Mastering Python

Chapter 1. Getting Started – One Environment per Project

There is one aspect of the Python philosophy that always has been, and always will be, the most important in the entire language—readability, or Pythonic code. This book will help you master writing Python the way it was meant to be: readable, beautiful, explicit, and as simple as possible. In short, it will be Pythonic code. That is not to say that complicated subjects will not be covered. Naturally, they will, but whenever the philosophy of Python is at stake, you will be warned when and where the technique is justified.

Most of the code within this book will function on both Python 2 and Python 3, but the main target is Python 3. There are three reasons for doing this:

Python 3 was released in 2008, which is a very long time in the rapidly changing software world. It's not a new thing anymore, it's stable, it's usable, and, most importantly, it's the future.
Development for Python 2 effectively stopped in 2009. Certain features have been backported from Python 3 to Python 2, but any new development will be for Python 3 first.
Python 3 has become mature. While I have to admit that Python 3.2 and older versions still had a few small issues that made it hard to write code that functions on both Python 2 and 3, Python 3.3 did improve greatly in that aspect, and I consider it mature. This is evidenced by the marginally modified syntax in Python 3.4 and 3.5 and a lot of very useful features, which are covered in this book.

To summarize, Python 3 is an improvement over Python 2. I have been a skeptic for a very long time myself, but I do not see any reason not to use Python 3 for new projects, and even porting existing projects to Python 3 is generally possible with only minor changes. With cool new features such as async with in Python 3.5, you will want to upgrade just to try it.

This first chapter will show you how to properly set up an environment, create a new isolated environment, and make sure you get similar results when running the same code on different machines. Most Python programmers are already using virtualenv to create virtual Python environments, but the venv command, introduced in Python 3.3, is a very nice alternative. It is essentially a clone of the virtualenv package but is slightly simpler and bundled with Python. While its usage is mostly analogous to virtualenv, there are a few changes that are interesting to know.

Secondly, we will discuss the pip command. The pip command is automatically installed when using venv through the ensurepip package, a package introduced in Python 3.4. This package automatically bootstraps pip into an existing Python library while maintaining independent versions of Python and pip. Before Python 3.4, venv came without pip and had to be installed manually.

Finally, we will discuss how packages created with distutils can be installed. While pure Python packages are generally easy to install, it can get challenging when C modules are involved.

In this chapter, the following topics are covered:

Creating a virtual Python environment using venv
Bootstrapping pip using ensurepip
Installing packages based on distutils (C/C++) with pip

Creating a virtual Python environment using venv

Most Python programmers are already be familiar with venv or virtualenv, but even if you're not, it's never too late to start using it. The venv module is designed to isolate your Python environments so that you can install packages specific to your current project without polluting your global namespace. For example, having a filename such as sys.py in your current directory can seriously break your code if you expect to have the standard Python sys library—your local sys libraries will be imported before the global one, effectively hiding the system library. In addition, because the packages are installed locally, you don't need system (root/administrator) access to install them.

The result is that you can make sure you have exactly the same version of a package on both your local development machine and production machines without interfering with other packages. For example, there are many Django packages around that require specific versions of the Django project. Using venv, you can easily install Django 1.4 for project A and Django 1.8 for project B without them ever knowing that there are different versions installed in other environments. By default, the environments are even configured in such a way that the global packages are not visible. The benefit of this is that to get an exact list of all installed packages within the environment, simply a pip freeze will suffice. The downside is that some of the heavier packages (for example, numpy) will have to be installed in every separate environment. Needless to say, which choice is the best for your project depends on the project. For most projects, I would keep the default setting of not having the global packages, but when messing around with projects that have lots of C/C++ extensions, it would be convenient to simply enable the global site packages. The reason is simple; if you do not have a compiler available, installing the package locally can be difficult, while the global install has an executable for Windows or an installable package for Linux/Unix available.

Note

The venv module (https://docs.python.org/3/library/venv.html) can be seen as a slightly simplified version of the virtualenv tool (https://virtualenv.pypa.io/), which has been bundled with Python since version 3.3 (refer to PEP 0405 -- Python Virtual Environments: https://www.python.org/dev/peps/pep-0405/).

