Select a way to obtain process information from the system.

    * `/proc` is used if supported.
    * The system `ps` utility is used as a fallback option.

def _get_process_mapping():
    """Select a way to obtain process information from the system.

    * `/proc` is used if supported.
    * The system `ps` utility is used as a fallback option.
    """
    for impl in (proc, ps):
        try:
            mapping = impl.get_process_mapping()
        except EnvironmentError:
            continue
        return mapping
    raise ShellDetectionFailure('compatible proc fs or ps utility is required')