EC
struct.error
PythonERRORNotableValue ErrorHIGH confidence

Binary struct pack/unpack failed

Production Risk

Common when parsing network protocols; always validate buffer length with struct.calcsize().

What this means

Raised by the struct module when a pack format string does not match the data, or when trying to unpack a buffer of the wrong size.

Why it happens
  1. 1Unpacking a buffer that is shorter than the format requires
  2. 2struct.pack() value out of range for the format (e.g., >255 for "B")
  3. 3Wrong format string for the binary data being parsed
How to reproduce

Unpacking a buffer shorter than the format requires.

trigger — this will error
trigger — this will error
import struct
struct.unpack('>IH', b'\x00\x01')  # Format needs 6 bytes, only 2 given

expected output

struct.error: unpack requires a buffer of 6 bytes

Fix

Check buffer length before unpacking

WHEN Parsing binary data from network or files

Check buffer length before unpacking
import struct

FORMAT = '>IHB'  # 4+2+1 = 7 bytes
SIZE = struct.calcsize(FORMAT)

def parse_packet(data):
    if len(data) < SIZE:
        raise ValueError(f"Packet too short: {len(data)} < {SIZE}")
    return struct.unpack(FORMAT, data[:SIZE])

Why this works

struct.calcsize() returns the exact byte count needed; always validate before unpack.

Code examples
Triggerpython
import struct
struct.unpack(">IH", b"\x00\x01")  # struct.error: unpack requires 6 bytes, got 2
Handle with try/exceptpython
import struct
FMT = ">IH"
try:
    vals = struct.unpack(FMT, data)
except struct.error as e:
    print(f"Bad binary data: {e}")
Avoid with calcsize checkpython
import struct
FMT = ">IH"
SIZE = struct.calcsize(FMT)
if len(data) >= SIZE:
    vals = struct.unpack(FMT, data[:SIZE])
Sources
Official documentation ↗

Python Docs — struct module

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