The virtualenv package can generally be used as a drop-in replacement for venv, which is especially relevant for older Python versions (below 3.3) that do not come bundled with venv.

Creating your first venv

Creating an environment is quite easy. The basic command comes down to pyvenv PATH_TO_THE_NEW_VIRTUAL_ENVIRONMENT, so let's give it a try. Note that this command works on Linux, Unix, and Mac; the Windows command will follow shortly:

# pyvenv test_venv
# . ./test_venv/bin/activate
(test_venv) #

Note

Some Ubuntu releases (notably 14.04 LTS) maim the Python installation by not including the full pyvenv package with ensurepip. The standard workaround is to call pyvenv --without-pip test_env, which requires a manual pip installation through the get_pip.py file available on the pip home page.

This creates an environment called test_venv, and the second line activates the environment.

On Windows, everything is slightly different but similar overall. By default, the pyvenv command won't be in your PATH, so running the command is slightly different. The three options are as follows:

Add the Python\Tools\Scripts\ directory to your PATH
Run the module:
```
python -m venv test_venv
```

Run the script directly:

python Python\Tools\Scripts\pyvenv.py test_venv

For convenience, I would recommend that you add the Scripts directory to your PATH anyhow, since many other applications/scripts (such as pip) will be installed there as well.

Here is the full example for Windows:

C:\envs>python -m venv test_venv
C:\envs>test_venv\Scripts\activate.bat
(test_venv) C:\envs>

Tip

When using Windows PowerShell, the environment can be activated by using test_venv\Scripts\Activate.ps1 instead. Note that you really do need backslashes here.

venv arguments

So far, we have just created a plain and regular venv, but there are a few, really useful flags for customizing your venv specifically to your needs.

First, let's look at the venv help:

Parameter	Description
`--system-site-packages`	It gives the virtual environment access to the `system-site-packages` directory
`--symlinks`	Try to use `symlinks` rather than copies when symlinks are not the default for the platform
`--copies`	Try to use copies rather than symlinks even when symlinks are the default for the platform
`--clear`	Delete the contents of the environment directory, if it exists, before environment creation
`--upgrade`	Upgrade the environment directory to use this version of Python, assuming that Python has been upgraded in-place
`--without-pip`	This skips installing or upgrading pip in the virtual environment (`pip` is bootstrapped by default)

The most important argument to note is --system-site-packages, which enables the global site packages within the environment. This means that if you have a package installed in your global Python version, it will be available within your environment as well. However, if you try to update it to a different version, it will be installed locally. Whenever possible, I would recommend disabling the --system-site-packages flag because it gives you a simple environment without too many variables. A simple update of the system packages could break your virtual environment otherwise, but worse, there is no way to know which packages are needed locally and which ones are just installed for other purposes.

To enable this for an existing environment, you can simply run the environment creation command again, but this time adding the --system-site-packages flag to enable the global site packages.

To disable it again, you can simply run the environment creation command without the flag. This will keep the locally (within the environment) installed packages available but will remove the global packages from your Python scope.

Tip

When using virtualenvwrapper, this can also be done with the toggleglobalsitepackages command from within the activated environment.

The --symlinks and --copies arguments can generally be ignored, but it is important to know the difference. These arguments decide whether the files will be copied from the base python directory or whether they will be symlinked.

Note

Symlinks are a Linux/Unix/Mac thing; instead of copying a file it creates a symbolic link that tells the system where to find the actual file.

By default, venv will try to symlink the files, and if that fails, it will fall back to copying. Since Windows Vista and Python 3.2, this is also supported on Windows, so unless you're using a very old system, you will most likely be using symlinks in your environment. The benefit of symlinks is that it saves disk space and stays in sync with your Python installation. The downside is that if your system's Python version undergoes an upgrade, it can break the packages installed within your environment, but that can easily be fixed by reinstalling the packages using pip.

Finally, the --upgrade argument is useful if your system Python version has been upgraded in-place. The most common use case for this argument is for repairing broken environments after upgrading the system Python with a copied (as opposed to symlinked) environment.

Differences between virtualenv and venv

Since the venv module is essentially a simpler version of virtualenv, they are mostly the same, but some things are different. Also, since virtualenv is a package that is distributed separately from Python, it does have some advantages.

