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Python programming

Which version of Python should I target?

Currently, Launchpad requires Python 3.5.

How should I format my docstrings?

First of all, thank you for writing docstrings. They make the code much easier to follow for the people that didn’t originally write it. To answer the question, you have the following options.

  • A single short sentence.

  • A single short sentence, blank line, further explanation.

  • A single short sentence, blank line, rst-style explanation of arguments and return value.

  • A single short sentence, blank line, further explanation with rst-style explanation of arguments and return value.

You may include a short doctest in the further explanation. See the examples in the Epydoc documentation. We’re using the rst or ReStructuredText format, and not using the Javadoc or Epytext formats.

See also: PEP-8, Style Guide for Python Code and Docstring Conventions.

Note that we’re not using the full expressiveness of ReStructuredText, so [[http://python.org/peps/pep-0287.html|PEP-287]] doesn’t apply.

How should I use assertions in Launchpad code?

See XXX AssertionsInLaunchpad.

What exceptions am I allowed to catch?

See XXX ExceptionGuidelines.

What exception should I raise when something passed into an API isn’t quite right?

In short, never raise ValueError, NameError or TypeError, and avoid subclassing these exceptions as well. The full instructions are at XXX ExceptionGuidelines.

In the case of NotFoundError, if you are going to catch this specific error in some other code, and then take some corrective action or some logging action, then seriously consider creating your own subclass. This allows your code to handle exactly the situation that you expect, and not be tricked into handling NotFoundErrors raised by code several levels in.

When writing docstrings, always think whether it makes things clearer to say which exceptions will be raised due to inappropriate values being passed in.

I have a self-posting form which doesn’t display the updated values after it’s submitted. What’s wrong?

For now, all self-posting forms have to call lp.services.database.sqlbase.flush_database_updates() after processing the form.

I need to define a database class that includes a column of dbschema values. How do I do this?

Use an XXX EnumCol.

I have received a security proxied list from some API. I need to sort it, but the sort() method is forbidden. What do I do?

When you get a list from a security proxied object, that list is protected against being altered. This is important, because you don’t know who else might have a reference to that list.

When programming in Python generally, it is a good idea to make a copy of a list you get from somewhere before altering it. The security proxies just enforce this good practice.

You can make a copy of the list by using the list constructor. Here is an example, using the launchpad API.

members = getUtility(ITeamParticipationSet).getAllMembers()
members = list(members)  # Get a mutable list.

You can also use the sorted builtin to do this.

members = sorted(getUtility(ITeamParticipationSet).getAllMembers())

SQL Result Set Ordering

If the ordering of an SQL result set is not fully constrained, then your tests should not be dependent on the natural ordering of results in the sample data.

If Launchpad does not depend on the ordering of a particular result set, then that result set should be sorted within the test so that it will pass for any ordering.

As a general rule, the result sets whose order we want to test are the ones that are displayed to the user. These should use an ordering that makes sense to the user, and the tests should ensure that happens.

How do I use a PostgreSQL stored procedure/expression as the order_by in Storm?

You have to wrap it in SQL():

from storm.expr import Desc, SQL

store.find(Person).order_by(SQL("person_sort_key(displayname, name)"))

store.find(Question, SQL("fti @@ ftq('firefox')")).order_by(
    Desc(SQL("rank(fti, ftq('firefox'))"))

How do I generate SQL commands safely?

The safest way is to let Storm’s query compiler do it:

results = list(
    store.find((Person.id, Person.name), Person.display_name == 'Stuart Bishop'))

If you can’t do that, perhaps because you’re doing something too complicated for the query compiler to manage, then the next safest way is to ensure that all data is passed in as parameters, the way the DB-API intended it:

results = list(store.execute(
    "SELECT id, name FROM Person WHERE displayname = %s",
    params=('Stuart Bishop',)))

If you need to embed your data in the SQL query itself, there is only one rule you need to remember - quote your data. Failing to do this opens up the system to an SQL injection attack, one of the more common and widely known security holes and also one of the more destructive. Don’t attempt to write your own quote method - you will probably get it wrong. The only two formats you can use are %d and %s, and %s should always be escaped, no exceptions!

from lp.services.database.sqlbase import quote
results = list(store.execute(
    "SELECT id, name FROM Person WHERE displayname = %s" % quote("Stuart Bishop")))

store.execute("SELECT * FROM Person WHERE name = %s" % quote(
    "'; drop table person cascade; insert into person(name) values ('hahaloser')"))

The second command in the previous example demonstrates a simple argument that might be passed in by an attacker.

