Python package installation options

Installing packages normally happens by looking at http://pypi.python.org/pypi for the desired module, but pip supports installing from version control, local projects, and from distribution files as well.

Python wheels are pre-built archives that can speed up the package installation process compared to installing from source files. They can be compared to installing pre-made binary applications for an operating system rather than building and installing source files.

Wheels were developed to replace Python eggs, which performed wheels' functions before the new packaging standards were developed. Wheels improve on eggs by specifying the .dist-info directory (a database of installed Python packages that is very close to the on-disk format) and by implementing package metadata (which helps identify software dependencies).

pip installs from wheels whenever possible, though this feature can be disabled using pip install --no-binary. If wheel files aren't available, pip will look for source files. Wheels can be downloaded from PyPI manually or pulled from a local repository; just tell pip where the local file is located.

How to do it...

1. Use pip to pull the latest version of the package directly from PyPI:

      $ pip install <package_name>

2. Alternately, a specific version of the package can be downloaded:

      $ pip install <package_name>==1.2.2

Here is an example of downgrading pygments from our earlier install in pipenv:

3. As a final option, a minimum version of a package can be downloaded; this is common when a package has a significant change between versions:

      $ pip install "<package_name> >= 1.1"

4. If a PyPI package has a wheel file available, pip will automatically download the wheel; otherwise, it will pull the source code and compile it.

      $ pip install <some_package>

5. To install a local wheel file, provide the full path to the file:

      $ pip install /local_files/SomePackage-1.2-py2.py3-none-any.whl

How it works...

The wheel file name format breaks down to <package_name>-<version>-<language_version>-<abi_tag>-<platform_tag>.whl. The package name is the name of the module to be installed, followed by the version of this particular wheel file.

The language version refers to Python 2 or Python 3; it can be as specific as necessary, such as py27 (any Python 2.7.x version) or py3 (any Python 3.x.x version).

The ABI tag refers to the Application Binary Interface. In the past, the underlying C API (Application Programming Interface) that the Python interpreter relies on changed with every release, typically by adding API features rather than changing or removing existing APIs. The Windows OS is particularly affected, where each Python feature release creates a new name for the Python Window's DLL.

The ABI refers to Python's binary compatibility. While changes to Python structure definitions may not break API compatibility, ABI compatibility may be affected. Most ABI issues occur from changes in the in-memory structure layout.

Since version 3.2, a limited set of API features has been guaranteed to be stable for the ABI. Specifying an ABI tag allows the developer to specify which Python implementations a package is compatible with, for example, PyPy versus CPython. Generally speaking, this tag is set to none, implying there is no specific ABI requirement.

The platform tag specifies which OS and CPU the wheel package is designed to run. This is normally any, unless the wheel's developer had a particular reason to limit the package to a specific system type.