The following are the advantages of venv over virtualenv:

venv is distributed with Python 3.3 and above, so no separate install is needed
venv is simple and straightforward with no features besides the bare necessities

Advantages of virtualenv over venv:

virtualenv is distributed outside of Python, so it can be updated separately.
virtualenv works on old Python versions, but Python 2.6 or a higher version is recommended. However, Python 2.5 support is possible with older versions (1.9.x or lower).
It supports convenient wrappers, such as virtualenvwrapper (http://virtualenvwrapper.readthedocs.org/)

In short, if venv is enough for you, use it. If you are using an old Python version or want some extra convenience, such as virtualenvwrapper, use virtualenv instead. Both projects essentially do the same thing, and efforts have been made to easily switch between them. The biggest and most significant difference between the two is the wide variety of Python versions that virtualenv supports.

Installing C/C++ packages

Most Python packages are purely Python and blissfully easy to install, just as a simple pip install packagename does the trick. However, there are cases where compilation is involved and installation goes from a simple pip install to searching for hours to see which dependencies are needed to install a certain package.

The specific error message will differ as per the project and environment, but there is a common pattern in these errors, and understanding what you are looking at can help a lot when searching for a solution.

For example, when installing pillow on a standard Ubuntu machine, you'll get a few pages full of errors, warnings, and other messages that end like this:

    x86_64-linux-gnu-gcc: error: build/temp.linux-x86_64-3.4/libImaging/Jpeg2KDecode.o: No such file or directory
    x86_64-linux-gnu-gcc: error: build/temp.linux-x86_64-3.4/libImaging/Jpeg2KEncode.o: No such file or directory
    x86_64-linux-gnu-gcc: error: build/temp.linux-x86_64-3.4/libImaging/BoxBlur.o: No such file or directory
    error: command 'x86_64-linux-gnu-gcc' failed with exit status 1

    ----------------------------------------
Command "python3 -c "import setuptools, tokenize;__file__='/tmp/pip-build-_f0ryusw/pillow/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-kmmobum2-record/install-record.txt --single-version-externally-managed --compile --install-headers include/site/python3.4/pillow" failed with error code 1 in /tmp/pip-build-_f0ryusw/pillow

Upon seeing messages like these, you might be tempted to search for one of the lines such as x86_64-linux-gnu-gcc: error: build/temp.linux-x86_64-3.4/libImaging/Jpeg2KDecode.o: No such file or directory. While this might give you some relevant results, most likely it will not. The trick with installations like these is to scroll up until you see messages about missing headers. Here is an example:

  In file included from libImaging/Imaging.h:14:0,
                   from libImaging/Resample.c:16:
  libImaging/ImPlatform.h:10:20: fatal error: Python.h: No such file or directory
   #include "Python.h"
                      ^
  compilation terminated.

The key message here is that Python.h is missing. These are part of the Python headers and are needed for the compilation of most C/C++ packages within Python. Depending on the operating system, the solutions will vary—unfortunately. So, I recommend that you skip all parts of this paragraph that are not relevant for your case.

Debian and Ubuntu

In Debian and Ubuntu, the package to be installed is python3-dev or python2-dev if you're still using Python 2. The command to execute is as follows:

# sudo apt-get install python3-dev

However, this installs the development headers only. If you want the compiler and other headers bundled with the install, then the build-dep command is also very useful. Here is an example:

# sudo apt-get build-dep python3

Red Hat, CentOS, and Fedora

Red Hat, CentOS, and Fedora are rpm-based distros that use the yum package manager to install the requirements. Most development headers are available through <package-name>-devel and are easily installable as such. To install the Python 3 development headers, use this line:

# sudo apt-get install python3-devel

To make sure you have all the requirements such as development headers and compilers to build packages such as Python, the yum-builddep command is available:

# yum-builddep python3

OS X

The install procedure on OS X consists of three steps before the actual package can be installed.

First, you have to install Xcode. This can be done through the OS X App Store at https://itunes.apple.com/en/app/xcode/id497799835?mt=12.

