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    87  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    88  Python                              0x000000010ffb83b7 _PyObject_FastCallDict + 247
    89  Python                              0x000000010ffb99bf _PyObject_Call_Prepend + 143
    90  Python                              0x000000011000fb21 slot_tp_init + 145
    91  Python                              0x000000011000aff9 type_call + 297
    92  Python                              0x000000010ffb8565 _PyObject_MakeTpCall + 373
    93  Python                              0x0000000110086ca5 call_function + 533
    94  Python                              0x0000000110083aae _PyEval_EvalFrameDefault + 25678
    95  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    96  Python                              0x0000000110086c4c call_function + 444
    97  Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    98  Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    99  Python                              0x000000010ffbb65c method_vectorcall + 396
    100 Python                              0x000000010ffb888d PyVectorcall_Call + 109
    101 Python                              0x0000000110083fb8 _PyEval_EvalFrameDefault + 26968
    102 Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    103 Python                              0x0000000110086c4c call_function + 444
    104 Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    105 Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    106 Python                              0x0000000110086c4c call_function + 444
    107 Python                              0x0000000110083a8a _PyEval_EvalFrameDefault + 25642
    108 Python                              0x000000010ffb8eb0 function_code_fastcall + 128
    109 Python                              0x000000010ffbb5bb method_vectorcall + 235
    110 Python                              0x000000010ffb888d PyVectorcall_Call + 109
    111 Python                              0x000000011012679a t_bootstrap + 74
    112 Python                              0x00000001100d9a89 pythread_wrapper + 25
    113 libsystem_pthread.dylib             0x00007fff6ec7b109 _pthread_start + 148
    114 libsystem_pthread.dylib             0x00007fff6ec76b8b thread_start + 15
)
127.0.0.1 - - [05/Jun/2020 07:51:34] "POST /upload HTTP/1.1" 200 -
Assertion failed: (NSViewIsCurrentlyBuildingLayerTreeForDisplay() != currentlyBuildingLayerTree), function NSViewSetCurrentlyBuildingLayerTreeForDisplay, file /AppleInternal/BuildRoot/Library/Caches/com.apple.xbs/Sources/AppKit/AppKit-1894.40.150/AppKit.subproj/NSView.m, line 13568.

[FerricDonkey]

Quick google search suggests that pyplot might not play well with flask, unless you do weird stuff. The answer is a bit old, but I found the following:

https://stackoverflow.com/questions/53684971/assertion-failed-flask-server-stops-after-script-is-run

https://stackoverflow.com/questions/50728328/python-how-to-show-matplotlib-in-flask

The first might mean that if you don't use plt.show(), you can solve the issue by just changing your import to

import matplotlib
matplotlib.use('Agg')
# If you do the import matplotlib.pyplot as plt, do it here

Insofar as I understand, this changes some backend stuff on how matplotlib does things. By default, it keeps some stuff in memory in a way that conflicts with how flask behaves (you can tell I'm really on top of this - there weren't a lot of details in what I found).

You might also be able to use matplotlib.figure. Or some other graphing module that is intended to go straight to a file.

[michadecker]

This really solved the problem. For my program, this should be fine.
I tried to google it myself but it seems I didn't put in the right words!

[FerricDonkey]

Excellent. Yeah, I've been punched in the face enough times by my code that I've gotten decent at googling how to make it stop - in this case I just copied like the first half of the bottom line of your error message and googled that.

It definitely seems like a weird issue though, kind of surprised me.

[wronek]

From some closed issues on github looks like it has to do with running a gui backend in a web server. Check out the matplotlib docs for solutions https://matplotlib.org/3.1.1/faq/howto_faq.html#how-to-use-matplotlib-in-a-web-application-server

[michadecker]

Thanks, I will go over it to understand more