encoding: use s.decode to trigger UnicodeDecodeError When calling encode on a str, the string is first decoded using the default encoding and then encoded. So s.encode('ascii') == s.decode().encode('ascii') We don't care about the encode step here -- we're just after the UnicodeDecodeError raised by decode if it finds a non-ASCII character. This way is also marginally faster since it saves the construction of the extra str object.