How to run PP4E book examples with Python 3.3

This page summarizes the 4 steps required to run the examples in the book Programming Python, 4th Edition with Python 3.3, and gives the corresponding changes applied to the new release 1.4 of the book's examples package, created October 15, 2013. In brief:

1. [Windows only] Set PY_PYTHON=3 for new Windows launcher
2. [Windows only] Fix #! lines unrecognized by new Windows launcher
3. Install 2-file PyMailGUI patch required for 3.3 email package change
4. Install the Pillow drop-in replacement for the PIL image library

The first 2 of these steps are needed for Windows only, and the new release 1.4 of the examples package incorporates the first 3 steps automatically. For example, the PyGadgets and PyDemos auto-launchers, as well as the larger stand-alone programs such as PyMailGUI and PyEdit should work on 3.3 out of the box. Still, you'll need to manually install Pillow instead of PIL for image examples such as PyPhoto, and may need to set your PY_PYTHON on Windows for general 3.X use.

Note that all these changes are backward-compatible with earlier 3.X releases, so release 1.4 of the book's examples package works with Pythons 3.0 through 3.3. Python 3.4 and later may or may not require additional patches, especially given the flux in the email package; watch for updates to be posted here as warranted. Also note that most book examples will run unchanged under 3.3; the steps outlined here are mainly required for specific programs only, and mostly for larger graphical programs.

1) [Windows only] Set PY_PYTHON=3 for new Windows launcher

As noted here, the new Windows launcher automatically installed with Python 3.3 runs scripts and commands with an installed 2.X by default, unless they give a more specific release number in #! patterns or command argument options. This effectively breaks programs meant to be used with 3.X if they use generic Python commands or have top-level script files with missing or release-ambiguous #! lines. As it's common practice on Windows to omit a #! line formerly useful only in some Unix usage modes, this can have broad impacts. Even on Unix, #! lines with a generic "python" resolved by file links is a norm, but will now fail if run by Python 3.3 on Windows.

To work around this, you can either:

1. Add a Unix #! line to the top of every Python 3.X script you run on Windows—a bizarre expectation that needlessly adds extra work, and seems just plain rude to people who use Python on Windows.
2. Set your PY_PYTHON environment variable to "3" to force the 3.3 launcher to default to 3.X globally for all programs that don't give a version explicitly.

• For code run in-process—via imports, exec(), etc.—the launcher script itself requires a "#!...python3" line, so that the entire process is started with 3.X.
• For code run in a spawned process—with os.system(), os.spawnv(), etc.—the launcher script may or may not require a "#!...python3" line for its own code, but must set PY_PYTHON=3 via os.environ for any version-generic spawned programs (os.environ settings are inherited by subprocesses).

In the book examples package

In release 1.4, the required changes were applied to the book's 4 auto-launcher scripts that I drag out as shortcuts to my Windows desktop and use on a regular basis, as well as in 1 utility module they leverage:

PP4E\Launch_PyGadgets_bar.pyw	Added #!/usr/bin/python3 to force 3.X execution of itself; this script runs Launcher.py tools in-process via imports.
PP4E\Launch_PyDemos.pyw	Added #!/usr/bin/python3 to force 3.X execution of itself; this script runs Launcher.py tools in-process via imports.
PP4E\Gui\TextEditor\pyedit.pyw	Added #!/usr/bin/python3 line to force 3.X execution of itself; this script runs the main PyEdit script in-process with exec().
PP4E\Internet\Email\PyMailGui\altconfigs\launch_PyMailGui.py	Added os.environ['PY_PYTHON']='3' to force 3.X execution of the PyMailGUI subprocess spawned with os.system(); a #! line for this script's own execution is irrelevant, as its code runs on both 2.X and 3.X.
PP4E\Launcher.py (utility)	Added both #!/usr/bin/python3 to force 3.X execution of itself (when run standalone), and os.environ['PY_PYTHON']='3' to force 3.X execution of any subprocess it spawns (see below).

Note that the last entry in this table, Launcher.py, sets PY_PYTHON=3 in os.environ, to be inherited by spawned programs. This may be moot for its immediate subprocesses, because this module runs explicit "python ..." command lines with os.spawnv() after setting Python and system paths automatically, thereby cutting out the 3.3 "py" launcher in the Windows registry; deeper program descendants, however, may still require this setting. Complex to be sure, but avoidable if you're willing to require an extra environment setting for using existing 3.X code under 3.3 on Windows—a manual task PP4E's top-level launchers were designed to obviate.

