[issue38837] struct.pack: Unable to pack more than 256 bytes at a time

[Dave Lotton]

Dave Lotton  added the comment:

Mark, you are absolutely correct.  I'm an idiot.  

Focused on wrong thing.  Thank you.

--
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue38837] struct.pack: Unable to pack more than 256 bytes at a time

[Mark Dickinson]

Mark Dickinson  added the comment:

The error you're getting is because you're trying to pack a *value* that's 
larger than 255, not because you're trying to pack more than 256 bytes. Packing 
more than 256 bytes at a time works fine for me.

>>> packed = struct.pack("<512B", *[n//2 for n in range(512)])  # fine
>>> packed = struct.pack("2B", *[255, 256])
Traceback (most recent call last):
  File "", line 1, in 
struct.error: ubyte format requires 0 <= number <= 255

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nosy: +mark.dickinson

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[issue38837] struct.pack: Unable to pack more than 256 bytes at a time

[Dave Lotton]

New submission from Dave Lotton :

Using struct.pack it is not possible (Python 3.6.8 and 2.7.15) to pack more 
than 256 bytes at a time.  

This seems like an arbitrarily small number, and seems to be inconsistent with 
the capabilities of the unpack function, which is able to unpack a larger 
number of bytes.

Below demonstrates the issue...

>>> # Create large data set to be packed
>>> data = range(0,512)

>>> # Demonstrate format string for packing
>>> '<%dB' % len(data)
'<512B'

>>> # Try to pack large data set
>>> packed = pack('<%dB' % len(data), *data)
Traceback (most recent call last):
  File "", line 1, in 
struct.error: ubyte format requires 0 <= number <= 255

>>> # Make data set <= 256 bytes
>>> data = range(0,256)
>>> packed = pack('<%dB' % len(data), *data)
>>> # Data successfully packed


>>> # Append another 256 bytes to packed data
>>> packed += pack('<%dB' % len(data), *data)

>>> # Unpack all 512 bytes
>>> unpacked = unpack('<%dB' % len(packed), packed)
>>> 
>>> unpacked
(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 
22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 
42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 
62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 
82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 
101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 
117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 
133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 
149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 
165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 
181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 
197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 
213, 214, 215, 216, 217, 218, 219, 220, 22
 1, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 
237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 
253, 254, 255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 
18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 
38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 
58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 
78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 
98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 
114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 
130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 
146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 
162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 
178, 179, 180, 181, 182, 183, 184, 185, 186,
  187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 19
7, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 
213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 
229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 
245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255)




The 256 byte limit on packing seems arbitrarily small and is inconsistent with 
the unpack function.  I'm wondering if there is a rationale for this limit, or 
it has simply been an oversight in the implementation.

I am using Mint Linux 19.2 64-bit (Ubuntu Bionic - based distro).  The problem 
is manifested on both Python 3.6.8 and 2.7.15 included in the distro.

--
components: Library (Lib)
messages: 356886
nosy: Dave Lotton
priority: normal
severity: normal
status: open
title: struct.pack: Unable to pack more than 256 bytes at a time
type: behavior
versions: Python 2.7, Python 3.6

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