- property: mylist
external_name: mylist
import_transform: “lambda x: len(x) > 0 and eval(x) or ””

Why it works:
In your csv, your list should export like so:
“[u'item1', u'item2', u'item3']“
Running an eval on this string, converts it to a list. The length check is like if/else statement, if len(x) > 0 convert it else use empty string.

With Django (what I am using), you use this command:
manage.py vacuum_indexes

What is nice about the process is that, for each index that has been removed from your index.yaml, it will confirm whether you want to delete it or not. So you get a second chance to make sure you are right. If you delete the wrong index, it’s going to take some time for it to rebuild, and portions of your site could be down.

Why do I need this:
I don’t think you would see a noticeable speed improvement to your data inserts, updates, or deletes by having a few less unused indexes, but it couldn’t hurt. Also you are limited to 100 indexes (as of April 2010) so this how you free them up.

appcfg.py update check failed: EOF occurred in violation of protocol

This error is related to not having the SSL module for python installed. The easy fix is to install it by running “easy_install ssl” from the command prompt. However, using Python 2.5, I received errors and it did not work.

The longer, more involved fix is to compile the SSL module for python. This was a hassle but after about an hour I finally got it working.

Here is the tutorial I used to install the ssl module:
http://beautifulisbetterthanugly.com/posts/2009/aug/19/compile-ssl-115-python-25-or-lower/

I also had to follow the troubleshooting section on this link:
http://wiki.cython.org/InstallingOnWindows
because I did not have the python library file (libpython25.a).

Steps to Add a Developer Email Address:

1. Log in to your Google App Engine Account.
2. Under Administration, click Permissions, and add the email address.
3. Log out, and check for the validation email.

More Info:
This is quite a hassle. I understand the reason is to cut back on spam but not only do you have to have a valid email account but you must create a google account for it.
In my case, I just wanted to send notifications out but I don’t monitor the sender address (but I do include a contact link in the email).
Darn spammers!

use urllib
# you have to pass in a dictionary
print urllib.urlencode({'id':'100', 'name':'john smith'})

# then you get 'id=100&name=john+smith'

Urlencoding Unicode and UTF8 Values

#this works fine
print urllib.urlencode({name:'José'})

#you have to be careful with unicode strings
# this will throw an error:
print urllib.urlencode({name:u'José'})
# UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in
# position 11: ordinal not in range

#since the default ascii conversion can't handle the 'é',
# you have to be explicit in what charset you are using
print urllib.urlencode({'name':u'José'.encode('utf8')})

Additional Info:
Python Doc for urllib module
Unicode Primer

from datetime import datetime
from datetime import timedelta

#Add 1 day
print datetime.now() + timedelta(days=1)

#Subtract 60 seconds
print datetime.now() - timedelta(seconds=60)

#Add 2 years
print datetime.now() + timedelta(days=730)

#Other Parameters you can pass in to timedelta:
# days, seconds, microseconds,
# milliseconds, minutes, hours, weeks

#Pass multiple parameters (1 day and 5 minutes)
print datetime.now() + timedelta(days=1,minutes=5)

Here is a python reference that gives more examples and advanced features:
http://docs.python.org/library/datetime.html

If you are coming from a .net or sql environment, here are the above examples in C# and SQL (Microsoft) for comparison:
C#

DateTime myTime = new DateTime();

--Add 1 day
myTime.AddDays(1);

--Subtract 60 seconds
myTime.AddSeconds(-60);

--Add 2 years
myTime.AddYears(2);

SQL

--Add 1 day
select DATEADD(day, 1, getdate())

--Subtract 60 seconds
select DATEADD(second, -60, getdate())

--Add 2 years
select DATEADD(Year, 2, getdate())

moduleName = 'myApp.models'
className = 'Blog'

#import the module by saying 'from myApp.models import Blog'
module = __import__(moduleName, {}, {}, className)

#now you can instantiate the class
obj = getattr(module, className )()

#set a property of the object using a string
setattr('Title','my first entry')

#call an existing method you already know
obj.save()

I was using Python 2.5, so it is probably safe to say the code above is at least Python 2.5+ compatible.

Some additional references:
Dive Into Python – The Power of Introspection
Python Manual – Built In Functions

myObj = None
print myObj or ''

This brought back a reference to a blurb in Dive Into Python regarding ternary operators, although at the time I didn’t think of using it this way.

In my case, it was an image tag that I included in the admin list display. In my custom field code, I did not handle the “None” case. So instead of displaying a broken link it was trying to load the None image in my admin directory. Generating the URL that was causing the issue:
/admin/myApp/myModel/None/

reload(myModule)

This assumes that you have imported the module at some point within your interpreter session.

When Is This Useful
It’s helpful if you are debugging code in a IDE like PyScripter. It’s quicker to test your programs within the IDE’s interpreter (it’s also the default). However if you have multiple files open and are making changes to them, your changes to files that have been imported already will not take affect. Unless you use the reload() command.

An Alternate Method
You can also programmatically remove the module from the cache. Then the next time your import is called, it will reload it:

if  'myModule' in sys.modules:
    del(sys.modules["myModule"])
import myModule