Python segfaulted. What should I do?

Python programs should never segfault, but it can happen if you trigger a bug in an extension module or the Python core itself. Since a segfault won’t give you a Python traceback, it can be a bit daunting trying to debug these sort of problems. If you run into this sort of problem, tell the list.

See XXX DebuggingWithGdb for some tips on how to narrow down where a segfault bug like this is occurring. In some cases you can even get a Python stack trace for this sort of problem.

I want an object to support __getitem__, what’s the best style?

Many Launchpad objects support __getitem__. For example, if you have a Foo, and want to support Foo()['bar'], you will implement __getitem__ for class Foo. Often, this is used along with GetitemNavigation in your browser code to ensure smooth traversal.

The __getitem__ code itself should not, however, contain the magic that fetches whatever needs to be fetched. It should instead call another method that does so, explicitly saying what it is getting. So for example:

class FooContentClass:

    def __getitem__(self, name):
        """See IFoo."""
        return self.getVersion(name)

    def getVersion(self, name):
        """See IFoo."""
        # blah blah blah
        return version

Note that generally, a __getitem__ method should give access to just one kind of thing. In the example above, it gives you access to versions with the given name. If your traversal needs to get two kinds of things, for example versions or changesets, then this is better put in the traversal code in the FooNavigation class than in the __getitem__ code of the database class.


Properties should be cheap. Using a property can make accessing fields or calculated results easier, but programmers expect properties to be usable without consideration of the internal code in the property. As such, a property that calls expensive routines such as disk resources, examining database joins or the like will violate this expectation. This can lead to hard to analyze performance problems because its not clear what is going on unless you are very familiar with the code.

Our code routinely contradicts this guideline. I remember I had issues in the past with TALES traversal when trying to use methods, and had to use properties instead. We have decorators such as @cachedproperty to help with the performance issues. Someone who knows what he talks about should update this FAQ to match reality. – DavidAllouche <<DateTime(2007-11-14T16:50:06Z)>>

Properties should always be used instead of __call__() semantics in TALES expressions. The rule is that in view classes, we don’t do this:

def foo(self):

We always do this:

def foo(self):


Questions about Storm usage.

XXX StormMigrationGuide document is highly recommended.

How to retrieve a store?

There are two ways of retrieving a storm ‘store’, before issuing a query using native syntax.

The first format retrieves the Store being used by another object. Use this method when you don’t need to make changes, but want your objects to interact nicely with objects from an unknown Store (such as a methods parameters):

from storm.store import Store

store = Store.of(object)
result = store.find(Person, Person.name == 'zeca')

You can also explicitly specify what Store you want to use. You get to choose the realm (Launchpad main, auth database) and the flavor (master or slave). If you are retrieving objects that will need to be updated, you need to use the master. If you are doing a search and we don’t mind querying data a few seconds out of date, you should use the slave.

from lp.services.webapp.interfaces import (
    IStoreSelector, MAIN_STORE, AUTH_STORE,

master_store = getUtility(IStoreSelector).get(MAIN_STORE, MASTER_FLAVOR)
master_obj = store.find(Person, Person.name == 'zeca')
slave_store = getUtility(IStoreSelector).get(MAIN_STORE, SLAVE_FLAVOR)
slave_obj = store.find(Person, Person.name == 'zeca')

If you don’t need to update, but require up-to-date data, you should use the default flavor. (e.g. most views - the object you are viewing might just have been created). This will retrieve the master unless the load balancer is sure all changes made by the current client have propagated to the replica databases.

from lp.services.webapp.interfaces import (

store = getUtility(IStoreSelector).get(MAIN_STORE, DEFAULT_FLAVOR)
result = store.find(Person, Person.name == 'zeca')

Security, authentication

See XXX LaunchpadAuthentication

How can I do get the current user in a database class?

You need to pass it in one of the parameter’s method. You shouldn’t use the ILaunchBag for this. In fact, you shouldn’t use the ILaunchBag in any database class.

The principle is that the database code must not rely on implicit state, and by that is meant state not present in the database object’s data nor in the arguments passed to the method call. Using ILaunchBag or check_permission would use this kind of implicit state.

