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Contrary to PEP-8 (https://www.python.org/dev/peps/pep-0008/#tabs-or-spaces) claiming "Python 3 disallows mixing the use of tabs and spaces for indentation", in reality Python 3 allows mixing tabs and spaces without warnings or errors. For example (with all spaces replaced with middle dot and all spaces replaced with a short arrow):
#!/usr/bin/python3
for·i·in·range(2):
··for·j·in·range(3):
➧··for·k·in·range(4):
·➧···print(i,j,k,sep=",")
Either the PEP-8 should be fixed or the parser/compiler should be fixed. 
The current behavior seems a bit unstable because the exact order of spaces and tabs causes "TabError: inconsistent use of tabs and spaces in indentation" to appear more or less random.
I'd prefer python3 to require that *all whitespace* at the start of the all the lines is tabs for the whole file or spaces for the whole file. And first indented line sets the preference for the whole file.
(Personally I'd prefer tabs contrary to PEP-8 language but this bug is not about the preference. This is PEP-8 claiming things that are not true.)
$ python3 --version
Python 3.5.2

I can verify this on 3.7.4.

I believe that the interpreter only requires that each block is consistent, not that all blocks in a module are consistent.

Thank you for your report Mikko. Yes, the heuristic used in Python 3 is unable to catch all inconsistencies in using tabs and spaces.
Such bug was already reported early. There should be an open issue.

Seems related issue24260 

Thanks, Serhiy. This has been reported in 24260, so I'm closing this issue.