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From: Travis Griggs <travisgriggs@gmail.com>
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Subject: Style help for a Smalltalk-hack
Date: Mon, 22 Oct 2012 17:43:00 -0700
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Xref: csiph.com comp.lang.python:31911

I'm writing some code that does a structured read from formatted binary =
file. The code I came up with looks like:

# get the first four bytes, the first gap field
chunk =3D byteStream.read(4)
while chunk:
    # interpret the gap bytes
    gap, =3D struct.unpack('>I', chunk)
    # suck off the valveCount
    valveCount, =3D struct.unpack('>I', byteStream.read(4))
    # collect the next valveCount signatures
    signatures =3D [struct.unpack('>I', byteStream.read(4))[0] for _ in =
range(valveCount)]
    self.script.append(ScriptSpan(gap=3Dgap, valveSet=3Dsignatures))
    # now get the next 4 bytes for the gap of the next iteration, it'll =
be empty if we're at end
    chunk =3D byteStream.read(4)

I can't help but thinking that there's some better way (i.e. more =
pythonic) to do this that doesn't involve having to use another module =
(Construct) or exploring generators or something like that. What bugs me =
about it is that there is two different styles for reading/decoding =
values from the byte stream. valveCount and signatures are both paired =
invocations of unpack() and read(). But to detect the end of the stream =
(file), I have to split the read() and unpack() of the gap value across =
3 different lines of the code, and they don't even sit adjacent to each =
other.

I'm wandering up the Python curve with a passel of Smalltalk experience =
under my belt, so I expect I'm struggling with trying to map something =
like this across to python

[byteStream atEnd] whileFalse: [
     gap :=3D (byteStream next: 4) asInteger.
     valveCount :=3D (byteStream next: 4) asInteger.
     signatures :=3D (1 to: valveCount) collect: [:_ | (byteStream next: =
4) asInteger].
     self script add: (ScriptSpan gap: gap valveSet: signatures).
]

The part that doesn't seem to be there in the standard python library is =
the idea of an atEnd message for streams, it's inferred as a byproduct =
of a read().

Please be gentle/kind. I'm still learning. :) TIA

--
Travis Griggs
"A vital ingredient of success is not knowing that what you're =
attempting can't be done." -Terry Pratchett