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Development Environment / Python

Python software is run using a Python interpreter.

Introduction

Python can be downloaded and installed from the Python website (python.org/downloads). If possible, install and use the latest 3.x version. However, the choice of Python version may depend on integration with an existing environment, and in some cases Python may be bundled with a software product.

To determine what version of Python is already installed and will run by default, open a command shell in the target computer operating system (Cygwin, Linux, or Windows), and check for the installed and default version:

$ python --version
Python 2.7.2

Even if nothing is listed, Python may still be installed on the system and may not have been configured in the PATH. Refer to the following sections for instructions to install Python on different operating systems:

  • Normal Installation - normal installations allow using Python for any purpose on a computer, see installation instructions for various operating systems below
  • Bundled Python Installations - when Python is packaged with a software product and is run independent of Python installed on the operating system
  • Virtual Python Environment - used to isolate Python package installs from operating system Python install

Bundled Python Installations

Python may be distributed with software products that depend on Python and need to control its installation so as to not break the products. This approach may be taken where significant Python libraries have been developed to integrate with the product. Bundled versions of Python are generally not configured as the default installation on a system and often require that a startup script is run to configure the Python environment. Examples of bundled Python distributions include:

  • Esri ArcGIS Geographic Information System, which includes ArcPy Python integration. On Windows and ArcGIS 10.4, Python is installed in the C:\Python27 folder and includes C:\Python27\ArcGIS10.4. The ArcGIS software provides configuration tools to select the Python that is used.
  • Quantum GIS (QGIS)) - open source Geographic Information System, which includes PyQGIS Python integration (for example Python may be installed in the C:\OSGeo4W64\apps\Python27 folder and the Python environment is initialized by running C:\OSGeo4W64\bin\python.exe)

Virtual Python Environment

Virtual environments are a newer approach that install Python packages into virtual environment folder so that the installation of add-on Python packages does not impact Python installations on the computer. This option may also be appropriate in cases when administrative privileges are not available on the computer. See additional information in the Development Tasks / Creating a Virtual Environment section.

Install Python

The Python installation depends on the operating system.

Cygwin Install Python on Cygwin

Python for Cygwin is installed by selecting the Python interpreter in the install package listing. The Python program is installed as /usr/bin/python or /usr/bin/python3 and supporting files are installed in /usr/lib/Python2.7/ and /usr/lib/Python3.4/ (the version numbers will increase as newer Python versions are released on Cygwin). The latest supported Python for each major version is installed when the Cygwin installer is run.

Use python to run Python 2 and python3 to run Python 3. Both are installed in /usr/bin.

Linux Install Python on Linux

Python for Linux is typically installed by following the instructions for the Linux distribution. For example, see instructions:

The Python program is typically installed as /usr/bin/python or /usr/bin/python3 and supporting files are installed in /usr/lib/Python2.7/ and /usr/lib/Python3.4/, for example. Consequently, the programs will typically be found in the PATH.

Windows Install Python on Windows

Python for Windows is installed by following the instructions:

Python can be installed in system location e.g.,C:\Program Files\Python37) or under user files (C:\Users\user\AppData\Local\Python\Python37). The former requires administrator privileges whereas the latter does not (although this may not be consistent?). Python versions prior to version 3.5 have default system installation folder C:\Python27, for example. The followin table summarizes changes in defaults for different versions of Python (need to update the table with more information).

Python Version System Installation Folder User Installation Folder
Python 2.7 C:\Python27 ?
Python 3.5+ C:\Program Files\... C:\Users\user\AppData\Local\Programs\Python\Python35

The installer will ask whether to modify the PATH. Newer Python installations also install a program py.exe in the Windows C:\Windows\py.exe folder, which is always in the PATH. In this case, specific Python programs such as python3 will not be in the path but bo exist in the installation folder. Verify that py is available as follows:

> where py

The py program will by default run the latest Python but can specify which Python to run:

  • py - run latest Python version
  • py -2 - run the latest Python 2 version
  • py -3 - run the latest Python 3 version
  • py somefile.py - run the specified Python module

The py program essentially wraps all Python versions and also provides a way to always run Python without conflicts in the PATH environment variable. This allows the default behavior to find a recent version of Python. However, it may be necessary to exert more control of how Python is run, for example:

  • Use a custom script to run a Python program, which can specify the exact version of Python to use.
  • Manually update the PATH environment variable to find a specific version of Python. For example, the Python installer may not result in necessary PATH environment variable. This can become more of an issue if additional Python-related programs are run, such as pip (see below), mkdocs for documentation, virtualenv, etc. It may make sense to update the PATH to include the most recent Python version. For example, use the Windows 10 Control Panel and then Environment Variables / Path / Edit... to set the PATH to find Python37 installed in user files, as follows.
    • %USERPROFILE%\AppData\Local\Programs\Python\Python37 - find python.exe itself (note that only python.exe may be availble, and not python3.exe)
    • %USERPROFILE%\AppData\Local\Programs\Python\Python37\Scripts - find Python-related scripts such as pip.exe (note that pip3.exe and pip.exe may both be available and equivalent), mkdocs.exe, and virtualenv.exe

Install pip to Install Add-on Packages

It is often necessary to install add-on packages that extend the basic Python functionality.

The pip software is used to install Python packages and is the preferred installation tool since older tools such as easy_install do not support current conventions. Therefore, in order to install third-party packages, install pip first. The pip software is often installed by default for new Python versions, but may require installation or updating. In addition to installing a specific package, pip will install necessary dependencies. See the following resources:

In summary:

  • Add-on packages should install into a location consistent with the Python software install location.
  • The pip utility should be used to install add-on packages.
  • It is possible to use pip to install modules when multiple versions of Python are installed. In general, make sure that the correct pip version is used by confirming the pip install location that is found in the PATH (using where on Windows, and which on Cygwin/Linux). If necessary, type the full path to the pip version to use. Also use pip3 if necessary to avoid confusion with Python2 pip. The pip and pip3 programs may be equivalent on a system where only Python3 is installed. However, pip and pip3 may be different programs depending on which Python versions are installed.
  • If pip prints messages about how to update itself, follow those instructions.
  • See the notes on installing Python (in particular Windows) to for information about setting the PATH to find pip.
  • See the examples below for specific operating systems.

Cygwin Install pip on Cygwin

The following uses a Cygwin bash shell. To check whether pip is already installed,:

pip --version
pip3 --version

If not installed, install with the following, repeating for each Python installation:

$ python -m ensurepip
$ python3 -m ensurepip

linux Install pip on Linux

The following uses a Linux bash shell. To check whether pip is already installed,:

pip --version
pip3 --version

If not installed, install with the following, repeating for each Python installation:

$ python -m ensurepip
$ python3 -m ensurepip

Windows Install pip on Windows

The following uses a Windows Command Shell. To check whether pip is already installed:

py -2 -m pip --version
pip 7.0.1 from C:\Python27\lib\site-packages (python 2.7)

py -3 -m pip --version
pip 8.1.2 from C:\Users\sam\AppData\Local\Programs\Python\Python35-32\lib\site-packages (python 3.5)

If not installed, install with the following, repeating for each Python installation:

py -2 -m ensurepip

py -3 -m ensurepip

Install Third-party Packages

Third party packages typically have their own websites with installation instructions. Follow those instructions within the configured Python environment that is to receive the installation. If necessary, check the Python version first to ensure compatibility with the package and run pip or pip3 accordingly.

Install Integrated Development Environment

An Integrated Development Environment (IDE) is helpful for development because it provides an integrated edit/run/test/build/deploy environment.

See the list of IDEs in the Overview.