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Python extension wrapping the ICU C++ API

Project description

README file for PyICU


Welcome to PyICU, a Python extension wrapping the ICU C++ libraries.

ICU stands for "International Components for Unicode". These are the i18n libraries of the Unicode Consortium. They implement much of the Unicode Standard, many of its companion Unicode Technical Standards, and much of Unicode CLDR.

The PyICU source code is hosted at

The ICU homepage is

See also the CLDR homepage at

Installing PyICU

PyICU is a python extension implemented in C++ that wraps the C/C++ ICU library. It is known to also work as a PyPy extension. Unless pkg-config and the ICU libraries and headers are already installed, building PyICU from the sources on PyPI involves more than just a pip call. Many operating systems distribute pre-built binary packages of ICU and PyICU, see below.

  • Mac OS X

    • Ensure ICU is installed and can be found by pkg-config (as icu-config was deprecated as of ICU 63.1), either by following ICU build instructions, or by using Homebrew:

      # install libicu (keg-only)
      brew install pkg-config icu4c
      # let discover keg-only icu4c via pkg-config
      export PATH="/usr/local/opt/icu4c/bin:/usr/local/opt/icu4c/sbin:$PATH"
      export PKG_CONFIG_PATH="$PKG_CONFIG_PATH:/usr/local/opt/icu4c/lib/pkgconfig"
    • Install PyICU with the same C++ compiler as your Python distribution (more info):

      # EITHER - when using a gcc-built CPython (e.g. from Homebrew)
      export CC="$(which gcc)" CXX="$(which g++)"
      # OR - when using system CPython or another clang-based CPython, ensure system clang is used (for proper libstdc++
      unset CC CXX
      # avoid wheels from previous runs or PyPI
      pip install --no-binary=:pyicu: pyicu
    • ICU and PyICU binaries are both available via Macports as well. The same limitations about mixing binaries may apply.

      # see versions available
      /opt/local/bin/port search pyicu
      sudo /opt/local/bin/port install ...
  • Debian

    apt-get update
    # EITHER - from apt directly
    apt-get install python3-icu
    # OR - from source
    apt-get install pkg-config libicu-dev
    pip install --no-binary=:pyicu: pyicu
  • Ubuntu: similar to Debian, there is a pyicu package available via apt.

  • Alpine Linux: there is a pyicu package available via apk.

  • NetBSD: there is a pyicu package available via pkg_add.

  • OpenBSD: there is a pyicu package available via pkg_add.

  • Other operating systems: see below.

Building PyICU

Please, refer to next section for building Python, ICU and PyICU from sources. The current section is about building only PyICU from sources, with all dependencies such as Python and ICU already present.

Before building PyICU the ICU libraries must be built and installed. Refer to each system's instructions for more information.

PyICU is built from sources with setuptools or with build and pip:

  • verify that pkg-config is available (the icu-config program is deprecated as of ICU 63.1)

    pkg-config --cflags --libs icu-i18n

    If this command returns an error or doesn't return the paths expected then ensure that the INCLUDES, LFLAGS, CFLAGS and LIBRARIES dictionaries in contain correct values for your platform. Starting with ICU 60, -std=c++11 must appear in your CFLAGS or be the default for your C++ compiler.

  • either build and install PyICU with setuptools

    python build
    sudo python install
  • or build PyICU with build and install it with pip

    python -m build
    sudo python -m pip install dist/PyICU-<version>-<platform>.whl
  • either test PyICU with setuptools

    python test
  • or test PyICU with pytest

    python -m pytest

Building PyICU, Python 3 and ICU from sources

The instructions at note_855 contain the complete steps for building everything from sources into a self-contained directory, without modifying any system directories. They were made and tested on an M1 Mac but they can be modified and reused for any unix environment. In particular, they outline how to build PyICU from sources without icu-config or pkg-config being present.

Running PyICU

  • Mac OS X Make sure that DYLD_LIBRARY_PATH contains paths to the directory(ies) containing the ICU libs.

  • Linux & Solaris Make sure that LD_LIBRARY_PATH contains paths to the directory(ies) containing the ICU libs or that you added the corresponding -rpath argument to LFLAGS.

  • Windows Make sure that PATH contains paths to the directory(ies) containing the ICU DLLs.

What's available

See the CHANGES file for an up to date log of changes and additions.

API Documentation

There is no API documentation for PyICU. The API for ICU is documented at and the following patterns can be used to translate from the C++ APIs to the corresponding Python APIs.


The ICU string type, UnicodeString, is a type pointing at a mutable array of UChar Unicode 16-bit wide characters and is described here. The Python 3 str type is described here and here. The Python 2 unicode type is described here.

Because of their differences, ICU's and Python's string objects are not merged into the same type when crossing the C++ boundary but converted.

ICU APIs taking UnicodeString arguments have been overloaded to also accept arguments that are Python 3 str or Python 2 unicode objects. Python 2 str objects are auto-decoded into ICU strings using the utf-8 encoding.

