I added PyQt5 to the Python3.6 installation on my iMac running Sierra 10.12.4 in a Terminal window with:
$pip3 PyQt5
Then I tried to run QtDesigner. It failed as follows:
$cd /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/PyQt5/Qt/lib/QtDesigner.framework/Versions/5
$./QtDesigner
-bash: ./QtDesigner: cannot execute binary file
The file description is:
$file QtDesigner
QtDesigner: Mach-O 64-bit dynamically linked shared library x86_64
I was pretty sure my iMac is a 64 bit machine but just to check:
file /usr/bin/mail
/usr/bin/mail: Mach-O 64-bit executable x86_64
Any ideas why QtDesigner won't run?