How can I protect a method based on one of its parameter?

You can’t! Only attribute access can be protected and only the attribute name and the current user is available when that check is made.

But there is a common pattern you can use: call in that method another method on the object passed as parameter. That method can be appropriately protected using the current security infrastructure. Since this auxiliary method is part of an object-collaboration scenario, it’s usually a good idea to start these methods with the notify verb. The method is notifying the other object that a collaboration is taking place.

This will often happen with methods that needs to operate on bugs - since you usually don’t want the operation to be allowed if it’s a private bug that the user doesn’t have access to.


def linkBug(self, bug):
# If this is a private bug that the user doesn't have access, it
# will raise an Unauthorized error.

Email Notifications

When I need to send a notification for a person/team, how do I know what email address(es) I have to send the notification to?

As you know, persons and teams are meant to be interchangeable in Launchpad, but when it comes to mail notification the rules differ a bit, see XXX TeamEmail for more information. In order to mask these rules, there’s a helper function called get_contact_email_addresses() in lib/lp/services/mail/helpers.py that you should always use to get the contact address of a given person/team. Please note that this function will always return a set of email addresses, which is perfectly suitable to be passed in to simple_sendmail().

Web UI

How do I perform an action after an autogenerated edit form has been successfully submitted?

You need to write a view’s class for this form, if you don’t have one already. In your view’s class, add a method changed().

def changed(self):
    # This method is called after changes have been made.

You can use this hook to add a redirect, or to execute some logging, for example.

How do I perform an action after an autogenerated add form has been successfully submitted?

You need to write a view’s class for this form, if you don’t have one already. In your view’s class, add a method createAndAdd().

def createAndAdd(self, data):
    # This method is called with the data from the form.

You can use this hook to create new objects based on the input from the user.

How can I redirect a user to a new object just created from an autogenerated add form?

You need to write a view’s class for this form, if you don’t have one already. In your view’s class, add a method nextURL().

def nextURL(self):
    # This method returns the URL where the user should be redirected.

How do I format dates and times in page templates?

Let’s use some object’s datecreated attribute as an example.

To format a date, use tal:content="context/datecreated/fmt:date.

To format a time, use tal:content="context/datecreated/fmt:time.

To format a date and time, use tal:content="context/datecreated/fmt:datetime.

How should I generate notices like “Added Bug #1234” to the top of the page?

response.addInfoNotification('Added <b>Bug #%(bug_id)d</d>', bug_id=bug.id)

There are other notification levels (Debug, Info, Notice, Warning, Error), as per XXX BrowserNotificationMessages.

Launchpad API

How do I add a new celebrity?

See XXX AddingLaunchpadCelebrity.

Global Configuration

How do I add items to launchpad-lazr.conf?

This is done by changing the file lib/lp/services/config/schema-lazr.conf.

  • Items should be created with the following syntax:

    # Comment describing the item.
    key_name: default_value

    key_name must be a valid Python identifier.

  • Subsections should be created with the following syntax:

    key_name: ...

    section_name must be a valid Python identifier.

How are these default values changed for specific environments?

The default configuration values are overridden in /configs/<environment>/launchpad-lazr.conf. Notable environments include: - production — the production environment; launchpad.net. - staging — the staging environment; staging.launchpad.net. - development — local development environment, used with make run and make run_all; launchpad.test. - testrunner — the environment used when running tests.

The syntax for overriding configuration values is the same as the syntax for defining them.

How do I use the items listed in launchpad-lazr.conf?

Once your items are added to the launchpad-lazr.conf file, you may use them as follows:

>>> from lp.services.config import config
>>> # We grab dbname from the default section
>>> dbname = config.dbname
>>> # We grab the dbuser from the gina subsection
>>> dbuser = config.gina.dbuser

How can I temporarily override configuration variables in tests?

Use lp.testing.TestCase.pushConfig.


What kind of tests should we use?

See the XXX TestsStyleGuide for the complete answer.

Short answer is that we favour the use of doctest in lib/lp/*/doc for API documentation and XXX PageTests for use-cases documentation. We use doctests and regular python unittest to complete the coverage.

How do I run just one doctest file, e.g. lib/lp/*/doc/mytest.txt?

Use the --test argument to name it:

bin/test --test=mytest.txt

What about running just one pagetest story, e.g. lib/lp/*/stories/initial-bug-contacts?

bin/test --test=stories/initial-bug-contacts

What about running a standalone pagetest, e.g. xx-bug-index.txt?

