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Notes on Python, Django, and web development on Ubuntu Linux


How to install grunt on Ubuntu 14.04

Grunt is a Javascript task runner that can be used to compile Sass, run JSHint, or run many other plugins. It depends on the Node.js package manager, npm.

If you use the standard Ubuntu Trusty Tahr repository to install nodejs/npm, you will get this error when running grunt:

/usr/bin/env: node: No such file or directory
Instead, use the chris-lea nodejs repository.

Install nodejs, npm, and grunt-cli

$ sudo add-apt-repository ppa:chris-lea/node.js 
$ sudo apt-get update 
$ sudo apt-get install nodejs 
$ sudo npm install -g grunt-cli 

Install grunt in your project directory

$ cd ~/myproject 
$ echo "{}" > package.json 
$ npm ...
... read more »

The old "%" string formatting and the new string .format() method handle unicode differently

Today I learned that the old style "%" string formatting and the new string .format() method behave differently when interpolating unicode strings. I was suprised to find out that one of these lines raised an error while one did not:

'%s' % u'O\u2019Connor'

The old style "%" formatting operation returns a unicode string if one of the values is a unicode string even when the format string is a non-unicode string:

Python 2.7.3 (default, Feb 27 2014, 19:58:35) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for ...
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Python logging filters do not propagate like handlers and levels do

Loggers are organized in a hierarchical fashion. A logger named '' is a child of a logger named 'foo'.

getLogger() returns a reference to a logger instance with the specified name if it is provided, or root if not. The names are period-separated hierarchical structures. Multiple calls to getLogger() with the same name will return a reference to the same logger object. Loggers that are further down in the hierarchical list are children of loggers higher up in the list. For example, given a logger with a name of foo, loggers with names of, ...

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Subdomain-based configuration for a Flask local development server

This example shows how to set up a Flask local development server to use a different configuration based on the subdomain of the request. The project I work on has several environments (dev, qa, staging, etc). Each environment has different database and API hostnames. I use this to switch between database and API environments quickly while using my local development server.

This assumes a create_app function is used to create the Flask application instance as described in the Application Factories Flask documentation.


Modify the create_app function to take a configobj argument and use it to override the default configuration ...

... read more »

How to add a margin around markers in the Google Static Maps API using Python

This example shows how to use Python to generate a Google Static Map URL for a map that contains markers within some dimensions which are smaller than the map image dimensions. This effectively allows for setting minimum X and Y margins around the markers in a map. This is useful for a "fluid" web design where a maximum map size is requested from Google and is then cut off at the edges for small browser windows.

The bulk of this solution is based on the Javascript code here:

import math

def generate_map_url(
        max_map_width_px ...
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Do you have a lot of short, single-use, private functions in your Python code?

Do you have a lot of short, single-use, private functions in your Python code? For example, below is some stubbed out authentication code I've been working on. It checks if a user's password is correct and updates the hash algorithm to use bcrypt. The 4 private functions with the leading underscore are from 1 to 10 lines long and are only used by the check_password function. These functions are part of a larger module with about 20 functions. I don't like that these 4 functions add clutter to the module and are not grouped with the function ...

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How to expose a Flask local development server to the public using SSH remote port forwarding

Here is how to run a Flask local development server on your local machine and expose it to the public via a remote server you have control over. This uses SSH remote port forwarding which is a converse of local port forwarding described here: How to run a Django local development server on a remote machine and access it in your browser on your local machine using SSH port forwarding

  1. On the remote host, edit the sshd_config file (mine was located at /etc/ssh/sshd_config) to allow remote hosts to connect to ports forwarded for the client:
    GatewayPorts yes
  2. On ...
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When is the try-finally block used in Python?

The finally block is used to define clean-up actions. Why is the finally block needed? Why can't the clean up actions be put after the try/except/else block? This works in some cases, but if there is a return, break, or continue, or an unhandled exception inside the try, except, or else clauses, that code will never be executed. The finally block executes even in these conditions.

    print 'Inside try'
    raise Exception
    print 'Inside finally'
print 'Never get here'


Inside try
Inside finally
Traceback (most recent call last):
  File "", line 13, in 
    raise ...
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Using Python's gzip and StringIO to compress data in memory

I needed to gzip some data in memory that would eventually end up saved to disk as a .gz file. I thought, That's easy, just use Python's built in gzip module.

However, I needed to pass the data to pycurl as a file-like object. I didn't want to write the data to disk and then read it again just to pass to pycurl. I thought, That's easy also-- just use Python's cStringIO module.

The solution did end up being simple, but figuring out the solution was a lot harder than I thought. Below is my ...

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How to start a long-running process in screen and detach from it

How to start a long-running process in screen, detach from it, and reattach to it later.

Start a long running process in screen and detach

  • Ssh to the remote host, myremote:
    eliot@mylocal:~$ ssh myremote 
  • Start a new screen session
    eliot@myremote:~$ screen 
  • Start a long running process, "sleep 3600":
    eliot@myremote:~$ sleep 3600 
  • Detach from the screen session:
    eliot@myremote:~$ CTRL-A : detach ENTER 
    (Hit [CTRL-A], then type a colon character, then type "detach", then hit [ENTER])
  • Exit your remote SSH session:
    eliot@myremote:~$ exit 

Reattach to the existing screen session

  • Ssh to the remote host again:
    eliot@mylocal:~$ ssh ...
... read more »
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