For special installation instructions regarding MS Windows and MacOS-X, see the specific sections below.
You need Python 2.4 or later.
Unless you are using a static binary distribution (e.g. a Windows binary egg from PyPI), you need to install libxml2 and libxslt, in particular:
Newer versions generally contain fewer bugs and are therefore recommended. XML Schema support is also still worked on in libxml2, so newer versions will give you better compliance with the W3C spec.
Get the easy_install tool and run the following as super-user (or administrator):
easy_install --allow-hosts=lxml.de,*.python.org lxml
On MS Windows, the above will install the binary builds that we provide. If there is no binary build of the latest release yet, please search PyPI for the last release that has them and pass that version to easy_install like this:
easy_install --allow-hosts=lxml.de,*.python.org lxml==2.2.2
On Linux (and most other well-behaved operating systems), easy_install will manage to build the source distribution as long as libxml2 and libxslt are properly installed, including development packages, i.e. header files, etc. Use your package management tool to look for packages like libxml2-dev or libxslt-devel if the build fails, and make sure they are installed.
On MacOS-X, use the following to build the source distribution, and make sure you have a working Internet connection, as this will download libxml2 and libxslt in order to build them:
STATIC_DEPS=true sudo easy_install --allow-hosts=lxml.de,*.python.org lxml
If you want to build lxml from the GitHub repository, you should read how to build lxml from source (or the file doc/build.txt in the source tree). Building from developer sources or from modified distribution sources requires Cython to translate the lxml sources into C code. The source distribution ships with pre-generated C source files, so you do not need Cython installed to build from release sources.
If you have read these instructions and still cannot manage to install lxml, you can check the archives of the mailing list to see if your problem is known or otherwise send a mail to the list.
If you want to use lxml together with the official libxml2 Python bindings (maybe because one of your dependencies uses it), you must build lxml statically. Otherwise, the two packages will interfere in places where the libxml2 library requires global configuration, which can have any kind of effect from disappearing functionality to crashes in either of the two.
To get a static build, either pass the --static-deps option to the setup.py script, or run easy_install with the STATIC_DEPS or STATICBUILD environment variable set to true, i.e.
STATIC_DEPS=true easy_install lxml
The STATICBUILD environment variable is handled equivalently to the STATIC_DEPS variable, but is used by some other extension packages, too.
For MS Windows, the binary egg distribution of lxml is statically built against the libraries, i.e. it already includes them. There is no need to install the external libraries if you use an official lxml build from PyPI.
Unless you know what you are doing, this means: do not install libxml2 or libxslt if you use a binary build of lxml. Just use easy_install by following the installation instructions above.
Only if you want to upgrade the libraries and/or compile lxml from sources, you should install a binary distribution of libxml2 and libxslt. You need both libxml2 and libxslt, as well as iconv and zlib.
A macport of lxml is available. Try port install py25-lxml.
If you want to use a more recent lxml release, you may have to build it yourself. Apple doesn't help here, as the system libraries of libxml2 and libxslt installed under MacOS-X are horribly outdated, and updating them is everything but easy. In any case, you cannot run lxml 2.x with the system provided libraries, so you have to use newer libraries.
Luckily, lxml's setup.py script has built-in support for building and integrating these libraries statically during the build. Please read the MacOS-X build instructions.
A number of users also reported success with updated libraries (e.g. using fink or macports), but needed to set the runtime environment variable DYLD_LIBRARY_PATH to the directory where fink keeps the libraries. In any case, this method is easy to get wrong and everything but safe. Unless you know what you are doing, follow the static build instructions above.