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. from a Windows binary installer), 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.
The best way to install lxml is to get the pip package management tool and run the following as super-user (or administrator):
pip install lxml
To install a specific version, either download the distribution manually and let pip install that, or pass the desired version to pip:
pip install lxml==3.1.2
For MS Windows, recent lxml releases feature community donated binary distributions, although you might still want to take a look at the related FAQ entry. If you fail to build lxml on your MS Windows system from the signed and tested sources that we release, consider using the binary builds from PyPI or the unofficial Windows binaries that Christoph Gohlke generously provides.
On Linux (and most other well-behaved operating systems), pip 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. Alternatively, setting STATIC_DEPS=true will download and build both libraries automatically.
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 pip install 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 pip with the STATIC_DEPS or STATICBUILD environment variable set to true, i.e.
STATIC_DEPS=true pip install lxml
The STATICBUILD environment variable is handled equivalently to the STATIC_DEPS variable, but is used by some other extension packages, too.
Most MS Windows systems lack the necessarily tools to build software, starting with a C compiler already. Microsoft leaves it to users to install and configure them, which is usually not trivial and means that distributors cannot rely on these dependencies being available on a given system. In a way, you get what you've paid for and make others pay for it.
Due to the additional lack of package management of this platform, it is best to link the library dependencies statically if you decide to build from sources. For that, use the binary distribution of libxml2 and libxslt. You need both libxml2 and libxslt, as well as iconv and zlib, which you can get from the same download site. Further build instructions are in the source build documentation.
A macport of lxml is available. Try something like 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 MacOS-X is so badly maintained by them that the pre-installed system libraries of libxml2 and libxslt tend to be horribly outdated, and updating them is everything but easy. In any case, you cannot run lxml 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.