The Tool Desk
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Convert an integer with int.to_bytes()
The method signature is n.to_bytes(length, byteorder, signed=False). The length sets the output width in bytes, and byteorder must be "big" or "little". The default for signed is False, so values are treated as unsigned unless you request signed encoding. See Python’s int.to_bytes() documentation.
n = 1024
encoded = n.to_bytes(2, byteorder="big")
print(encoded) # b"x04x00"Choose the width and order required by the receiving file format or protocol. Python raises OverflowError when the integer cannot be represented in the specified number of bytes.
Choose the byte length
If you need the smallest unsigned representation of a nonnegative integer, calculate its width from its bit length:
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n = 1024
length = (n.bit_length() + 7) // 8
encoded = n.to_bytes(length, byteorder="big")This produces a compact representation, but it may not match a format that requires a fixed-width field. For example, a protocol may require two bytes even when the value could fit in one; the extra leading zero byte is part of that representation.
Zero has a bit length of zero, so the formula gives a zero-byte result. If your format requires at least one byte, use:
length = max(1, (n.bit_length() + 7) // 8)Choose big-endian or little-endian
Byte order determines which end of the value is stored first. Big-endian places the most significant byte first; little-endian places it last. For the same value and width, the byte sequence differs:
n = 1000
big = n.to_bytes(2, byteorder="big")
little = n.to_bytes(2, byteorder="little")
print(big) # b"x03xe8"
print(little) # b"xe8x03"Use the order specified by the format you are reading or writing. sys.byteorder is appropriate only when you specifically need the current machine’s native order; it is not a substitute for a fixed byte order in data exchanged between systems.
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Negative values cannot be represented in the default unsigned mode. Set signed=True to use two’s-complement encoding and choose a width large enough for the value:
encoded = (-1024).to_bytes(2, byteorder="big", signed=True)
print(encoded) # b"xfcx00"The signed setting affects positive values too: the same high-bit pattern can be interpreted differently depending on whether decoding is signed or unsigned. Keep the width and signedness aligned with the format.
Decode the bytes and check a round trip
Use int.from_bytes() to turn a bytes-like value back into an integer. Supply the same byte order and signedness used for the encoding:
n = 1000
raw = n.to_bytes(2, byteorder="little")
restored = int.from_bytes(raw, byteorder="little")
print(restored) # 1000For signed values, pass signed=True to int.from_bytes() as well. The decoder also accepts a bytes-like object or an iterable that yields byte values. See Python’s int.from_bytes() documentation.
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Know the Python version behavior
int.to_bytes() and int.from_bytes() have been available since Python 3.2. Python 3.11 added defaults that let to_bytes() omit its length and byte order, and let from_bytes() omit its byte order. The no-argument form of to_bytes() is a one-byte shorthand: (65).to_bytes() returns b"A", but values above 255 raise OverflowError. For code that must also run on Python 3.10 or earlier, provide the required arguments explicitly.
Quick Recap
Quick checks when conversion fails or looks wrong
- If you see
OverflowError, check that the selected width can represent the value and that a negative value is encoded withsigned=True. - If the bytes appear reversed, verify whether the format expects
"big"or"little". - If a decoded integer is wrong, check that decoding uses the encoding’s byte order, width, and signedness.
- If a leading zero byte is missing, use the fixed width required by the destination format instead of the minimum computed width.
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