Quick start¶

This page walks you through the two things you will do most often: calculating a checksum and verifying one.

Prerequisites

Make sure the library is installed.

Calculate a checksum¶

Pick one of the predefined configurations, create a Calculator and hand it your data:

from crc import Calculator, Crc8

calculator = Calculator(Crc8.CCITT)  # (1)!

checksum = calculator.checksum(b"123456789")
assert checksum == 0xF4

Crc8.CCITT is just one of many configurations shipping with the library. See configurations for the full list of available CRC8, CRC16, CRC32 and CRC64 configurations.

Verify a checksum¶

Instead of comparing checksums yourself, let the calculator do it:

from crc import Calculator, Crc8

calculator = Calculator(Crc8.CCITT)

assert calculator.verify(b"123456789", 0xF4)

Make it faster¶

Pass optimized=True to use a lookup table based implementation. This is the option you want when checksumming a lot of data:

from crc import Calculator, Crc8

calculator = Calculator(Crc8.CCITT, optimized=True)

assert calculator.checksum(b"123456789") == 0xF4

Create it once, reuse it

Building the lookup table happens when the calculator is created. Create the calculator once and reuse it, rather than creating a new one per checksum.

Checksum a file¶

checksum and verify accept more than just bytes — a file object works just as well, and is read in chunks so large files do not need to fit into memory:

from crc import Calculator, Crc8

calculator = Calculator(Crc8.CCITT, optimized=True)

with open("afile.txt", "rb") as f:  # (1)!
    checksum = calculator.checksum(f)

Make sure to open the file in binary mode ("rb").

Where to go next¶

Calculating checksums

All the ways to calculate and verify checksums, and which input types are supported.

Calculating checksums
Configurations

Use a predefined configuration or define your own.

Configurations
Raw registers

Drive the CRC calculation yourself for full control.

Raw registers