There are a few code formatting tools I like to use in just about any Python (and Django) project: black, isort, and flake8. These all serve slightly different purposes, and there are alternatives to each. A full discussion/comparison of code formatting tools in Python is beyond the scope of this post*, but in brief:

• Black is great because it automatically formats my code with a well recognized style. I have my editor (currently VSCode) set to run black when I save a file. This makes my code consistent, and is great in teams I've worked in because it means we don't have to worry about styling in code reviews.
• isort is great for sorting imports. Even though I tend to do a bit of this sorting as I go, I inevitably swap an import or leave one out of place. This organizes them so I don't have to.
• flake8 catches minor (and sometimes not so minor) code issues like unused variables.

There are a few ways to configure these libraries, but for now, I have them in their own individual config files. These should all be placed at the root of your Django project.

For black, I use the default config. In some other projects, I've extended the line length, but currently I'm leaving black as default.

For isort, isort.cfg is the filename and mine looks like this:

[settings]
skip_glob=env/*
extend_skip_glob=**/migrations
profile=black

Where env is the name of your environment. The extended skip for migrations isn't necessary, but I wanted to avoid changing my migration files. The black profile is nice to avoid conflicts between black and isort. Skipping your environment is likely required. When I've left this out (or accidentally run isort without the config) isort runs against the libaries in my environment and ends up breaking them by creating circular dependencies. One way to make sure you aren't going to do this is to run isort --check-only . to make sure it's only running against your files before actually running it.

For flake8 .flake8 is the filename and mine looks like this:

[flake8]
exclude =
    migrations
    __pycache__
    manage.py
    settings.py
    env
    .pytest_cache
    .vscode

These exclude all of the files in the folders migrations, __pycache__ and env (where env is the name of your environment). You may want to exclude more, or not exclude some of these, but I've found this to work for me.

Finally, on other projects I've used all of these with pre-commit, which is a library that uses git hooks to prevent you from committing code that doesn't use the standards you and your team have set, such as black, isort, and flake8, or some other combination of code formatting tools. It also allows you to use a single command to run all three tools.

Unfortunately, I can't figure out how to use pre-commit in my current setup with my python projects in WSL and my git client (Fork) in Windows. Perhaps this is a sign I should start using the git CLI more. But if you know of a good way to use precommit with this setup, let me know!

*between writing and publishing this I read James Bennett covering similar ground. He goes a bit deeper on some of the tools I mention (and other related tools), so I wanted to link this for further reading.

I found the documentation for presigned URLS on AWS using boto to be insufficient, so here is how I formed the request.

I wanted to have a file private in S3, but expose it to a user for download if they were authorized to access it. I found the easiest way to do that was leave the object in S3 as private, but use the AWS API to generate a pre-signed URL with a low timeout (a minute or so). Unfortunately, I found the documentation not so straightforward and cobbled together a few Stack Overflow answers to come up with exactly what I needed. In particular, I needed to add the ‘ResponseContentType’ to get the file to download correctly, and needed to specify my AWS credentials.

I’m using Django to serve up the download via a get request:

class GetDownloadURL(APIView):

    def get(self, request):
        # Get the service client.
        session = boto3.session.Session(profile_name="AWSUserName")
        s3 = session.client("s3")

        # Generate the URL to get 'key-name' from 'bucket-name'
        url = s3.generate_presigned_url(
            ClientMethod="get_object",
            Params={
                "Bucket": "your-s3-bucket",
                "Key": "SampleDLZip.zip",
                "ResponseContentType": "application/zip",
            },
            ExpiresIn=100,
        )

        return Response(url)

Notes: You will need to update the Key and Bucket params to match what you have in S3. Depending how you have set up your AWS credentials, you may be able to omit the ‘profile_name="AWSUserName"’ parameter. I prefer to be explicit in my config because I’ve run into issues when I have used the default in the past.

I am creating a Django application using PostgreSQL (PSQL) for my database and was nearly finished with the API when I discovered some strange behavior. After successfully testing the API in the Django app, I decided to run some basic queries on the database. I received the following error for nearly every field in the app:

    select MaxSceneKey from game_progress_gameplaykeys;
    ERROR:  column "maxscenekey" does not exist
    LINE 1: select MaxSceneKey from game_progress_gameplaykeys;
                   ^
    HINT:  Perhaps you meant to reference the column "game_progress_gameplaykeys.MaxSceneKey".

I was getting the same result for every field in the table that I tried (and when I try to include the table name as the hint suggests), except for ‘user_id’ and ‘objective'.

I confirmed that the fields existed using \d+ game_progress_gameplaykeys, tried changing some of their field types, and even upgraded from Postgres 9.5 to 10.5 (I was planning to do this anyway).

After a bunch of searching, I found the issue:

“All identifiers (including column names) that are not double-quoted are folded to lower case in PostgreSQL.” from https://stackoverflow.com/questions/20878932/are-postgresql-column-names-case-sensitive

I created camelCase field names in my Django app based on what the field names were previously in my application (written in C#).

I decided to fix this (for now) by fixing my models to all use snake_case and using https://github.com/vbabiy/djangorestframework-camel-case to switch the keys from camelCase to snake_case when they come into the API. One issue solved!

While debugging that issue, I decided to update my laptop’s code + postgres version since I hadn’t worked on it in a while and wanted to see if the issue was just on my desktop. When I reinstalled PSQL, I couldn’t seem to log into it using the user I was creating. Using the postgres user was fine, though.

I finally figured out the issue was that PSQL was running on port 5433, not 5432 (the default). After that, I was puzzling over what could be running on 5432 since ‘netstat’ and ‘lsof’ revealed nothing else running on my WSL Ubuntu VM. As I was searching around, I saw someone mention that really only PSQL should be running on that port, and I realized I had installed PSQL on Windows on that machine before I moved over to WSL. I uninstalled that, switched back to 5432 in Linux, restarted PSQL, and boom, good to go.

While I was debugging that issue, I learned some good information about PSQL along the way:

/etc/postgresql/10/main/postgresql.conf allows you to set and check the port that PSQL is running on.

/etc/postgresql/10/main/pg_hba.conf allows you to set different security protocols for connections to PSQL. Notable for local development: set the local connection lines to ‘trust’ so you don’t have to enter a password when logging in.

Note: you need to restart the PSQL server for either of these changes to take effect. Note 2: MORE IMPORTANT NOTE: Don’t use trust anywhere other than a local version of PSQL. Ever.

These are the lines I had to change to get that to work (may be different in versions of PSQL other than 10.5):

# "local" is for Unix domain socket connections only
local   all             all                                     trust
# IPv4 local connections:
host    all             all             127.0.0.1/32            trust
# IPv6 local connections:
host    all             all             ::1/128                 trust