On 16-Sep-07, at 12:38 PM, Guido van Rossum wrote:
> I'm not doubting that *your* subclass works well enough. The problem
> is that it must robust in the light of *any* subclass, no matter how
> crazy.
I understand that, but I'm not sure what kind of problems can be
created by crazy subclasses. But my imagination of "crazy subclass"
is pretty limited.
> I'd have to understand more about your app to see whether subclassing
> truly makes sense.
I didn't want to flood too many pointless details into the
discussion, so here's the minimum that I think is relevant. The
project is pyPdf, a library for reading and writing PDF files. I've
been working on making the library support unicode text strings
within PDF documents.
In a PDF file, a "string" can either be a text string, or a byte
string. A string is a text string if it starts with a UTF-16BE BOM
marker, or if it can be decoded using an encoding called
PDFDocEncoding (which is specified by the PDF reference, similar to
Latin-1 but different just to make life difficult). pyPdf needs to
be capable of reading and writing these string objects. Whether a
string is a byte or a text string, writing out the raw bytes is the
same process after the text has been encoded. This lends itself to a
common StringObject base class:
class StringObject(PdfObject):
# contains common behavior for both types of strings, such as
the ability to serialize out a byte array, encrypt/decrypt strings
for "secure" PDF files
# also contains reading code that attempts to autodetect whether
the string is a byte or text string
class ByteStringObject(bytes, StringObject):
# adds the byte array storage, and passes self back to
StringObject for serialization output
class TextStringObject(str, StringObject):
# overrides the default output serialization to encode the
unicode string to match PDF's requirements,
# passes the resulting byte array up for serialization.
(complete source code, if you're interested: http://hg.pybrary.net/
pyPdf-py3/file/fe0dc2014a1b/pyPdf/generic.py)
Deriving from the bytes type provides storage, and also direct & easy
access to the byte array content. I think in this case using bytes
as a base type makes sense, at least as much as using str as a base
type. pyPdf derives from list and dict for different PDF object
types in a similar manner as well.
Mathieu Fenniak
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