Matt Leaverton

Software and Electronics.

Currently Senior Software Engineer at Glowforge

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Python EXE Debugging Trick

There comes a time in every Python developer's life when they are ready for the next level: freezing scripts into an EXE for deployment. Much ink and blood has been shed on the topic and I have nothing to contribute directly to that today other than PyInstaller is a very reasonable tool for the job.

If you are still reading after that earth-shattering news, you are in for a treat. Not long after the aforementioned time comes, another time comes along - the EXE does not work, and you are stumped.

Thanks to Jason R. Coombs and Steven Kryskalla in this StackOverflow answer from 2009, here is an excellent software nugget that can ease debugging particularly sticky PyInstaller issues.

import code

With no external libraries or fancy code, this drops you directly into the Python REPL (Read Evaluate Print Loop). This should feel familiar as it is rather like the standard Python command line interface, which makes for a powerful debugging tool for a frozen environment.

A couple notes:

  • First, nothing is imported by default, so import everything you want to use.
  • Second, when frozen, only the bundled libraries are available for access so not Python library installed locally on your system will be available. The Python standard library is bundled in with PyInstaller - run sys.builtin_module_names (after importing sys) to see what is available.
  • Third, especially if using PyInstaller in --onefile mode, paths get complicated, so take care to understand where you are and where things are relative to you if that is important to your debugging.
  • Last, simply use sys.exit() to exit when finished - make sure to import sys first.

To see how this can assist in debugging, consider the following scenario: you are developing a script to help a bakery advertise their delicious foods. This script was not developed with deployment in mind, but you have people showing interest, and you wish to share an EXE. The script works just fine when run it on your machine in test mode:

Welcome to Buntastic Bakery - please enjoy your 'Placeholder Croissant'

So you say, great, bundle it into an EXE and ship it to your dozens of friends who enjoy fine baked goods.

Soon, reports are rolling in that instead of information about fine baked confections, your friends are getting the unappetizing test message:

Welcome to Buntastic Bakery - please enjoy your 'Crusty Test Croissant'

Apparently you neglected to run the EXE before shipping, because you see the same message on your machine when you test. Time to debug. With test mode enabled, the message should come from place_holder_baked_good so that seems like a good place to start looking.

Adding the code snippet from above to the code yields the following:

def placeholder_baked_good(confection: str) -> str:
    """ Test mode - find and return a test baked good """

    import code

    baked_good = importlib.import_module('library.test_data')
    return baked_good.TEST_STRING.format(confection)

Note: Play along at home with this code by getting your own copy from Github

Build into an EXE and run it again to find yourself in the interactive Python console

> pyinstaller main.spec
> .\dist\main.exe
Python 3.10.2 (tags/v3.10.2:a58ebcc, Jan 17 2022, 14:12:15) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.

The invalid output message indicates that the code inside the ModuleNotFoundError exception is run, so test out the import manually to see if that is in fact the issue.

>>> import library.test_data as baked_good
Traceback (most recent call last):
  File "<console>", line 1, in <module>
ModuleNotFoundError: No module named 'library'

As suspected - Python reports that it cannot find our library module. It is clearly present in the repo and this code works when run in script format, so something must be missing when translating into the EXE. One last check to confirm - sys.modules shows all modules that the Python import system and PyInstaller module loaders have found and processed (as noted before, if you're looking for built-in libraries, use sys.builtin_module_names). If the package you want to import is not shown in sys.modules then Python has not seen the library we are looking for yet.

>>> import sys
>>> from pprint import pprint
>>> pprint(sys.modules)
'copyreg': <module 'copyreg' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\\\copyr...
'data': <module 'data' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\data\\__init__.pyc'>,
'data.data_manager': <module 'data.data_manager' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\dat...
'dataclasses': <module 'dataclasses' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\dataclasses.pyc...
'io': <module 'io' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\\\io.pyc'>,
'itertools': <module 'itertools' (built-in)>,
'keyword': <module 'keyword' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\\\keywo...
'linecache': <module 'linecache' from 'C:\\Users\\MATTLE~1\\AppData\\Local\\Temp\\_MEI175642\\\\l...
'marshal': <module 'marshal' (built-in)>,

First, you can see that the data and data.data_manager modules where the placeholder_baked_good and get_baked_good methods are located are already processed - PyInstaller found those and brought them along. You can also see that library is completely missing.

PyInstaller can perform a lot of magic in deciding what to bundle, but it is not perfect. This example case is contrived to deliberately bamboozle PyInstaller with a dynamic import that it cannot see. It demonstrates that if your use case requires code trickery, care must be taken to give PyInstaller its best chance at success.

In this case, the solution is simple - give PyInstaller a hand by adding library.test_data to the hiddenimports list in main.spec:

a = Analysis(
pyz = PYZ(a.pure, a.zipped_data, cipher=None)

Upon removal of the debug code and a rebuild, the EXE now works as expected.

> pyinstaller main.spec
> .\dist\main.exe
Welcome to Buntastic Bakery - please enjoy your 'Placeholder Croissant'

Astute readers may note - the simplest solution to the exact problem presented is to remove the dynamic import and place from library import test_data as baked_good, which will work and also hint to PyInstaller to bring along library.test_data with no extra help. Let this serve as both a recommendation to try to work with PyInstaller as often as possible, but also a way to get out of sticky situations if you need it.

A neat PyInstaller specific trick to see exactly what modules could be available for import, and not just those processed already is to dig into the custom importers that PyInstaller creates to load code from the EXE archive it creates. Anyone can write extensions Python import system, details of which are for another day. The "path finders" - used to process an import statement and determine if there is anything available to load - can be viewed at sys.meta_path:

>>> sys.meta_path
[<class '_frozen_importlib.BuiltinImporter'>, <class '_frozen_importlib.FrozenImporter'>, 
 <pyimod02_importers.FrozenImporter object at 0x000002172DA54DF0>, <class '_frozen_importlib_external.PathFinder'>]

pyimod02_importers.FrozenImporter here is the PyInstaller path finder responsible for reporting whether an import statement can load any module from its bundle. On load, this finder builds a table of contents containing all available libraries, and we can take a peek:

>>> import sys
>>> from pprint import pprint
>>> pprint(sys.meta_path[2].toc)
>>> import library.test_data as baked_good
>>> print(baked_good.TEST_STRING.format("Muffin"))
Placeholder Muffin

After fixing our dynamic import issue, library.test_data is now available and can be successfully imported and used.

PyInstaller provides several official solutions to problematic scenarios. In case those do not fit your needs, let this trick be another tool in your belt for debugging deployments.