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 🙂