CPython uses reference counting, so an object is garbage collected as
soon as there are no references to it, but that's just an implementation
detail.
Other implementations, such as Jython and IronPython, don't use
reference counting, so you don't know when an object will be garbage
collected, which means that the file might remain open for an unknown
time afterwards in case 1 above.
Most people use CPython, so it's not surprising that case 1 is so
common.
Additionally, many scripts just use a small number of files (say,
1-5 files) so having a file-handle open for the duration of the
run it minimal overhead.
On the other hand, when processing thousands of files, I always
explicitly close each file to make sure I don't exhaust some
file-handle limit the OS or interpreter may enforce.
-tkc
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