crcmod

library1.7pypypi✓ verified 26d ago

CRCmod is a Python module for calculating Cyclic Redundancy Checks (CRCs). It provides functionality to generate custom CRC algorithms and access a wide range of predefined CRC algorithms, such as CRC-CCITT and CRC-32. The current version is 1.7. The library has a slow release cadence, with updates primarily focused on bug fixes and stability.

serialization

pip install crcmod

package ↗homepage ↗

INSTALL

IMPORT

SIG · CRCMOD

crcmod

serializationpythonv1.7

Install

2.5s avg

Import

—

Disk

17MB

Pass rate

10/ 10

Env Coverage10 / 10

glibc

3.9–3.13

musl

3.9–3.13

Install & Compatibility

Where this runs

tested against v1.7 · pip install

no network on importno background threads

Install × environment matrix

Each cell = how many times install + import succeeded across repeated harness runs. Partial = flaky.
glibc = Debian/Ubuntu slim · musl = Alpine Linux

musl

py 3.10–3.95 runs

installs and imports cleanly · install 0.0s · import 0.000s · 19.3MB

glibc

py 3.10–3.95 runs

installs and imports cleanly · install 2.5s · import 0.000s · 20MB

17MB installed

● package 17MB

Code

Verified usage

Verified import paths — ran on the pinned version, not inferred.

crcmod

✓ import crcmod

Crc

✓ from crcmod.predefined import Crc

✗ from crcmod import Crc

While `crcmod.Crc` exists, `crcmod.predefined.Crc` is commonly used for standard CRC algorithms.

mkCrcFun

✓ from crcmod import mkCrcFun

✗ crcmod.mkCrcFun()

`mkCrcFun` is a function that *returns* a CRC calculation function, it's not a method to be called directly on the module.

This quickstart demonstrates how to calculate CRCs using both predefined algorithms and custom parameters. It shows both the object-oriented approach (`Crc` objects) and the functional approach (`mkCrcFun` functions) for flexibility.

import crcmod import crcmod.predefined # 1. Using a predefined CRC algorithm (object-oriented approach) # Get a CRC-16-CCITT-FALSE calculator crc16_ccitt = crcmod.predefined.Crc('crc-ccitt-false') data_bytes = b'Hello, world!' crc16_ccitt.update(data_bytes) print(f"CRC-16-CCITT-FALSE for '{data_bytes.decode()}': {hex(crc16_ccitt.crcValue)}") # You can reset and reuse the object crc16_ccitt.reset() crc16_ccitt.update(b'Another test') print(f"CRC-16-CCITT-FALSE for 'Another test': {hex(crc16_ccitt.crcValue)}") # 2. Creating a custom CRC function (functional approach) # Example: CRC-32 (Ethernet) - polynomial 0x104C11DB7, initial 0xFFFFFFFF, xorOut 0xFFFFFFFF, revIn/Out True crc32_func = crcmod.mkCrcFun(0x104C11DB7, initCrc=0xFFFFFFFF, xorOut=0xFFFFFFFF, rev=True) data_bytes_32 = b'123456789' result_32 = crc32_func(data_bytes_32) print(f"CRC-32 for '{data_bytes_32.decode()}': {hex(result_32)}") # 3. Using a predefined CRC algorithm (functional approach) crc32_predef_func = crcmod.predefined.mkCrcFun('crc-32') result_predef_32 = crc32_predef_func(b'123456789') print(f"CRC-32 (predefined func) for '123456789': {hex(result_predef_32)}")

Debug

Known issues

gotchaInput data to CRC functions (e.g., `update`, `mkCrcFun`) MUST be bytes (`bytes`), not strings (`str`). Passing a Python string will result in a `TypeError`.

fix

Always encode strings to bytes before passing them to crcmod functions, e.g., `my_string.encode('utf-8')`.

affects: All versions

gotchaThere are different ways to use crcmod: `crcmod.predefined.Crc` (object-oriented for standard CRCs), `crcmod.mkCrcFun` (functional for custom CRCs), and `crcmod.predefined.mkCrcFun` (functional for standard CRCs). Confusing these can lead to errors.

fix

Use `crcmod.predefined.Crc('crc-name')` for an object to update incrementally with a standard CRC. Use `crcmod.mkCrcFun(...)` or `crcmod.predefined.mkCrcFun('crc-name')` if you need a simple function that calculates the CRC of an entire byte string in one call.

affects: All versions

gotcha`crcmod.predefined.Crc` objects, used for incremental CRC calculations, do not provide a `reset()` method. Attempting to call `reset()` will result in an `AttributeError`.

fix

If you need to restart a CRC calculation from its initial state, you must create a new `crcmod.predefined.Crc` object. For example, instead of `my_crc.reset()`, use `my_crc = crcmod.predefined.Crc('crc-name')` to get a fresh instance.

affects: All versions

Errors

Common errors & fixes

ModuleNotFoundError: No module named 'crcmod'

The `crcmod` library is not installed in the active Python environment or there is a conflict between Python 2 and Python 3 installations.

fix

Install the library using pip, specifying the Python version if necessary: `pip install crcmod` or `pip3 install crcmod`. Ensure your virtual environment is activated if applicable.

TypeError: Unicode-objects must be encoded before calculating a CRC

In Python 3, `crcmod` functions expect byte strings (e.g., `b'data'`) as input, but a standard Unicode string (e.g., `'data'`) was provided.

fix

Encode the Unicode string to bytes before passing it to the `crcmod` function, for example: `crc_function('your_string'.encode('utf-8'))`.

CommandException: Downloading this composite object requires integrity checking with CRC32c, but your crcmod installation isn't using the module's C extension, so the hash computation will likely throttle download performance.

The C extension for `crcmod` failed to compile during installation because necessary development tools (like a C compiler and Python development headers) were missing, causing `crcmod` to fall back to its slower pure-Python implementation.

fix

Ensure a C compiler (e.g., `gcc`) and Python development headers (e.g., `python-dev` or `python3-devel` depending on your Python version and OS) are installed on your system, then reinstall `crcmod` using `pip install --force-reinstall crcmod`.

ValueError: The only polynomials allowed are those that generate 8, 16, 24, 32, or 64 bit CRCs.

The custom polynomial provided to `crcmod.mkCrcFun` does not correspond to a valid CRC width (8, 16, 24, 32, or 64 bits), or its hexadecimal representation is incorrect for the desired bit length.

fix

Either use a predefined CRC polynomial from `crcmod.predefined` or ensure your custom polynomial correctly represents a generator for an 8, 16, 24, 32, or 64-bit CRC (e.g., an 8-bit polynomial should be between `0x100` and `0x1FF` inclusive).

Upgrade

Version history

1.7latest on PyPI · released Jun 27, 2010

Audit

Dependencies

No dependency data recorded yet.

Agent activity

26 hits · last 30 days

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Resources

packagehttps://pypi.org/project/crcmod/ ↗

homepagehttp://crcmod.sourceforge.net/ ↗