Like this:

bin/test --test=xx-bug-index

And if I want to execute all tests except one?

bin/test '!test_to_ignore'

How can I examine my test output with PDB?

bin/test’s -D argument is everyone’s friend.

If your test raises any exceptions or failures, then the following will open a pdb shell right where the failure occurred:

bin/test -D -vvt my.test.name

Where can I get help on running tests?

Try this:

bin/test --help

How can I check test coverage?

The bin/test script has a --coverage option that will report on code coverage.

How can I run only the tests for the page test layer?

bin/test --layer=PageTestLayer

Where should I put my tests: in a test_foo.py module, or a foo.txt doctest file?

You should prefer doctests. A good rule of thumb is that test_*.py modules are best for tests that aren’t useful for documentation, but instead for increasing test coverage to obscure or hard-to-reach code paths.

It is very easy to write test code that says “check foo does bar”, without explaining why. Doctests tend to trick the author into explaining why.

However resist the temptation to insert tests into the system doctests (lib/lp/*/doc/*.txt) that reduce their usefulness as documentation. Tests which confuse rather than clarify do not belong here. To a lesser extent, this also applies to other doctests too.

How to I setup my tests namespace so I can remove unwanted import statements and other noise?

For XXX DocFileSuite tests, such as the system documentation tests, you can pass in setUp and tearDown methods. You can stuff values into the namespace using the setUp method.

from zope.component import getUtility

def setUp(test):
    test.globs['getUtility'] = getUtility

Why is my page test failing mysteriously?

This is often due to a bug in the doctest code that means that ellipses (...) don’t match blank lines (<BLANKLINE>). Inserting blank lines in the right parts of the page test should fix it.

If you are running a single test and getting odd database failures, chances are you haven’t run make schema. When running a single test the database setup step is skipped, and you need to make sure you’ve done it before.

I’m writing a pagetest in the standalone directory that changes some objects. Will my changes be visible to other pagetests in this directory?

No. The database is reset between each standalone pagetest.

Why is my page test not failing when adding extra whitespace chars inside a textarea?

Because by default, the page test ignores them so you don’t need to take care of the indentation. Sometimes, the indentation matters, e.g. inside <pre> and <textarea> tags, and if you want to test those, you will need to append to the test #doctest: -NORMALIZE_WHITESPACE, for instance:

>>> print(line)  

What does the number in square brackets after a doctest name mean?

When you get a warning or an error from a doctest, or you see it in a traceback, you’ll see the name written with a number in square brackets, like this:

<doctest gpg-encryption.txt[16]>

The number in the text above is “16”, which is the index of the “doctest example” in that file. What this means is that the first >>> in the file is example zero. The next >>> is example one, and so on. So [16] refers to the seventeenth line starting with >>> in the doctest file gpg-encrpytion.txt.

How do I find a backtrace?

Backtraces are kept on chinstrap in the /srv/gangotri-logs directory.

Should I rely on the order of os.listdir?

No. The order in which results are returned by os.listdir is not defined. Wrap your calls with sorted() if the order is important to passing the test.

How do I find the tests that cover the code I’ve changed?

Although you’ve written the tests first before changing code, a lot of code is also exercised by many other tests and it’s not immediately obvious which those might be without running the whole test suite. There is an experimental tool to help with this at XXX TestsFromChanges.

Sample Data

Sampledata should be innocuous, such that if it is viewed by people outside Canonical, it will not be embarrassing or reveal company-confidential information.

Where can I find a list of the sample users?

As an aid to testing, the sample database provides a set of sample users with differing memberships and authorization levels.

How to I make changes to the sample Launchpad database?

If you are making a schema change, see XXX PolicyAndProcess/DatabaseSchemaChangesProcess.

You shouldn’t make changes to sample data unless you are submitting your changes to db-devel. You can save the state of the development site’s database and replace the current-dev.sql so that everyone can see a working site.

  • make schema to create a clean db.

  • Do one of these next two.
    • If you have a patch, say database/schema/patch-2208-99-0.sql, apply the patch to launchpad_ftest_playground and launchpad_dev.

