Python module after installing

Installing Python Modules¶

As a popular open source development project, Python has an active supporting community of contributors and users that also make their software available for other Python developers to use under open source license terms.

This allows Python users to share and collaborate effectively, benefiting from the solutions others have already created to common (and sometimes even rare!) problems, as well as potentially contributing their own solutions to the common pool.

This guide covers the installation part of the process. For a guide to creating and sharing your own Python projects, refer to the distribution guide .

For corporate and other institutional users, be aware that many organisations have their own policies around using and contributing to open source software. Please take such policies into account when making use of the distribution and installation tools provided with Python.

Key terms¶

• pip is the preferred installer program. Starting with Python 3.4, it is included by default with the Python binary installers.
• A virtual environment is a semi-isolated Python environment that allows packages to be installed for use by a particular application, rather than being installed system wide.
• venv is the standard tool for creating virtual environments, and has been part of Python since Python 3.3. Starting with Python 3.4, it defaults to installing pip into all created virtual environments.
• virtualenv is a third party alternative (and predecessor) to venv . It allows virtual environments to be used on versions of Python prior to 3.4, which either don’t provide venv at all, or aren’t able to automatically install pip into created environments.
• The Python Package Index is a public repository of open source licensed packages made available for use by other Python users.
• the Python Packaging Authority is the group of developers and documentation authors responsible for the maintenance and evolution of the standard packaging tools and the associated metadata and file format standards. They maintain a variety of tools, documentation, and issue trackers on both GitHub and Bitbucket.
• distutils is the original build and distribution system first added to the Python standard library in 1998. While direct use of distutils is being phased out, it still laid the foundation for the current packaging and distribution infrastructure, and it not only remains part of the standard library, but its name lives on in other ways (such as the name of the mailing list used to coordinate Python packaging standards development).

Changed in version 3.5: The use of venv is now recommended for creating virtual environments.

Basic usage¶

The standard packaging tools are all designed to be used from the command line.

The following command will install the latest version of a module and its dependencies from the Python Package Index:

python -m pip install SomePackage

For POSIX users (including macOS and Linux users), the examples in this guide assume the use of a virtual environment .

For Windows users, the examples in this guide assume that the option to adjust the system PATH environment variable was selected when installing Python.

It’s also possible to specify an exact or minimum version directly on the command line. When using comparator operators such as > , < or some other special character which get interpreted by shell, the package name and the version should be enclosed within double quotes:

python -m pip install SomePackage==1.0.4 # specific version python -m pip install "SomePackage>=1.0.4" # minimum version

Normally, if a suitable module is already installed, attempting to install it again will have no effect. Upgrading existing modules must be requested explicitly:

python -m pip install --upgrade SomePackage

More information and resources regarding pip and its capabilities can be found in the Python Packaging User Guide.

Creation of virtual environments is done through the venv module. Installing packages into an active virtual environment uses the commands shown above.

How do I …?¶

These are quick answers or links for some common tasks.

… install pip in versions of Python prior to Python 3.4?¶

Python only started bundling pip with Python 3.4. For earlier versions, pip needs to be “bootstrapped” as described in the Python Packaging User Guide.

… install packages just for the current user?¶

Passing the —user option to python -m pip install will install a package just for the current user, rather than for all users of the system.

… install scientific Python packages?¶

A number of scientific Python packages have complex binary dependencies, and aren’t currently easy to install using pip directly. At this point in time, it will often be easier for users to install these packages by other means rather than attempting to install them with pip .

… work with multiple versions of Python installed in parallel?¶

On Linux, macOS, and other POSIX systems, use the versioned Python commands in combination with the -m switch to run the appropriate copy of pip :

python2 -m pip install SomePackage # default Python 2 python2.7 -m pip install SomePackage # specifically Python 2.7 python3 -m pip install SomePackage # default Python 3 python3.4 -m pip install SomePackage # specifically Python 3.4

Appropriately versioned pip commands may also be available.

