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bpo-26175: Fix SpooledTemporaryFile IOBase abstract #3249

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bpo-26175: Fix SpooledTemporaryFile IOBase abstract #3249

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83d6152
Fix SpooledTemporaryFile IOBase abstract
GFernie Aug 30, 2017
3b69baa
Add news entry for bpo-26175 patch
GFernie Aug 30, 2017
753b4fb
Update 2017-08-30-23-14-17.bpo-26175.GOaj9y.rst
merwok Sep 20, 2017
a70113e
Make SpooledTemporaryFile subclass IOBase
GFernie Sep 30, 2017
fb28362
Refactor tests to more reliably test abstract
GFernie Sep 30, 2017
bc9d0a9
Return new absolute position from underlying file on seek
GFernie Sep 9, 2018
b588dec
Do nothing on __del__
GFernie Sep 9, 2018
ab49dd1
Return new file size from underlying file on truncate
GFernie Sep 9, 2018
4251b21
Merge remote-tracking branch 'origin/master' into bpo-26175_fix-Spool…
GFernie Oct 30, 2019
19f4c08
Add versionchanged tag
GFernie Oct 30, 2019
4519b9c
Use :class: instead of :py:data:
GFernie Oct 30, 2019
554aad6
Revert "Do nothing on __del__"
GFernie Nov 23, 2019
be094b3
Add test for rolled file finalisation
GFernie Nov 23, 2019
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17 changes: 16 additions & 1 deletion Lib/tempfile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -626,7 +626,7 @@ def TemporaryFile(mode='w+b', buffering=-1, encoding=None,
_os.close(fd)
raise

class SpooledTemporaryFile:
class SpooledTemporaryFile(_io.IOBase):
"""Temporary file wrapper, specialized to switch from BytesIO
or StringIO to a real file when it exceeds a certain size or
when a fileno is needed.
Expand Down Expand Up @@ -685,6 +685,9 @@ def __exit__(self, exc, value, tb):
def __iter__(self):
return self._file.__iter__()

def __del__(self):
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@pppery pppery Jun 6, 2018 •
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This shouldn't be added: deleting the SpooledTemporaryFile will null out the reference to the underline file, and therefor call its __del__

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@vadmium vadmium Jun 9, 2018 •
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I think if you inherit the default IOBase.__del__ implementation, it will call close and defeat any ResourceWarning that might otherwise be emitted. Perhaps it is better to make __del__ do nothing, or set it to object.__del__. [Seems that last option doesn’t exist.]

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I agree: the underlying file should not be explicitly deleted as this is not expected behaviour. I can imagine a situation where someone is deliberately holding a reference to the underlying file and they would not expect/want it to be closed until their own reference has fallen out of scope. I've changed the method to do nothing and added a docstring to reflect this.

Thanks for your feedback

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I agree: the underlying file should not be explicitly deleted as this is not expected behaviour

The doc says: """This function operates exactly as TemporaryFile() does, except [irrelevant differences]."""

And then, about TemporaryFile: """On completion of the context or destruction of the file object the temporary file will be removed from the filesystem."""

So it seems the underlying file should be deleted when the file object disappears.

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The point is that IOBase implements a __del__ which has some side-effects. Those side effects are not desirable here. Any underlying buffers being wrapped, e.g. the TemporaryFile or the BytesIO, will be gc'd and handled as they should.

I think this implementation is correct.

return self._file.__del__()
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Hmm, calling the __del__ method directly like that is not correct. __del__ is only to be called when all external references are gone. Instead I would suggest something like the following:

    def __del__(self):
        if not self.closed:
            import warnings
            warnings.warn('unclosed file %r' % (self,), ResourceWarning,
                          stacklevel=2, source=self)
            self.close()

(this is adapted from FileIO.__del__ in Lib/pyio.py)


def close(self):
self._file.close()

Expand Down Expand Up @@ -737,6 +740,12 @@ def newlines(self):
def read(self, *args):
return self._file.read(*args)

def readable(self):
return self._file.readable()

def readinto(self, b):
return self._file.readinto(b)

def readline(self, *args):
return self._file.readline(*args)

Expand All @@ -746,6 +755,9 @@ def readlines(self, *args):
def seek(self, *args):
self._file.seek(*args)

def seekable(self):
return self._file.seekable()

@property
def softspace(self):
return self._file.softspace
Expand All @@ -767,6 +779,9 @@ def write(self, s):
self._check(file)
return rv

def writable(self):
return self._file.writable()

def writelines(self, iterable):
file = self._file
rv = file.writelines(iterable)
Expand Down
16 changes: 16 additions & 0 deletions Lib/test/test_tempfile.py
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Original file line number Diff line number Diff line change
Expand Up @@ -982,6 +982,22 @@ def test_basic(self):
f = self.do_create(max_size=100, pre="a", suf=".txt")
self.assertFalse(f._rolled)

def test_is_iobase(self):
# SpooledTemporaryFile should implement io.IOBase
self.assertIsInstance(self.do_create(), io.IOBase)

def test_iobase_interface(self):
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Those tests are not very interesting. It would be better to test the methods operate properly.

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It literally delegates everything to the underlying file.

The existing tests should cover the behaviors. The new behaviors are intended to flesh out the API so that SpooledTemporaryFile, and these tests seem to cover that.

# SpooledTemporaryFile should implement the io.IOBase interface.
# IOBase does not declare read(), readinto(), or write(), but
# they should be considered part of the interface.
iobase_abstract = {'read', 'readinto', 'write'}
spooledtempfile_abstract = set(dir(tempfile.SpooledTemporaryFile))
missing_attributes = iobase_abstract - spooledtempfile_abstract
self.assertFalse(
missing_attributes,
'io.IOBase attributes missing from SpooledTemporaryFile'
)

def test_del_on_close(self):
# A SpooledTemporaryFile is deleted when closed
dir = tempfile.mkdtemp()
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2017-08-30-23-14-17.bpo-26175.GOaj9y.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fully implement io.IOBase interface for tempfile.SpooledTemporaryFile.
Patch by Gary Fernie.