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126 lines
4.2 KiB
ReStructuredText
126 lines
4.2 KiB
ReStructuredText
File Handling in Pillow
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=======================
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When opening a file as an image, Pillow requires a filename,
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pathlib.Path object, or a file-like object. Pillow uses the filename
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or Path to open a file, so for the rest of this article, they will all
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be treated as a file-like object.
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The first four of these items are equivalent, the last is dangerous
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and may fail::
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from PIL import Image
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import io
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import pathlib
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im = Image.open('test.jpg')
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im2 = Image.open(pathlib.Path('test.jpg'))
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f = open('test.jpg', 'rb')
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im3 = Image.open(f)
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with open('test.jpg', 'rb') as f:
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im4 = Image.open(io.BytesIO(f.read()))
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# Dangerous FAIL:
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with open('test.jpg', 'rb') as f:
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im5 = Image.open(f)
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im5.load() # FAILS, closed file
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The documentation specifies that the file will be closed after the
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``Image.Image.load()`` method is called. This is an aspirational
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specification rather than an accurate reflection of the state of the
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code.
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Pillow cannot in general close and reopen a file, so any access to
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that file needs to be prior to the close.
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Issues
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------
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The current open file handling is inconsistent at best:
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* Most of the image plugins do not close the input file.
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* Multi-frame images behave badly when seeking through the file, as
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it's legal to seek backward in the file until the last image is
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read, and then it's not.
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* Using the file context manager to provide a file-like object to
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Pillow is dangerous unless the context of the image is limited to
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the context of the file.
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Image Lifecycle
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---------------
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* ``Image.open()`` called. Path-like objects are opened as a
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file. Metadata is read from the open file. The file is left open for
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further usage.
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* ``Image.Image.load()`` when the pixel data from the image is
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required, ``load()`` is called. The current frame is read into
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memory. The image can now be used independently of the underlying
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image file.
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* ``Image.Image.seek()`` in the case of multi-frame images
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(e.g. multipage TIFF and animated GIF) the image file left open so
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that seek can load the appropriate frame. When the last frame is
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read, the image file is closed (at least in some image plugins), and
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no more seeks can occur.
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* ``Image.Image.close()`` Closes the file pointer and destroys the
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core image object. This is used in the Pillow context manager
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support. e.g.::
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with Image.open('test.jpg') as img:
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... # image operations here.
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The lifecycle of a single frame image is relatively simple. The file
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must remain open until the ``load()`` or ``close()`` function is
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called.
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Multi-frame images are more complicated. The ``load()`` method is not
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a terminal method, so it should not close the underlying file. The
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current behavior of ``seek()`` closing the underlying file on
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accessing the last frame is presumably a heuristic for closing the
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file after iterating through the entire sequence. In general, Pillow
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does not know if there are going to be any requests for additional
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data until the caller has explicitly closed the image.
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Complications
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-------------
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* TiffImagePlugin has some code to pass the underlying file descriptor
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into libtiff (if working on an actual file). Since libtiff closes
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the file descriptor internally, it is duplicated prior to passing it
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into libtiff.
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* ``decoder.handles_eof`` This slightly misnamed flag indicates that
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the decoder wants to be called with a 0 length buffer when reads are
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done. Despite the comments in ``ImageFile.load()``, the only decoder
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that actually uses this flag is the Jpeg2K decoder. The use of this
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flag in Jpeg2K predated the change to the decoder that added the
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pulls_fd flag, and is therefore not used.
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* I don't think that there's any way to make this safe without
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changing the lazy loading::
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# Dangerous FAIL:
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with open('test.jpg', 'rb') as f:
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im5 = Image.open(f)
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im5.load() # FAILS, closed file
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Proposed File Handling
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----------------------
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* ``Image.Image.load()`` should close the image file, unless there are
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multiple frames.
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* ``Image.Image.seek()`` should never close the image file.
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* Users of the library should call ``Image.Image.close()`` on any
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multi-frame image to ensure that the underlying file is closed.
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