On Windows, use the py Python launcher in combination with the -m switch:

py -2 -m pip install SomePackage # default Python 2 py -2.7 -m pip install SomePackage # specifically Python 2.7 py -3 -m pip install SomePackage # default Python 3 py -3.4 -m pip install SomePackage # specifically Python 3.4

Common installation issues¶

Installing into the system Python on Linux¶

On Linux systems, a Python installation will typically be included as part of the distribution. Installing into this Python installation requires root access to the system, and may interfere with the operation of the system package manager and other components of the system if a component is unexpectedly upgraded using pip .

On such systems, it is often better to use a virtual environment or a per-user installation when installing packages with pip .

Pip not installed¶

It is possible that pip does not get installed by default. One potential fix is:

python -m ensurepip --default-pip

There are also additional resources for installing pip.

Installing binary extensions¶

Python has typically relied heavily on source based distribution, with end users being expected to compile extension modules from source as part of the installation process.

With the introduction of support for the binary wheel format, and the ability to publish wheels for at least Windows and macOS through the Python Package Index, this problem is expected to diminish over time, as users are more regularly able to install pre-built extensions rather than needing to build them themselves.

Some of the solutions for installing scientific software that are not yet available as pre-built wheel files may also help with obtaining other binary extensions without needing to build them locally.

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ImportError after successful pip installation [duplicate]

@BhargavRao Yikes. I’m not entirely on-board with the duplicate closure here, simply because the question at the dupe target is a horrible mess full of UPDATE and EDIT sections. Tonight I’ll see if I can tidy it up. If I can, then I think it’ll probably make sense to merge the questions so that cel’s (good) answer from here can get exposed to the larger audience of the other question.

@AndrasDeak, thanks, that seems to be a more comprehensive target for this particular question, given that the answers also point to the same solution. I’m now thinking of a different way to dupe this question to your target, and leave the other question alone.

@AndrasDeak That’s a legit criticism, and it’d be nice to be able to somehow curate the answers down on this question or the previous dupe target to just 4, each succinctly covering one of these cases.

@Mark, Cool, I’ll change the duplicate to that, but I’m still not certain if a merge is needed here. (Merging is a small pain as it is almost irreversible). The OP seems to be active too. I think it is better to wait for a few more users to confirm once.

@BhargavRao, and all in the conversation: I am generally fine with improving by merging duplicates. I think it’s important that the question/answer pair is available in some form in an easy searchable way. I will trust in your judgement on how to merge/improve the question.

2 Answers 2

TL;DR: There are often multiple versions of python interpreters and pip versions present. Using python -m pip install instead of pip install will ensure that the library gets installed into the default python interpreter.

Please also note: From my personal experience I would advice against using sudo pip install to install packages into system’s default python interpreter. This can lead to a various messy issues. Whenever you are tempted to call pip with sudo , please check first if a virtualenv is not a better option for you.

Most modern systems ship multiple python interpreters. Each interpreter maintains its own set of installed packages. When installing new packages, it is important to understand into which interpreter those packages are actually installed.

On unix systems the shell can be used to understand what exactly is happening.

Typing which -a python shows all interpreters that in your PATH . The first line corresponds to the interpreter that is used when you run python from the command line.

/private/tmp/py32/bin/python /usr/local/bin/python /usr/bin/python 

Each pip version belongs to exactly one interpreter. which -a pip shows all pip versions. Again the first line is what will be called when you type pip in your shell.

/usr/local/bin/pip /usr/bin/python 

Note that in this case python belongs to the interpreter installed in /private/tmp/py32/ , but pip installs into the interpreter /usr/local/bin . After a successful install of a library, you will not be able to import it in your default python interpreter.

So how do you import the installed library?

Your first option is to start the desired interpreter with its full path. So if you type /usr/local/bin/python , you will be able to import the library.

The second — often preferred — option is to specifically invoke the right version of pip. To do so, you can use python -m pip install instead of pip install . This will call the pip version that belongs to your default python interpreter.

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