Then you have to install the Xcode command-line tools:

# xcode-select --install

Finally, you need to install the Homebrew package manager. The steps are available at http://brew.sh/, but the install command is as follows:

# /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Note

Other package managers, such as Macports, are also possible, but Homebrew is currently the OS X package manager with the most active development and community.

Once all of these steps have been completed, you should have a working Homebrew installation. The working of Homebrew can be verified using the brew doctor command. If there are no major errors in the output, then you should be ready to install your first packages through brew. Now we simply need to install Python and we're done:

# brew install python3

Windows

On Windows, manual compilation of C Python packages is generally a non-trivial task to say the least. Most packages have been written with Linux/Unix systems in mind (OS X falls under the Unix category), and Windows is a nice-to-have for developers. The result is that packages are difficult to compile on Windows because there are few people testing them and many of the libraries require manual installation, making it a very tedious task. So, unless you really have to, try and stay away from manually compiling Python packages on Windows. Most packages are available as installable binary downloads with a bit of searching, and there are alternatives such as Anaconda that include binary packages for most important C Python packages.

If you still feel inclined to manually compile C Python packages, then there is another option, and it is generally an easier alternative. The Cygwin project (http://cygwin.com/) attempts to make Linux applications run natively on Windows. This is generally an easier solution than making packages work with Visual Studio.

If you do wish to take the Visual Studio path, I would like to point you towards Chapter 14, Extensions in C/C++, System Calls, and C/C++ Libraries, which covers manual writing of C/C++ extensions and some information on which Visual Studio versions you need for your Python version.

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Randall Degges Jun 20, 2016

So, I was one of the technical editors of this book. I'm a bit biased, but I still think my review is useful to any potential readers!This is an incredibly useful and interesting book on advanced Python programming. Rick covers a lot of advanced Python topics in depth, with many examples, and does a good job of clearly explaining concepts along the way.This book covers:- Writing good project documenting using Sphinx and reStructuredText.- AsyncIO in Python 3.- Testing and logging.- Advanced debugging with pdb and trace.- Performance optimization and profiling.- Multiprocessing to solve CPU-bound issues and get around the GIL.- Metaclassing.- Writing Python extensions in C / C++, and using system libraries (especially useful for people doing performance based Python).- Packaging your Python code into clean, re-usable libraries.- And lots more!If any of the above topics are of interest to you, I highly recommend giving this book a read as it covers each of these topics in a lot of depth, and has some really useful information about each! Through the process of editing this book, I actually learned several really cool things I did not know previously, particularly around new Python 3 async stuff!If you're already a Python programmer, and want to upgrade your skill set to the next level, this is an excellent book to take you there.

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Boaz Jun 16, 2016

Mastering Python is a good stepping stone for experienced Python developers who want to learn about the advanced features that Python has to offer.What I particularly like about this book is that it explains these subjects in a concise manner, while still discussing details, best practices, common pitfalls and the differences in Python versions.In addition, the author uses small code samples to explain the various topics that the book covers.That makes this book easy to read and allows it to be used as an advanced Python reference.

Rafal D. Dec 07, 2017

First three chapters touch on basics of Python and how to get everything all setup on your computer. Then it quickly goes on towards more intermediate and advance topics like decorators, generators, async, metaclasses, etc. Python has this whole Zen thing going for it, and this book emphasizes on correct coding practices and avoiding common pitfalls. Great reference for anyone working with Python.

Chandran Anjur Narasimhan Feb 21, 2018

This is one of the best python books that gives your an complete coverage of the programming language, tools needed and very detailed. I love this book.

Praveen Aug 31, 2016

excellent book for mastering python

Mastering Python: Master the art of writing beautiful and powerful Python by using all of the features that Python 3.5 offers

What do you get with eBook?

Mastering Python

Chapter 1. Getting Started – One Environment per Project

Creating a virtual Python environment using venv

Note

Creating your first venv

Note

Tip

venv arguments

Tip

Note

Differences between virtualenv and venv

Bootstrapping pip using ensurepip

ensurepip usage

Manual pip install

Tip

Installing C/C++ packages

Debian and Ubuntu

Red Hat, CentOS, and Fedora

OS X

Note

Windows

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Creating your first venv

venv arguments

Differences between virtualenv and venv

Tip

Key benefits

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Who is this book for?

What you will learn

Product Details

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