## 4.9.3 `encodings.idna` -- Internationalized Domain Names in Applications

New in version 2.3.

This module implements RFC 3490 (Internationalized Domain Names in Applications) and RFC 3492 (Nameprep: A Stringprep Profile for Internationalized Domain Names (IDN)). It builds upon the `punycode` encoding and `stringprep`.

These RFCs together define a protocol to support non-ASCII characters in domain names. A domain name containing non-ASCII characters (such as ``www.Alliancefrançaise.nu'') is converted into an ASCII-compatible encoding (ACE, such as ``www.xn-alliancefranaise-npb.nu''). The ACE form of the domain name is then used in all places where arbitrary characters are not allowed by the protocol, such as DNS queries, HTTP Host: fields, and so on. This conversion is carried out in the application; if possible invisible to the user: The application should transparently convert Unicode domain labels to IDNA on the wire, and convert back ACE labels to Unicode before presenting them to the user.

Python supports this conversion in several ways: The `idna` codec allows to convert between Unicode and the ACE. Furthermore, the `socket` module transparently converts Unicode host names to ACE, so that applications need not be concerned about converting host names themselves when they pass them to the socket module. On top of that, modules that have host names as function parameters, such as `httplib` and `ftplib`, accept Unicode host names (`httplib` then also transparently sends an IDNA hostname in the Host: field if it sends that field at all).

When receiving host names from the wire (such as in reverse name lookup), no automatic conversion to Unicode is performed: Applications wishing to present such host names to the user should decode them to Unicode.

The module `encodings.idna` also implements the nameprep procedure, which performs certain normalizations on host names, to achieve case-insensitivity of international domain names, and to unify similar characters. The nameprep functions can be used directly if desired.

`nameprep`( | label)

  Return the nameprepped version of label. The implementation currently assumes query strings, so `AllowUnassigned` is true.

`ToASCII`( | label)

  Convert a label to ASCII, as specified in RFC 3490. `UseSTD3ASCIIRules` is assumed to be false.

`ToUnicode`( | label)

  Convert a label to Unicode, as specified in RFC 3490.