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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_ca_certs(environ=os.environ):
 """ Retrieve a list of CAs at either the DEFAULT_CERTS_PATH or the env override, TXAWS_CERTS_PATH. In order to find .pem files, this function checks first for presence of the TXAWS_CERTS_PATH environment variable that should point to a directory containing cert files. In the absense of this variable, the module-level DEFAULT_CERTS_PATH will be used instead. Note that both of these variables have have multiple paths in them, just like the familiar PATH environment variable (separated by colons). """

    cert_paths = environ.get("TXAWS_CERTS_PATH", DEFAULT_CERTS_PATH).split(":")
    certificate_authority_map = {}
    for path in cert_paths:
        if not path:
            continue
        for cert_file_name in glob(os.path.join(path, "*.pem")):
            # There might be some dead symlinks in there, so let's make sure
            # it's real.
            if not os.path.exists(cert_file_name):
                continue
            cert_file = open(cert_file_name)
            data = cert_file.read()
            cert_file.close()
            x509 = load_certificate(FILETYPE_PEM, data)
            digest = x509.digest("sha1")
            # Now, de-duplicate in case the same cert has multiple names.
            certificate_authority_map[digest] = x509
    values = certificate_authority_map.values()
    if len(values) == 0:
        raise exception.CertsNotFoundError("Could not find any .pem files.")
    return values