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imshow() Python/macOS not showing the image

I'm literally just starting out, but already stuck .. I'm trying to get an image to display, but imshow() only opens an empty window. I don't get any errors, and the program doesn't hang (pressing a key closes the window just fine) - it's just that there is no image in the window.

I know that the image is loaded correctly, because I'm printing out the arrays and image data looks right. Also I can see that the window always has the correct _measurements_ for whichever image file I'm picking.

import numpy as np
import cv2

img = cv2.imread('butterfly.jpg', 0)
print(img)

cv2.imshow('Butterfly', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Things I have tried (to no avail):

using an absolute path instead
adding cv2.startWindowThread(); cv2.namedWindow('Butterfly') before the imshow() command
reading lots of posts with similar problems

I'm using Python3.6, on macOS (Mojave), cv2 v.4.0.1

Any ideas what I might be doing wrong?

imshow() Python/macOS not showing the image

import numpy as np
import cv2

img = cv2.imread('butterfly.jpg', 0)
print(img)

cv2.imshow('Butterfly', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Things I have tried (to no avail):

using an absolute path instead
adding cv2.startWindowThread(); cv2.namedWindow('Butterfly') before the imshow() command
reading lots of posts with similar problems

I'm using Python3.6, on macOS (Mojave), cv2 v.4.0.1

Any ideas what I might be doing wrong?

-- Edit -- print(cv2.getBuildInformation()) gives me this:

General configuration for OpenCV 4.0.1 =====================================
  Version control:               unknown

  Extra modules:
    Location (extra):            /tmp/opencv-20190105-31032-o160to/opencv-4.0.1/opencv_contrib/modules
    Version control (extra):     unknown

  Platform:
    Timestamp:                   2019-01-05T01:41:15Z
    Host:                        Darwin 18.2.0 x86_64
    CMake:                       3.13.2
    CMake generator:             Unix Makefiles
    CMake build tool:            /usr/local/Homebrew/Library/Homebrew/shims/mac/super/gmake
    Configuration:               Release

  CPU/HW features:
    Baseline:                    SSE SSE2 SSE3 SSSE3 SSE4_1 POPCNT SSE4_2
      requested:                 DETECT
      disabled:                  SSE4_1 SSE4_2 AVX AVX2
    Dispatched code generation:  SSE4_1 SSE4_2 FP16 AVX AVX2 AVX512_SKX
      requested:                 SSE4_1 SSE4_2 AVX FP16 AVX2 AVX512_SKX
      SSE4_1 (0 files):          +
      SSE4_2 (0 files):          +
      FP16 (0 files):            + FP16 AVX
      AVX (4 files):             + AVX
      AVX2 (11 files):           + FP16 FMA3 AVX AVX2
      AVX512_SKX (1 files):      + FP16 FMA3 AVX AVX2 AVX_512F AVX512_SKX

  C/C++:
    Built as dynamic libs?:      YES
    C++ Compiler:                /usr/local/Homebrew/Library/Homebrew/shims/mac/super/clang++  (ver 10.0.0.10001145)
    C++ flags (Release):         -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -fdiagnostics-show-option -Wno-long-long -Qunused-arguments -Wno-semicolon-before-method-body -ffunction-sections -fdata-sections  -fvisibility=hidden -fvisibility-inlines-hidden -DNDEBUG  -DNDEBUG
    C++ flags (Debug):           -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -fdiagnostics-show-option -Wno-long-long -Qunused-arguments -Wno-semicolon-before-method-body -ffunction-sections -fdata-sections  -fvisibility=hidden -fvisibility-inlines-hidden -g  -O0 -DDEBUG -D_DEBUG
    C Compiler:                  /usr/local/Homebrew/Library/Homebrew/shims/mac/super/clang
    C flags (Release):           -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -fdiagnostics-show-option -Wno-long-long -Qunused-arguments -Wno-semicolon-before-method-body -ffunction-sections -fdata-sections  -fvisibility=hidden -fvisibility-inlines-hidden -DNDEBUG  -DNDEBUG
    C flags (Debug):             -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-narrowing -Wno-delete-non-virtual-dtor -Wno-unnamed-type-template-args -Wno-comment -fdiagnostics-show-option -Wno-long-long -Qunused-arguments -Wno-semicolon-before-method-body -ffunction-sections -fdata-sections  -fvisibility=hidden -fvisibility-inlines-hidden -g  -O0 -DDEBUG -D_DEBUG
    Linker flags (Release):
    Linker flags (Debug):
    ccache:                      NO
    Precompiled headers:         NO
    Extra dependencies:
    3rdparty dependencies:

  OpenCV modules:
    To be built:                 aruco bgsegm bioinspired calib3d ccalib core datasets dnn dnn_objdetect dpm face features2d flann fuzzy gapi hfs highgui img_hash imgcodecs imgproc java_bindings_generator line_descriptor ml objdetect optflow phase_unwrapping photo plot python2 python3 python_bindings_generator reg rgbd saliency shape stereo stitching structured_light superres surface_matching tracking video videoio videostab xfeatures2d ximgproc xobjdetect xphoto
    Disabled:                    text world
    Disabled by dependency:      -
    Unavailable:                 cnn_3dobj cudaarithm cudabgsegm cudacodec cudafeatures2d cudafilters cudaimgproc cudalegacy cudaobjdetect cudaoptflow cudastereo cudawarping cudev cvv freetype hdf java js matlab ovis sfm ts viz
    Applications:                apps
    Documentation:               NO
    Non-free algorithms:         YES

  GUI:
    Cocoa:                       YES

  Media I/O:
    ZLib:                        /usr/lib/libz.dylib (ver 1.2.11)
    JPEG:                        build-libjpeg-turbo (ver 1.5.3-62)
    WEBP:                        build (ver encoder: 0x020e)
    PNG:                         /usr/local/lib/libpng.dylib (ver 1.6.36)
    TIFF:                        /usr/local/lib/libtiff.dylib (ver 42 / 4.0.10)
    OpenEXR:                     /usr/local/lib/libImath.dylib /usr/local/lib/libIlmImf.dylib /usr/local/lib/libIex.dylib /usr/local/lib/libHalf.dylib /usr/local/lib/libIlmThread.dylib (ver 2.2.0)
    HDR:                         YES
    SUNRASTER:                   YES
    PXM:                         YES
    PFM:                         YES

  Video I/O:
    FFMPEG:                      YES
      avcodec:                   YES (ver 58.35.100)
      avformat:                  YES (ver 58.20.100)
      avutil:                    YES (ver 56.22.100)
      swscale:                   YES (ver 5.3.100)
      avresample:                YES (ver 4.0.0)
    AVFoundation:                YES

  Parallel framework:            TBB (ver 2019.0 interface 11003)

  Trace:                         YES (with Intel ITT)

  Other third-party libraries:
    Intel IPP:                   2019.0.0 Gold [2019.0.0]
           at:                   /tmp/opencv-20190105-31032-o160to/opencv-4.0.1/build/3rdparty/ippicv/ippicv_mac/icv
    Intel IPP IW:                sources (2019.0.0)
              at:                /tmp/opencv-20190105-31032-o160to/opencv-4.0.1/build/3rdparty/ippicv/ippicv_mac/iw
    Lapack:                      YES (/usr/local/opt/openblas/lib/libopenblas.dylib /usr/local/opt/openblas/lib/libopenblas.dylib)
    Eigen:                       YES (ver 3.3.7)
    Custom HAL:                  NO
    Protobuf:                    build (3.5.1)

  OpenCL:                        YES (no extra features)
    Include path:                NO
    Link libraries:              -framework OpenCL

  Python 2:
    Interpreter:                 /usr/local/opt/python@2/bin/python (ver 2.7.15)
    Libraries:                   /usr/local/Cellar/python@2/2.7.15_1/Frameworks/Python.framework/Versions/2.7/lib/libpython2.7.dylib (ver 2.7.15)
    numpy:                       /usr/local/lib/python2.7/site-packages/numpy/core/include (ver 1.15.4)
    install path:                lib/python2.7/site-packages/cv2/python-2.7

  Python 3:
    Interpreter:                 /usr/local/opt/python/bin/python3 (ver 3.7.2)
    Libraries:                   /usr/local/opt/python/Frameworks/Python.framework/Versions/3.7/lib/python3.7/config-3.7m-darwin/libpython3.7.dylib (ver 3.7.2)
    numpy:                       /usr/local/lib/python3.7/site-packages/numpy/core/include (ver 1.15.4)
    install path:                lib/python3.7/site-packages/cv2/python-3.7

  Python (for build):            /usr/local/opt/python@2/bin/python

  Java:
    ant:                         NO
    JNI:                         /Library/Java/JavaVirtualMachines/openjdk-11.0.1.jdk/Contents/Home/include /Library/Java/JavaVirtualMachines/openjdk-11.0.1.jdk/Contents/Home/include/darwin /Library/Java/JavaVirtualMachines/openjdk-11.0.1.jdk/Contents/Home/include
    Java wrappers:               NO
    Java tests:                  NO

  Install to:                    /usr/local/Cellar/opencv/4.0.1
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