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Marco: I agree "Python reports an error" would have been simpler. That is what I meant to say. Anyway, perhaps we should put
Python raises :exc:`IndentationError` if mixed tabs and spaces are causing problems in leading whitespace.
In general, the exception seems to be IndentationError. TabError is a subclass, but is not raised in all circumstances. It seems the compiler assumes a particular tab model, so the exact exception depends on the amount of tabs and spaces:
>>> exec("if True:\n" + " "*8 + "1\n" "\t2\n")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<string>", line 3
 2
 ^
TabError: inconsistent use of tabs and spaces in indentation
>>> exec("if True:\n" + " "*9 + "1\n" "\t2\n")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<string>", line 3
 2
 ^
IndentationError: unindent does not match any outer indentation level