Internationalized Domain Names in Applications (IDNA)

Support for Internationalized Domain Names in Applications (IDNA) and Unicode IDNA Compatibility Processing. It supersedes the standard library's encodings.idna, which only implements the 2003 specification, offering broader script coverage and limiting domains with known security vulnerabilities.

Usage

Package may be installed from PyPI via the typical methods (e.g. python3 -m pip install idna)

For typical usage, the encode and decode functions will take a domain name argument and perform a conversion to ASCII-compatible encoding (known as A-labels), or to Unicode strings (known as U-labels) respectively.

>>> import idna
>>> idna.encode('ドメイン.テスト')
b'xn--eckwd4c7c.xn--zckzah'
>>> print(idna.decode('xn--eckwd4c7c.xn--zckzah'))
ドメイン.テスト

Conversions can be applied at a per-label basis using the ulabel or alabel functions for specialized use cases.

Compatibility Mapping (UTS #46)

This library provides support for Unicode IDNA Compatibility Processing which normalizes input from different potential ways a user may input a domain prior to performing the IDNA conversion operations. This functionality, known as a mapping, is considered by the specification to be a local user-interface issue distinct from IDNA conversion functionality.

For example, "Königsgäßchen" is not a permissible label as capital letters are not allowed. UTS 46 will convert this into lower case prior to applying the IDNA conversion.

>>> import idna
>>> idna.encode('Königsgäßchen')
...
idna.core.InvalidCodepoint: Codepoint U+004B at position 1 of 'Königsgäßchen' not allowed
>>> idna.encode('Königsgäßchen', uts46=True)
b'xn--knigsgchen-b4a3dun'
>>> idna.decode('xn--knigsgchen-b4a3dun')
'königsgäßchen'

When performing a decode operation for display purposes, decode() accepts a display=True argument that leaves any xn-- label that fails to decode unchanged. This is useful for user interface display where a domain is in use, the A-label form can be presented when it is not a valid IDN.

Exceptions

All errors raised during conversion derive from the idna.IDNAError base class. The more specific exceptions are:

• idna.IDNABidiError — raised when a label contains an illegal combination of left-to-right and right-to-left characters.
• idna.InvalidCodepoint — raised when a label contains a codepoint that is INVALID for IDNA.
• idna.InvalidCodepointContext — raised when a CONTEXTO or CONTEXTJ codepoint appears in a position whose contextual requirements are not satisfied.

Exceptions carry machine-readable attributes so that applications do not need to parse the message: code is a short, stable identifier for the rule that failed (listed below); and, when the failure can be attributed to a particular character, text (the label, or domain for UTS 46 processing, being validated), codepoint (the offending codepoint as an integer) and position (its 1-based index within text, as quoted in the message) are set. Each is None when it does not apply. Message wording is not part of the API and may change.

>>> try:
...     idna.encode('Königsgäßchen')
... except idna.IDNAError as err:
...     print(err.code, err.codepoint, err.position, err.text)
disallowed_codepoint 75 1 Königsgäßchen

Command-line tool

The package supports command-line usage to convert domain names between their Unicode and ASCII-compatible forms. It can be run either as a module (python3 -m idna) or, once installed (such as with uv tool or pipx), via the idna script:

$ uv tool install idna
$ idna xn--e1afmkfd.xn--p1ai
пример.рф
$ idna пример.рф
xn--e1afmkfd.xn--p1ai

Mode can be specified with -e/--encode or -d/--decode, otherwise it will be chosen automatically based on the first input. Multiple domains can be supplied either as arguments or through standard input. UTS #46 mapping is applied by default, which lets the tool accept inputs that aren't strictly valid IDNA 2008 by normalising them first, pass --strict to disable UTS #46.

Conversion failures are reported on stderr together with the offending input; processing continues with the remaining domains and the tool exits with a non-zero status if any conversion failed.

Additional Notes

• Version support. This library supports Python 3.9 and higher. As this library serves as a low-level toolkit for a variety of applications, we strive to support all versions of Python that are not beyond end-of-life.

• Unicode version. The IDNA and UTS #46 lookup tables are generated from a specific Unicode release, available as idna.unicode_version (and shown by idna --version). Some checks — NFC normalisation, bidirectional classes and combining classes — still come from the running Python's unicodedata module, so on an older Python a character new to Unicode may be rejected (typically with the unknown_codepoint or bidi_unknown_direction error codes) even though the tables know about it.

• Thread safety and free-threaded Python. The library holds no mutable global state — the lookup tables are read-only — so all functions may be called concurrently from multiple threads, and the incremental codec classes keep their state per instance. The test suite runs on free-threaded CPython builds (3.14t) with the GIL disabled and includes a concurrency test.

• Emoji. It is an occasional request to support emoji domains in this library. Encoding of symbols like emoji is expressly prohibited by the IDNA technical standard, and emoji domains are broadly phased out across the domain industry due to associated security risks.

• Regenerating lookup tables. The IDNA and UTS 46 functionality relies upon pre-calculated lookup tables, generated using the idna-data script in tools/.