[Python-Dev] Why can't I encode/decode base64 without importing a module?
mal at egenix.com
Wed Apr 24 10:22:42 CEST 2013
On 23.04.2013 19:24, Guido van Rossum wrote:
> On Tue, Apr 23, 2013 at 9:04 AM, M.-A. Lemburg <mal at egenix.com> wrote:
>> On 23.04.2013 17:47, Guido van Rossum wrote:
>>> On Tue, Apr 23, 2013 at 8:22 AM, M.-A. Lemburg <mal at egenix.com> wrote:
>>>> Just as reminder: we have the general purpose
>>>> encode()/decode() functions in the codecs module:
>>>> import codecs
>>>> r13 = codecs.encode('hello world', 'rot-13')
>>>> These interface directly to the codec interfaces, without
>>>> enforcing type restrictions. The codec defines the supported
>>>> input and output types.
>>> As an implementation mechanism I see nothing wrong with this. I hope
>>> the codecs module lets you introspect the input and output types of a
>>> codec given by name?
>> At the moment there is no standard interface to access supported
>> input and output types... but then: regular Python functions or
>> methods also don't provide such functionality, so no surprise
>> there ;-)
> Not quite the same though. Each function has its own unique behavior.
> But codecs support a standard interface, *except* that the input and
> output types sometimes vary.
The codec system itself
>> It's mostly a matter of specifying the supported type
>> combinations in the codec documentation.
>> BTW: What would be a use case where you'd want to
>> programmatically access such information before calling
>> the codec ?
> As you know, in Python 3, most code working with bytes doesn't also
> work with strings, and vice versa (except for a few cases where we've
> gone out of our way to write polymorphic code -- but users rarely do
> so, and any time you use a string or bytes literal you basically limit
> yourself to that type).
> Suppose I write a command-line utility that reads a file, runs it
> through a codec, and writes the result to another file. Suppose the
> name of the codec is a command-line argument (as well as the
> filenames). I need to know whether to open the files in text or binary
> mode based on the name of the codec.
Ok, so you need to know which codecs your tool can support and
which of those need text input and which bytes input.
I've been thinking about this some more: I think that type
information alone is not flexible enough to cover such
In your use case you'd want to only permit use of a certain
set of codecs, not simply all of them, since some might
not implement what you actually want to achieve with the tool,
e.g. a user might have installed a codec set that adds
support for reading and writing image data, but your
intended use was to only support text data.
So what we need is a way to allow the codecs to say e.g.
"I work on text", "I support encoding bytes and text",
"I encode to bytes", "I'm reversible", "I transform
input data", "I support bytes and text, and will create
same type output", "I work on image data", "I work on
X509 certificates", "I work on XML data", etc.
In other words, we need a form of tagging system, with a
set of standard tags that each codec can publish and
which also allows non-standard tags (which can then at
some point be made standard, if there's agreement on them).
Given a codec name you could then ask the codec registry for
the codec tags and verify that the chosen codec handles
text data, needs bytes or text encoding input and
creates bytes as encoding output. If the registry returns
codec tags that don't include the "I work on text" tag,
the tool could then raise an error.
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