# PycURL Installation

NOTE: You need Python and libcurl installed on your system to use or build pycurl. Some RPM distributions of curl/libcurl do not include everything necessary to build pycurl, in which case you need to install the developer specific RPM which is usually called curl-dev.

## From a source distribution

Build and install pycurl with the following commands:

    (if necessary, become root)
    tar -zxvf pycurl-$VER.tar.gz
    cd pycurl-$VER
    python -m pip install .

$VER should be substituted with the pycurl version number, e.g. 7.10.5.

Note that the installation script assumes that ‘curl-config’ can be located in your path setting. If curl-config is installed outside your path or you want to force installation to use a particular version of curl-config, set the `PYCURL_CURL_CONFIG` environment variable to the location of curl-config. Example:

    PYCURL_CURL_CONFIG=/usr/local/bin/curl-config python -m pip install .

If libcurl is linked dynamically with pycurl, you may have to alter the LD_LIBRARY_PATH environment variable accordingly. This normally applies only if there is more than one version of libcurl installed, e.g. one in /usr/lib and one in /usr/local/lib.

### SSL

PycURL requires that the SSL library that it is built against is the same one libcurl, and therefore PycURL, uses at runtime. PycURL’s `setup.py` uses `curl-config` to attempt to figure out which SSL library libcurl was compiled against, however this does not always work. If PycURL is unable to determine the SSL library in use it will print a warning similar to the following:

    src/pycurl.c:137:4: warning: #warning "libcurl was compiled with SSL support, but configure could not determine which " "library was used; thus no SSL crypto locking callbacks will be set, which may " "cause random crashes on SSL requests" [-Wcpp]

It will then fail at runtime as follows:

    ImportError: pycurl: libcurl link-time ssl backend (openssl) is different from compile-time ssl backend (none/other)

To fix this, set the `PYCURL_SSL_LIBRARY` environment variable to the SSL backend that libcurl is using:

    PYCURL_SSL_LIBRARY=openssl python -m pip install .

Substitute the appropriate backend name (`openssl`, `gnutls`, `nss`, `mbedtls`, `wolfssl`, `sectransp`, or `schannel`).

Advanced users invoking `python -m build` or `setup.py` directly can also have it obtain SSL backend information from an installed libcurl shared library by passing `--libcurl-dll=libcurl.so` (an unqualified `libcurl.so` would use the system libcurl, or a full path can be specified). When installing via `pip`, prefer `PYCURL_SSL_LIBRARY` above.

## pip

    pip install pycurl

If you need to specify an alternate curl-config, it can be done via an environment variable:

    export PYCURL_CURL_CONFIG=/usr/local/bin/curl-config
    pip install pycurl

The same applies to the SSL backend, if you need to specify it (see the SSL note above):

    export PYCURL_SSL_LIBRARY=[openssl|gnutls|nss|mbedtls|sectransp|schannel]
    pip install pycurl

### pip and cached pycurl package

If you have already installed pycurl and are trying to reinstall it via pip with different SSL options for example, pip may reinstall the package it has previously compiled instead of recompiling pycurl with newly specified options. More details are given in this Stack Overflow post.

To force pip to recompile pycurl, run:

    # upgrade pip if necessary
    pip install --upgrade pip

    # remove current pycurl
    pip uninstall pycurl

    # set PYCURL_SSL_LIBRARY
    export PYCURL_SSL_LIBRARY=nss

    # recompile and install pycurl
    pip install --compile pycurl

## Windows

There are currently no official binary Windows packages. You can build PycURL from source or use third-party binary packages.

### Building From Source

Building PycURL from source is not for the faint of heart due to the multitude of possible dependencies and each of these dependencies having its own directory structure, configuration style, parameters and quirks. Additionally different dependencies have different settings for MSVCRT usage, and an application must have all of its parts agreeing on a single setting.

If you are compiling PycURL from source it is recommended to compile all of its dependencies from source as well. Using precompiled libraries may lead to multiple MSVCRT versions mixed in the resulting PycURL binary, which will not be good.

If PycURL is to be linked statically against its dependencies, OpenSSL must be patched to link to the DLL version of MSVCRT.

For a minimum build you will just need libcurl source. Follow its Windows build instructions to build either a static or a DLL version of the library, then configure PycURL as follows to use it:

    python setup.py --curl-dir=c:\dev\curl-7.33.0\builds\libcurl-vc-x86-release-dll-ipv6-sspi-spnego-winssl --use-libcurl-dll

Note that `--curl-dir` must point not to libcurl source but rather to headers and compiled libraries.

