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

I was busy debugging a rest services I’m building in Flask and Flask-Restplus. I needed a way to log the request and response. I found some code that can be used as a wrapper of the WSGI application that does just this.

Create a file called dubug.py that contains the following code.

import pprint

class RequestLoggingWrapper(object):
    def __init__(self, app):
        self._app = app

    def __call__(self, environ, resp):
        errorlog = environ['wsgi.errors']
        pprint.pprint(('REQUEST', environ), stream=errorlog)

        def log_response(status, headers, *args):
            pprint.pprint(('RESPONSE', status, headers), stream=errorlog)
            return resp(status, headers, *args)

        return self._app(environ, log_response)

This wrapper works independently from Flask and wraps the WSGI application. It shows exactly what request is going in and what response is going out.

When running Flask with the built-in server you can use it as follows.

from debug import RequestLoggingWrapper

if __name__ == '__main__':
    app.wsgi_app = RequestLoggingWrapper(app.wsgi_app)
    app.run()

The output goes to the wgi.error stream. For the built Flask server it is printed to stderr.

I needed to debug the Flask request. After googeling around for a while I ran into a cool trick on Stack Overflow using the pprint module. The pprint module provides a capability to “pretty-print” arbitrary Python data structures in a form which can be used as input to the interpreter.

import pprint
str = pprint.pformat(request.environ, depth=5)

This same trick can be used with all the Flask variables.

• request.args: the key/value pairs in the URL query string
• request.form: the key/value pairs in the body, as sent by a HTML POST form
• request.files: the files in the body, which Flask keeps separate from form
• request.values: combined args and form, preferring args if keys overlap