contrib/python-zstandard/zstandard/__init__.py

# Copyright (c) 2017-present, Gregory Szorc
# All rights reserved.
#
# This software may be modified and distributed under the terms
# of the BSD license. See the LICENSE file for details.

"""Python interface to the Zstandard (zstd) compression library."""

from __future__ import absolute_import, unicode_literals

# This module serves 2 roles:
#
# 1) Export the C or CFFI "backend" through a central module.
# 2) Implement additional functionality built on top of C or CFFI backend.

import os
import platform

# Some Python implementations don't support C extensions. That's why we have
# a CFFI implementation in the first place. The code here import one of our
# "backends" then re-exports the symbols from this module. For convenience,
# we support falling back to the CFFI backend if the C extension can't be
# imported. But for performance reasons, we only do this on unknown Python
# implementation. Notably, for CPython we require the C extension by default.
# Because someone will inevitably want special behavior, the behavior is
# configurable via an environment variable. A potentially better way to handle
# this is to import a special ``__importpolicy__`` module or something
# defining a variable and `setup.py` could write the file with whatever
# policy was specified at build time. Until someone needs it, we go with
# the hacky but simple environment variable approach.
_module_policy = os.environ.get('PYTHON_ZSTANDARD_IMPORT_POLICY', 'default')

if _module_policy == 'default':
    if platform.python_implementation() in ('CPython',):
        from zstd import *
        backend = 'cext'
    elif platform.python_implementation() in ('PyPy',):
        from zstd_cffi import *
        backend = 'cffi'
    else:
        try:
            from zstd import *
            backend = 'cext'
        except ImportError:
            from zstd_cffi import *
            backend = 'cffi'
elif _module_policy == 'cffi_fallback':
    try:
        from zstd import *
        backend = 'cext'
    except ImportError:
        from zstd_cffi import *
        backend = 'cffi'
elif _module_policy == 'cext':
    from zstd import *
    backend = 'cext'
elif _module_policy == 'cffi':
    from zstd_cffi import *
    backend = 'cffi'
else:
    raise ImportError('unknown module import policy: %s; use default, cffi_fallback, '
                      'cext, or cffi' % _module_policy)