If libcurl and Python are not linked against the same exact C runtime (version number, static/dll, single-threaded/multi-threaded) you must use `--avoid-stdio` option (see below).

Additional Windows setup.py options:

- `--use-libcurl-dll`: build against libcurl DLL, if not given PycURL will be built against libcurl statically.

- `--libcurl-lib-name=libcurl_imp.lib`: specify a different name for libcurl import library. The default is `libcurl.lib` which is appropriate for static linking and is sometimes the correct choice for dynamic linking as well. The other possibility for dynamic linking is `libcurl_imp.lib`.

- `--with-openssl`: use OpenSSL/LibreSSL/BoringSSL crypto locks when libcurl was built against these SSL backends.

- `--with-ssl`: legacy alias for `--with-openssl`.

- `--openssl-lib-name=""`: specify a different name for OpenSSL import library containing CRYPTO_num_locks. For OpenSSL 1.1.0+ this should be set to an empty string as given here.

- `--avoid-stdio`: on Windows, a process and each library it is using may be linked to its own version of the C runtime (MSVCRT). FILE pointers from one C runtime may not be passed to another C runtime. This option prevents direct passing of FILE pointers from Python to libcurl, thus permitting Python and libcurl to be linked against different C runtimes. This option may carry a performance penalty when Python file objects are given directly to PycURL in CURLOPT_READDATA, CURLOPT_WRITEDATA or CURLOPT_WRITEHEADER options. In practice, Python 3 file objects do not expose C library FILE pointers, so this option is recognized but does nothing. You can also give `--avoid-stdio` option in PYCURL_SETUP_OPTIONS environment variable as follows:

      PYCURL_SETUP_OPTIONS=--avoid-stdio pip install pycurl

A good `setup.py` target to use is `bdist_wininst` which produces an executable installer that you can run to install PycURL.

You may find the following mailing list posts helpful:

- https://curl.haxx.se/mail/curlpython-2009-11/0010.html

- https://curl.haxx.se/mail/curlpython-2013-11/0002.html

### Using PycURL With Custom Python Builds

As of version 7.21.5, the official binary packages of PycURL are linked statically against all of its dependencies except MSVCRT. This means that as long as your custom Python build uses the same version of MSVC as the corresponding official Python build as well as the same MSVCRT linking setting (/MD et. al.), an official PycURL package should work.

If your Python build uses different MSVCRT settings or a different MSVC version from the official Python builds, you will need to compile PycURL from source.

If the C runtime library (MSVCRT.DLL) versions used by PycURL and Python do not match, you will receive a message like the following one when trying to import `pycurl` module:

    ImportError: DLL load failed: The specified procedure could not be found.

To identify which MSVCRT version your Python uses use the application profiling feature of Dependency Walker and look for msvcrt.dll variants being loaded. You may find the entire thread starting here helpful.

## Git Checkout

In order to build PycURL from a Git checkout, some files need to be generated. On Unix systems it is easiest to build PycURL with `make`:

    make

To specify which curl or SSL backend to compile against, use the same environment variables as easy_install/pip, namely `PYCURL_CURL_CONFIG` and `PYCURL_SSL_LIBRARY`.

To generate generated files only you may run:

    make gen

This might be handy if you are on Windows. Remember to run `make gen` whenever you change sources.

To generate documentation, run:

    make docs

Generating documentation requires Sphinx to be installed.

## A Note Regarding SSL Backends

libcurl’s functionality varies depending on which SSL backend it is compiled against. For example, users have reported problems with GnuTLS backend. As of this writing, generally speaking, OpenSSL backend has the most functionality as well as the best compatibility with other software.

If you experience SSL issues, especially if you are not using OpenSSL backend, you can try rebuilding libcurl and PycURL against another SSL backend.

## SSL Certificate Bundle

libcurl, and PycURL, by default verify validity of HTTPS servers’ SSL certificates. Doing so requires a CA certificate bundle, which libcurl and most SSL libraries do not provide.

Here is a good resource on how to build your own certificate bundle. certifie.com also has a prebuilt certificate bundle. To use the certificate bundle, use `CAINFO`, `CAPATH` , `PROXY_CAINFO`, and `PROXY_CAPATH` PycURL options.