Also keep in mind that this patch applies to the selected auto-launcher scripts only. When using 3.3 and later, you'll still probably want to manually set your PY_PYTHON environment variable eventually in order to run other version-agnostic 3.X scripts on Windows with 3.X instead of 2.X. This includes any book examples not updated by this patch; see the Advanced system settings in your Control Panel's System entry to set the version default globally.

Three caveats on this process

Although this patch suffices in the normal case, there are three issues that may impact your Windows launcher experience worth noting here:

• Unix portability paradox?: It's not impossible that hardcoding a "python3" in a #! line for use on Windows may not be portable to some Unix systems, which would virtually force a manual PY_PYTHON=3 setting on Windows machines with 2.X installs. If this impacts you, you'll have to modify the patched scripts as needed—set PY_PYTHON, and either use a generic "python" in #! lines, or avoid #! lines altogether (per the next bullet, PATH can do similar work with a "python" #! on 3.4). All Unix portability bets are off, of course, if you use a #! pattern not common on Unix; and abbreviated "#!python3" may be a prime suspect.
  
  
• PATH checked in 3.4 (sometimes): The Windows launcher has been recently modified in 3.4 to give a PATH variable setting priority over searching the registry for the highest-numbered Python, when just a generic "python" appears in a #! line. This better emulates the /usr/bin/env paradigm on Unix (and may allow for more portable #! lines in general), but is apparently employed only when generic "python" #! lines are present—not when #! lines are absent altogether, the normal case on Windows, and an additional case addressed by PY_PYTHON. For more background on this change, see Python issue reports: here and here.
  
  
• Filename associations bug?: for unknown reasons, Python 3.3+ Windows MSI installers can sometimes fail to properly set filename association in the Windows registry for launcher use. This should automatically associate .py and .pyw file types (plus bytecode) with the launcher's py.exe and pyw.exe executables, respectively. However, I've noticed this fail on 2 of 6 Windows machines tested—on 2 machines running Windows Vista and 7, Python files are incorrectly associated with a former python.exe and pythonw.exe after installing 3.3; on 4 other machines running XP, Vista, 7, and 8, the associations are correctly set to the launcher's programs instead. All these machines had prior Pythons installed, of various versions (3.2 and 2.6 where associations failed; 3.2, 3.1, and 2.5 where they worked).
  
  Without the expected filename associations, the launcher is never invoked for scripts run without explicit "py ..." command lines—scripts will instead run under any Python named in the registry, and will happily ignore any #! lines completely, despite 3.3's best intentions. I noticed this when 3.2 was still inexplicably running scripts after a 3.3 install, despite #! directives; more generally, you can detect this problem in simpler terms by running a script from the command line like the following, shown here on a system with incorrect associations:

  c:\PP4E> type t.py
  #!/usr/bin/python3.3
  import sys
  print(sys.version)
  
  c:\PP4E> t.py
  3.2.2 (default, Sep  4 2011, 09:07:29) [MSC v.1500 64 bit (AMD64)]
  
  c:\PP4E> py t.py
  3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:06:53) [MSC v.1600 64 bit (AMD64)]
  

  On another failer, this turns out even worse:

  C:\PP4E> type t.py
  #!/usr/bin/python3
  import sys
  print(sys.version)
  
  C:\PP4E> t.py
  2.6 (r26:66721, Oct  2 2008, 11:35:03) [MSC v.1500 32 bit (Intel)]
  
  C:\PP4E> py t.py
  3.4.0a2 (v3.4.0a2:9265a2168e2c+, Sep  8 2013, 19:41:05) [MSC v.1600 32 bit (Intel)]
  

  If the first command doesn't run the same highest-numbered 3.X Python installed and print the same version as the second, your associations are probably incorrect. In this case, the association reported by "assoc .py" and "ftype Python.File" command-line tools seem correct, but are apparently overridden by registry settings, and link to "python.exe" instead of "Python Launcher for Windows" in the Default Programs GUI.
  