      # psql -f database/schema/patch-2208-99-0.sql launchpad_ftest_playground # psql -f database/schema/patch-2208-99-0.sql launchpad_dev

    • Or, if you just want to change some sample data, using your browser or make harness

      # make the minimum changes needs to correct or add data to create the expected state. # If you you can undo you mistakes using the UI and harness for many actions, but sometimes you need to start over.

  • make newsampledata
    • This creates database/sampledata/newsampledata.sql and database/sampledata/newsampledata-dev.sql.

  • Review the changes and delete any additions to the karma table or backdate them to 2005 so that the site does not change as the data ages.
    • diff database/sampledata/current.sql database/sampledata/newsampledata.sql

    • diff database/sampledata/current-dev.sql database/sampledata/newsampledata-dev.sql

  • Move the new files into the right places.
    • mv database/sampledata/newsampledata.sql database/sampledata/current.sql

    • mv database/sampledata/newsampledata-dev.sql database/sampledata/current-dev.sql

  • make schema again.

  • Review the web site to verify your changes are correct.

  • git commit

Note that as a side effect of the sampledata being automatically generated, you will often get difficult to resolve conflicts if you have modified the sample data and attempt to merge in another branch that has also modified it. To work around this, it is recommended that your sampledata changes are always maintained as list of statements in a .sql file so that you can easily reset your current.sql to the launchpad trunk and replay your changes against it (psql -d launchpad_dev -f mysampledatachanges.sql).

I’ve only made minor changes to the sample data, but the diff is huge!

This often happens when you have modified the schema. Don’t worry - if the tests pass, then the modified sample data is good.

How do I set up connection multiplexing for my SSH session?

See XXX SshConnectionMultiplexing for details on how you might do this.


See XXX ReleaseCycles for more information.

When will my code changes end up on production?

PQM is frozen sometime on, or shortly after, Friday of week 3. Authorized Release Critical changes are then processed until Wednesday afternoon when the roll-out usually happens.

Bug fixes discovered in this release on the staging server may be cherry picked into the release. Further discussion can be found on the XXX StagingServer page.

I have an urgent bug fix

Get it reviewed and committed to rocketfuel as normal. If it’s during Week 4, contact the release manager (usually kiko). Otherwise, follow the PolicyandProcess/EChangePolicy

What is the staging server?

The staging server is rebuilt daily using a copy of the production database and the HEAD of launchpad. It lets you see if your code changes actually work on the production system and performance is good. It lives at https://staging.launchpad.net/.

Changing the launchpad dependency debs

The launchpad deb dependencies are in the ~launchpad PPA: https://launchpad.net/~launchpad/+archive/ppa.

To change them, follow the instructions on the XXX LaunchpadPpa#launchpad-dependencies (Launchpad PPA page).

How do I run scripts with the Python environment of Launchpad?

Use bin/py in your trunk, or whatever other branch you want to use. You might also want to use bin/ipy, if you like IPython.


We use virtualenv and pip for our build system.

Where can I read about it?

Also see the explanation about our pip setup.

You can read more general information on https://pip.pypa.io/en/stable/.

How can I find out what we are using via pip?

For direct dependencies, look in setup.py in the top directory of Launchpad. If it is in the install_requires argument, then we are getting it from pip.

For direct and indirect dependencies, look in constraints.txt in the top directory of Launchpad. Note that this constrains more packages than we actually use.

How can I find out what we are using via sourcecode?

The canonical answer is to recursively find all symlinks in lib/ and brzplugins/.

Here’s a find incantation that will list the results:

find lib brzplugins -lname '*/sourcecode/*' -ls

You should also be able to look at utilities/sourcedeps.conf. This controls what is updated when sources are updated, for instance, via rocketfuel-get.

However, it is maintained manually, so it could be behind the times if someone makes a mistake.

You do NOT answer this question by looking in the sourcecode directory. The sourcecode directory is typically a shared resource for all your branches, so it does not necessarily reflect what the current branch is using. It is also not cleaned out by any of our scripts.

Is the ultimate goal to completely get rid of sourcecode, so that all packages come from pip?

That was our original goal. We still want to shrink it to a very small size.

How do we make custom distributions?

See doc/pip.txt, mentioned above. Also see XXX PolicyForDocumentingCustomDistributions.

Working with our open-source components (lazr.*)

See XXX HackingLazrLibraries.

Where to go for other help?

If you have encountered a problem, maybe someone else has and has already documented it on the XXX SolutionsLog. Go have a look! If have a problem and you find a solution, document it there!