capybara/virtualenv/lib/python2.7/site-packages/pip/download.py
def _check_download_dir(link, download_dir):
    """ Check download_dir for previously downloaded file with correct hash
        If a correct file is found return its path else None
    """
    download_path = os.path.join(download_dir, link.filename)
    if os.path.exists(download_path):
        # If already downloaded, does its hash match?
        logger.info('File was already downloaded %s', download_path)
        if link.hash:
            download_hash = _get_hash_from_file(download_path, link)
            try:
                _check_hash(download_hash, link)
            except HashMismatch:
                logger.warning(
                    'Previously-downloaded file %s has bad hash, '
                    're-downloading.',
                    download_path
                )
                os.unlink(download_path)
                return None
        return download_path
    return None
def _check_download_dir(link, download_dir):
    """ Check download_dir for previously downloaded file with correct hash
        If a correct file is found return its path else None
    """
    download_path = os.path.join(download_dir, link.filename)
    if os.path.exists(download_path):
        # If already downloaded, does its hash match?
        logger.info('File was already downloaded %s', download_path)
        if link.hash:
            download_hash = _get_hash_from_file(download_path, link)
            try:
                _check_hash(download_hash, link)
            except HashMismatch:
                logger.warning(
                    'Previously-downloaded file %s has bad hash, '
                    're-downloading.',
                    download_path
                )
                os.unlink(download_path)
                return None
        return download_path
    return None
Check download_dir for previously downloaded file with correct hash
        If a correct file is found return its path else None