I upgraded the python interpreter in one of the components I ran in production from 3.5 to 3.7. Main reason for that was testing dataclasses functionality. Sometime ago I wrote a post about how to build a simply automatic initializer, and eventually it’s implemented in the standard library, which I believe is a great new feature!

The upgrade was quite smooth, even though I had an issue due to a non backwards compatible change in how a generator should flag it’s exhausted.

Making a long story short: use return instead of StopIteration when you want to stop sending items from your generator.

Next is the long story, things I learned while investigating the issue and the reason why I didn’t capture it before hand.

Lesson 1: Read the release notes

This may sound obvious, but…

Remember, a few hours of trial and error can save you several minutes
of looking at the README.

I am devloper (7 Nov 2018)

Python 3.7 release notes explains very clearly changes that this new version brings into python behavior, being one of them PEP-479. This PEP introduces a non backwards compatible change in how a generator behaves.

I was so focused on the new feature I wanted to test, I didn’t go over the release notes.

Lesson 2: The process

When you have a containerized environment, it’s quite straight forward to change/upgrade a specific component execution platform. We’ve been using Python 3.5 as our default runtime for a while, but I was intrigued to upgrade to newest versions and check the new functionalities provided by the language.

To be specific, I read some months ago about dataclasses and really looked to me like a nice functionality to reduce boilerplate code. Also, PEP563 - Postponed evaluation of annotations is a great new feature, in case you’re using annotations in your code.

In order to test the python interpreter upgrade, I chose as candidate an offline component which main goal is accessing gDrive API. Our library implements a tiny wrapper over the official python client library.

Find below a simplified snippet that, given a gDrive folderId, returns a generator which includes every file under that folder. As filenames are returned in several pages to prevent a very long response, we prefer using a generator instead of a list, so it’s up to the client to iterate as much as it’s needed (and therefore request additional pages under the hood):

def get_files_in_folder(folder_id, next_page_token=None):
    # Request a page
    _files, next_page_token = _get_files_page(folder_id, next_page_token)

    # Return every file included in the results
    for f in _files:
        yield f

    if next_page_token is None:
        # Flag generator is exhausted
        raise StopIteration
        # Request next page
        yield from get_files_in_folder(folder_id, next_page_token=next_page_token)

def _get_files_page(folder_id, page_token):
    results = get_service().files().list(
        q="'{0}' in parents".format(folder_id)

    return results.get('files', []), results.get('nextPageToken')

Right, probably raising a StopIteration instead of simply return was not a good idea, but this code works properly in python 3.5, but does not in 3.7 due to the change introduced in the mentioned PEP479.

Simply changing:

# Flag generator is exhausted
raise StopIteration


# Flag generator is exhausted

solved the issue.

Lesson 3: Check silent deprecation warnings

PEP-479 describes very clearly the transition plan in regards to the backwards incompatible change:

  • Python 3.5: Enable new semantics under future import; silent deprecation warning if StopIteration bubbles out of a generator not under future import.
  • Python 3.6: Non-silent deprecation warning.
  • Python 3.7: Enable new semantics everywhere.

As I upgraded from 3.5 to 3.7, I didn’t get any deprecation warning. Configuring PYTHONWARNINGS environment variable is a straight forward way to get this kind of warnings.

Lesson 4: Bring future default behaviour to your code

I took this opportunity to review what exactly __future__ statements tackles.

In regards to StopIteration breaking change, Python 3.5 introduced the chance to write your code in the way that is required in 3.7 (check the first point in the transition plan above). It’s really convenient to be ready for your next interpreter/library upgrade. In this specific case, all I needed to do was adding the following line on top of the gdrive API wrapper:

from __future__ import generator_stop

That way, running the code in a python 3.5 will behave as python 3.7, hence raising an error unless StopIteration is changed by return.

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