I have been dealing with a really frustrating problem today. Yesterday my code was working and today it was failed without any code changes.

This is the piece of code causing the problem.

app = Flask(__name__, instance_relative_config=True)
app.config.from_object('config')
app.config.from_pyfile('config.cfg')

And this is the error.

Traceback (most recent call last):
  File "run.py", line 1, in 
    from project import app
  File "C:\Python27\env\vod_test\src\project\__init__.py", line 37, in 
    load_config()
  File "C:\Python27\env\vod_test\src\project\__init__.py", line 33, in load_config
    app.config.from_pyfile('config.cfg')
  File "C:\Python27\env\vod_test\lib\site-packages\flask\config.py", line 129, in from_pyfile
    with open(filename) as config_file:
IOError: [Errno 2] Unable to load configuration file (No such file or directory): 'C:\\Python27\\env\\vod_test\\var\\project-instance\\config.cfg'

Flask is trying to load the config file from C:\\Python27\\env\\vod_test\\var\\project-instance\\config.cfg instead of from C:\\Python27\\env\\vod_test\\project\\instance\\config.cfg. Where is the var coming from? And why is there a hyphen between project and instance?

The strange thing was that the problem only occurred when I was running my test cases from the command line. In PyCharm the code was working. How frustrating. It turned out that PyCharm was using a different virtualenv than the command line. After comparing the output of ‘pip freeze’ I discovered that some libraries had different versions.

To solve the problem I added the following code.

import os
from flask import Flask


def load_instance_config_fix(filename):
    config = {}
    with app.open_instance_resource(filename) as f:
        for line in f:
            if line[0] == "#":
                continue
            name, value = line.partition("=")[::2]
            config[name.strip()] = value.strip()
    return config


def handle_incorrect_instance_path(f):
    def handle_load_instance_config():
        try:
            f()
        except IOError:
            app.instance_path = os.path.join(os.path.abspath(os.curdir), 'instance')
            config = load_instance_config_fix('config.cfg')
            app.config.update(config)

    return handle_load_instance_config


@handle_incorrect_instance_path
def load_config():
    app.config.from_object('config')
    app.config.from_pyfile('config.cfg')


app = Flask(__name__, instance_relative_config=True)

load_config()


@app.route('/')
def hello_world():
    return 'Hello, World!'

Just some nice syntactic sugar with a decorator to keep the code nice and concise.

Another way to solve the problem is by setting the instance_path when initializing Flask. What fun would that be 🙂

import os
from flask import Flask


app = Flask(__name__, instance_path=os.path.join(os.path.abspath(os.curdir), 'instance'), instance_relative_config=True)
app.config.from_object('config')
app.config.from_pyfile('config.cfg')


@app.route('/')
def hello_world():
    return 'Hello, World!'

Back to real coding 🙂

I had a little fight with Marshmallow and the ordered=True in class Meta. When ordered=True is set the fields should be outputed in the order specified in the property fields of the class Meta.

Example:

class AuthorSchema(ma.Schema):
    id = base_fields.Int(dump_only=True)

    absolute_url = ma.AbsoluteURLFor('api.author', id='<id>')

    links = ma.Hyperlinks({
        'self': ma.URLFor('api.author', id='<id>'),
        'collection': ma.URLFor('api.authors')
    })

    @post_dump(pass_many=True)
    def wrap(self, data, many):
        key = 'authors' if many else 'author'
        return {
            key: data
        }

    class Meta:
        fields = (
            'id',
            'name',
            'links',
            'absolute_url',
        )
        ordered = True

This was the output.

{
  "author": {
    "absolute_url": "http://localhost:8080/api/v4/authors/123",
    "id": 123,
    "links": {
      "collection": "/api/v4/authors/",
      "self": "/api/v4/authors/123"
    },
    "name": "Fred Douglass"
  }
}

After some Googeling I found out that the culprit was Flask.jsonify. It orders output alphabetically by default. The following setting switches this off.

app.config['JSON_SORT_KEYS'] = False

After this the output was in the specified order.

{
  "author": {
    "id": 123,
    "name": "Fred Douglass",
    "links": {
      "self": "/api/v4/authors/123",
      "collection": "/api/v4/authors/"
    },
    "absolute_url": "http://localhost:8080/api/v4/authors/123"
  }
}

Hope this saves someone else time.

If you need to know which Flask routes are configured you can execute the following command.

print flask_app.url_map