  To fix: open your Default Programs in Control Panel (or similar), and manually associate all 4 Python file extensions with the launcher's py.exe/pyw.exe executables (normally installed in C:\Windows), a regrettable and error-prone step, but a one-time event. This may or may not be a "bug" in the Windows launcher (the registry is notoriously fragile), but it's certainly an issue; it is especially difficult to justify a mandatory change that requires extra work and can break existing programs, if that change itself cannot be relied on to work as advertised.

For more on the 3.3 Windows launcher in general, see the new Appendix B in Learning Python, 5th Edition, or its early draft here. The often prescribed alternative to environment settings mentioned earlier—adding a Unix #! line at the top of every script on Windows—seems rude and extreme, especially given other launcher issues like those of the former and following sections.

2) [Windows only] Fix #! lines unrecognized by new Windows launcher

As described here, the new Windows launcher installed with Python 3.3 fails when a script begins with a Unix #! line that it cannot recognize. Some book example scripts used a "#!/bin/env python" pattern that was ignored on Windows by every Python through 3.2, but is now treated as an error by 3.3. Such #! forms are valid and even required on some Unix systems, but must be changed to a launcher-accepted pattern such as "#!/usr/bin/python" or "#!/usr/bin/env python" in order to run under 3.3 on Windows—a platform where such lines are otherwise irrelevant.

Note that this is true even if you use "python3" in the #! line or set your PY_PYTHON=3 per the prior section; an unrecognized #! pattern fails in 3.3 even if it's not version-ambiguous, and even if it's valid on a Unix system.

In the book examples package

In release 1.4, this fix was applied to the #! lines at the top of the following 10 files, changing their "/bin/env" to "/usr/bin/env"; no other code or environment changes are required:

• PP4E\Launch_PyDemos.pyw (also changed for prior section)
• PP4E\Launch_PyGadgets_bar.pyw (also changed for prior section)
• PP4E\LaunchBrowser.pyw
• PP4E\Internet\Ftp\Mirror\cleanall.py
• PP4E\Internet\Ftp\Mirror\downloadflat.py
• PP4E\Internet\Ftp\Mirror\downloadflat_modular.py
• PP4E\Internet\Ftp\Mirror\ftptools.py
• PP4E\Internet\Ftp\Mirror\uploadall.py
• PP4E\Internet\Ftp\Mirror\uploadflat.py
• PP4E\Internet\Ftp\Mirror\uploadflat_modular.py

You can locate and edit these files yourself in older example packages by using the Search/Grep tool in the book's PyEdit GUI example on the package's PP4E root directory, or by running one of the directory search and edit/replace utilities presented in the System Programming part of the book (e.g., see search_all.py and Visitor subclasses later in Chapter 6). Fixing any additional unrecognized #! patterns lurking in the examples package is an officially suggested exercise.

For more on the 3.3 Windows launcher in general, see the new Appendix B in Learning Python, 5th Edition, or its early draft here.

3) Install 2-file PyMailGUI patch required for 3.3 email package change

As covered here, Python 3.3 changed a standard library email utility that the book's largest example, PyMailGUI, depends on to format non-ASCII email address names for display. The patch is simple, and spans just two files:

• PP4E\Internet\Email\PyMailGui\py33patch.py, a new file that restores the prior version of the email utility
• PP4E\Internet\Email\PyMailGui\SharedNames.py, a modified file which runs an import of the patch to affect the change

In the book examples package

The new and modified file of the patch are automatically included in PyMailGUI's source code directory of the new release 1.4 of the examples package; no other code or environment changes are required.

4) Install the Pillow drop-in replacement for the PIL image library

As mentioned here, the third-party PIL image-processing library used by examples in the book has been subsumed by the Pillow fork—an open source drop-in replacement for PIL which is being actively supported for use in newer Python releases. You'll want to fetch and install Pillow to use most of the book's image examples, including the PyPhoto viewer, which employs image display, thumbnail generation, and resize operations. I've also used Pillow successfully for EXIF photo metadata tag processing.

Pillow is currently available at this site. For example, all image-based book examples run fine under on Python 3.3 after installing the following on my Windows 32- and 64-bit systems, respectively; see PIL's site for other Pythons and platforms:

• Pillow-2.2.1.win32-py3.3.exe
• Pillow-2.2.1.win-amd64-py3.3.exe

In the book examples package

No fix was applied, because the example package doesn't ship 3rd-party systems as bundled items as a rule; you'll want to fetch and install Pillow for your Python and platform, from its official site above.