Can't install lxml in python2.7

Question

Im trying to install lxml within a virtualenv with sudo pip install lxml and also sudo pip install --upgrade lxml but getting the following in both cases:

x86_64-linux-gnu-gcc -pthread -shared -Wl,-O1 -Wl,-Bsymbolic-functions -Wl,-z,
relro -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes    
-D_FORTIFY_SOURCE=2 -g -fstack-protector --param=ssp-buffer-size=4 -Wformat 
-Werror=format-security build/temp.linux-x86_64-2.7/src/lxml/lxml.etree.o -lxslt 
-lexslt -lxml2 -lz -lm -o build/lib.linux-x86_64-2.7/lxml/etree.so

/usr/bin/ld: cannot find -lz

collect2: error: ld returned 1 exit status
error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

----------------------------------------
Cleaning up...
Command /usr/bin/python -c "import setuptools, 
tokenize;__file__='/tmp/pip_build_root/lxml/setup.py';
exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), 
__file__, 'exec'))" install --record /tmp/pip-nmFOYf-record/install-record.txt 
--single-version-externally-managed --compile failed with error code 1 in 
/tmp/pip_build_root/lxml
Storing debug log for failure in /root/.pip/pip.log

I have tried all the posted solutions here, which implies that I have libxml2-dev, libxslt-dev and python-dev installed and I also installed build-essential
I'm currently running Linux Mint 17 Debian Based which uses apt-get as package manager.
python-lxml was already preinstalled.

Answer 1

lxml depends on various C libraries, and you have to have those C libraries installed—including their development files (headers, .so or .a libraries, etc.)—to build lxml. The installation docs explain what prerequisites you need to build on your particular platform.

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This error:

/usr/bin/ld: cannot find -lz

… means that the prerequisite you're missing is libz, aka zlib.

Since you're not on Windows, it's incredibly unlikely that you actually don't have zlib at all… but pretty plausible that you don't have the development files for zlib. On some platforms, most notably many linux distros, packages are typically split into separate pieces. For example, the parts of zlib needed at runtime may be in a package named zlib, while the parts needed for building other programs that need zlib in a package named zlib-dev or zlib-devel. The exact details depend on your exact platform.

That being said, if you don't have the zlib development files, you probably don't have the libxml2 or libxslt development files either, because I don't know of any platform where installing those wouldn't have pulled in the zlib files as well.

At any rate, since you haven't told us what platform you're one (and distro, if linux), I don't know what package manager to use, what the packages are named, etc., but do whatever's appropriate for your platform.

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Also:

I already have python-lxml installed

You really shouldn't install the same package both with your distro's package manager and with pip; that's a great way to confuse yourself.

But at any rate, most likely you installed python-lxml from a binary package, not from source, which means you didn't need the build requirements. Now you're trying to build it from source, which means you do.