To convert a Python 3 bytes or a Python 2 str object encoded in an encoding other than utf-8 to an ICU UnicodeString use the UnicodeString(str, encodingName) constructor.

ICU's C++ APIs accept and return UnicodeString arguments in several ways: by value, by pointer or by reference. When an ICU C++ API is documented to accept a UnicodeString reference parameter, it is safe to assume that there are several corresponding PyICU python APIs making it accessible in simpler ways:

For example, the 'UnicodeString &Locale::getDisplayName(UnicodeString &)' API, documented here, can be invoked from Python in several ways:

  1. The ICU way

     >>> from icu import UnicodeString, Locale
     >>> locale = Locale('pt_BR')
     >>> string = UnicodeString()
     >>> name = locale.getDisplayName(string)
     >>> name
     <UnicodeString: 'Portuguese (Brazil)'>
     >>> name is string
     True                  <-- string arg was returned, modified in place
  2. The Python way

     >>> from icu import Locale
     >>> locale = Locale('pt_BR')
     >>> name = locale.getDisplayName()
     >>> name
     'Portuguese (Brazil)'

    A UnicodeString object was allocated and converted to a Python str object.

A UnicodeString can be converted to a Python unicode string with Python 3's str() or Python 2's unicode() constructor. The usual len(), comparison, `[]and[:]`` operators are all available, with the additional twists that slicing is not read-only and that ``+=`` is also available since a UnicodeString is mutable. For example:

>>> name = locale.getDisplayName()
'Portuguese (Brazil)'
>>> name = UnicodeString(name)
>>> name
<UnicodeString: 'Portuguese (Brazil)'>
>>> str(name)
'Portuguese (Brazil)'
>>> len(name)
>>> str(name)
'Portuguese (Brazil)'
>>> name[3]
>>> name[12:18]
<UnicodeString: 'Brazil'>
>>> name[12:18] = 'the country of Brasil'
>>> name
<UnicodeString: 'Portuguese (the country of Brasil)'>
>>> name += ' oh joy'
>>> name
<UnicodeString: 'Portuguese (the country of Brasil) oh joy'>

error reporting

The C++ ICU library does not use C++ exceptions to report errors. ICU C++ APIs return errors via a UErrorCode reference argument. All such APIs are wrapped by Python APIs that omit this argument and throw an ICUError Python exception instead. The same is true for ICU APIs taking both a ParseError and a UErrorCode, they are both to be omitted.

For example, the 'UnicodeString &DateFormat::format(const Formattable &, UnicodeString &, FieldPosition &, UErrorCode &)' API, documented here is invoked from Python with:

>>> from icu import DateFormat, Formattable
>>> df = DateFormat.createInstance()
>>> df
<SimpleDateFormat: M/d/yy h:mm a>
>>> f = Formattable(940284258.0, Formattable.kIsDate)
>>> df.format(f)
'10/18/99 3:04 PM'

Of course, the simpler 'UnicodeString &DateFormat::format(UDate, UnicodeString &)' documented here can be used too:

>>> from icu import DateFormat
>>> df = DateFormat.createInstance()
>>> df
<SimpleDateFormat: M/d/yy h:mm a>
>>> df.format(940284258.0)
'10/18/99 3:04 PM'


ICU uses a double floating point type called UDate that represents the number of milliseconds elapsed since 1970-jan-01 UTC for dates.

In Python, the value returned by the time module's time() function is the number of seconds since 1970-jan-01 UTC. Because of this difference, floating point values are multiplied by 1000 when passed to APIs taking UDate and divided by 1000 when returned as UDate.

Python's datetime objects, with or without timezone information, can also be used with APIs taking UDate arguments. The datetime objects get converted to UDate when crossing into the C++ layer.


Many ICU API take array arguments. A list of elements of the array element types is to be passed from Python.


An ICU StringEnumeration has three next methods: next() which returns str objects, unext() which returns str objects in Python 3 or unicode objects in Python 2 and snext() which returns UnicodeString objects. Any of these methods can be used as an iterator, using the Python built-in iter function.

For example, let e be a StringEnumeration instance:

e = TimeZone.createEnumeration()
[s for s in e] # a list of 'str' objects
[s for s in iter(e.unext, '')] # a list of 'str' or 'unicode' objects
[s for s in iter(e.snext, '')] # a list of 'UnicodeString' objects


The ICU TimeZone type may be wrapped with an ICUtzinfo type for usage with Python's datetime type. For example:

from datetime import datetime
tz = ICUtzinfo(TimeZone.createTimeZone('US/Mountain'))

or, even simpler:

tz = ICUtzinfo.getInstance('Pacific/Fiji')

To get the default time zone use:

defaultTZ = ICUtzinfo.getDefault()

To get the time zone's id, use the tzid attribute or coerce the time zone to a string:

ICUtzinfo.getInstance('Pacific/Fiji').tzid -> 'Pacific/Fiji'
str(ICUtzinfo.getInstance('Pacific/Fiji')) -> 'Pacific/Fiji'

Further Reading

The unit tests have more examples of actual PyICU usage.

There are also a few samples ported from ICU C/C++.

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