It's running on the MBP now. Apparently the F-Secure anti-virus software running on the machine doesn't like Sage and blocks it's execution (even though it doesn't say so in its logs). Removing the previous 10.12.3 installation and reinstalling if from scratch with F-Secure disabled solved the issue. Anyway, the next thing to resolve is how to automatically start the jupyter interface rather than the old notebook, but I guess that's another thread...
BTW, the iMac had the same issue. It starts the app now, but unfortunately doesn't like the 10.12.3 dmg. When starting the app, it opens two terminal windows, one of which shows the error messages copied below. I'll try the 10.11.6 dmg later.
10.12.3 dmg error message on a late 2012 iMac:
iMac-of-Tony:~ tony$ '/Applications/SageMath-7.6.app/Contents/Resources/sage/sage' --notebook=sagenb
Fatal Python error: Py_Initialize: unable to load the file system codec
File "/Applications/SageMath-7.6.app/Contents/Resources/sage/local/lib/python/encodings/__init__.py", line 123
raise CodecRegistryError,\
^
SyntaxError: invalid syntax
Current thread 0x00007ffff04043c0 (most recent call first):
/Applications/SageMath-7.6.app/Contents/Resources/sage/sage: line 130: 11912 Abort trap: 6 "$SAGE_ROOT/relocate-once.py"
Traceback (most recent call last):
File "/Applications/SageMath-7.6.app/Contents/Resources/sage/src/bin/sage-notebook", line 7, in <module>
import argparse
File "/Applications/SageMath-7.6.app/Contents/Resources/sage/local/lib/python/argparse.py", line 85, in <module>
import collections as _collections
File "/Applications/SageMath-7.6.app/Contents/Resources/sage/local/lib/python/collections.py", line 21, in <module>
from operator import itemgetter as _itemgetter, eq as _eq
ImportError: dlopen(/Applications/SageMath-7.6.app/Contents/Resources/sage/local/lib/python2.7/lib-dynload/operator.so, 2): Symbol not found: __PyUnicodeUCS4_AsDefaultEncodedString
Referenced from: /Applications/SageMath-7.6.app/Contents/Resources/sage/local/lib/python2.7/lib-dynload/operator.so
Expected in: flat namespace
in /Applications/SageMath-7.6.app/Contents/Resources/sage/local/lib/python2.7/lib-dynload/operator.so
iMac-of-Tony:~ tony$
Hmm, what happens if you launch it directly from the Applications directory? You do need to move it from the dmg (I think you already did that). Also, do you have a fairly old piece of hardware running this with 10.12? In that case you might want to try the 10.11.6 dmg just in case.
Yes, I did move it from the dmg. When I open a terminal session and run /Applications/Sage-7.6.app/sage, eventually the sage: prompt appears and the browser-based notebook interface can be started by means of the notebook() command. It's ugly, but it works. The iMac is late 2012 and the MBP is early 2015, so not that old I would say...
Huh, I would have expected you to say that the terminal didn't work. I'm sorry that we don't have any tools to remotely diagnose this for you. The 2012 one with 10.12 I would expect to have different instruction sets but that shouldn't affect the app part.