testing an open file

[Gribouillis]

what cases where it may not have a mode attribute?

When the (pseudo) file object was not opened by the function open(), for example

>>> import io
>>> f = io.StringIO()
>>> f.mode
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: '_io.StringIO' object has no attribute 'mode'