launchpadlib

• https://launchpad.net/launchpadlib

launchpadlib is an open-source Python library that lets you treat the HTTP resources published by Launchpad's web service as Python objects responding to a standard set of commands. With launchpadlib you can integrate your applications into Launchpad without knowing a lot about HTTP client programming.

This document shows how to use a Python client to read and write Launchpad's data using the launchpadlib library. It doesn't cover the HTTP requests and responses that go back and forth behind the scenes: for that, see the "hacking" document. This document also doesn't cover the full range of what's possible with Launchpad's web service: for that, see the [[http://edge.launchpad.net/+apidoc/| web service reference documentation]].

Launchpad's web service currently exposes the following major parts of Launchpad:

• People and teams
• Team memberships
• Bugs and bugtasks
• The project registry
• Hosted files, such as bug attachments and mugshots.

As new features and capabilities are added to the web service, you'll be able to access most of them without having to update your copy of launchpadlib. You _will_ have to upgrade launchpadlib to get new client-side features (like support for uploaded files). The Launchpad team will put out an announcement whenever a server-side change means you should upgrade launchpadlib.

Installation

The launchpadlib library depends on wadllib, another open-source library released by the Launchpad team. Get a copy of the wadllib source with bzr and install it.

  $ bzr branch lp:wadllib
  $ cd wadllib
  $ sudo ./setup.py install

Then do the same for launchpadlib.

  $ bzr branch lp:launchpadlib
  $ cd launchpadlib
  $ sudo ./setup.py install

Getting started

The first step towards using Launchpad's web service is to choose a cache directory. The documents you retrieve from Launchpad will be stored here, which will save you a lot of time. Run this code in a Python session, substituting an appropriate directory on your computer:

    cachedir = "/home/me/.launchpadlib/cache/"

The next step is to set up credentials for your client.

    from launchpadlib.launchpad import Launchpad, STAGING_SERVICE_ROOT
    launchpad = Launchpad.get_token_and_login('just testing', STAGING_SERVICE_ROOT, cachedir)

The first argument to Launchpad.get_token_and_login() is a string that identifies the web service client. We use this string to gauge client popularity and detect buggy or inefficient clients. Here, though, we're just testing.

The second argument is the URL to the web service. Here, we're using STAGING_SERVICE_ROOT, which is "https://api.staging.launchpad.net/beta/". These examples run against the staging data, so that we don't change real data when we're just trying to see how launchpadlib works.

Once you're ready to write a launchpadlib program that changes real data, you can replace STAGING_SERVICE_ROOT with EDGE_SERVICE_ROOT.

You'll see a message like this:

 The authorization page
 (https://staging.launchpad.net/+authorize-token?oauth_token=xxxxxxxxx)
 should be opening in your browser. After you have authorized this
 program to access Launchpad on your behalf you should come back here
 and press <Enter> to finish the authentication process.

Your web browser will open to a page at launchpad.net. You'll be asked to login to Launchpad, and grant some level of access to your new credentials. The level of access you choose will determine how much you can do through launchpadlib with these credentials. This lets your users delegate a portion of their Launchpad permissions to your program, without having to trust it completely.

Once you grant access, hit Enter within your Python session. You'll get back a Launchpad object. Now you can access the web service.

If you get a failure at this point, it's probably because you're not a member of the launchpad beta testers team. Be sure to sign up both on the main Launchpad site and on staging, or you'll have to wait for the main database to be mirrored to the staging server before your client will work.

Here's some code that retrieves a bug from Launchpad and prints its title.

    bug_one = launchpad.bugs[1]
    print bug_one.title
    # Microsoft has a majority market share

Before proceeding further, save your credentials so that you don't have to do the get_token_and_login dance again the next time you need to access the web service.

    launchpad.credentials.save(file("some-file.txt", "w"))

Let's create a new Launchpad object from saved credentials right now.

    from launchpadlib.credentials import Credentials
    credentials = Credentials()
    credentials.load(open("some-file.txt"))
    launchpad = Launchpad(credentials, STAGING_SERVICE_ROOT, cachedir)

    bug_one = launchpad.bugs[1]
    print bug_one.title
    # Microsoft has a majority market share

If you did this correctly, you should notice that the second Launchpad object takes a lot less time to create than the one you created with Launchpad.get_token_and_login(). That's the cache at work. Note that the cache isn't thread-safe. If you've got multiple threads or processes running at once, give each one its own cache directory.

Getting help

If you don't know the capabilities of one of the objects you've got, you can call dir() on it. You'll see all of its fields and all the custom methods it supports. Unfortunately, you'll also see a bunch of launchpadlib-specific junk that you don't care about. That's why we've made available these four lists:

• lp_attributes: Data fields of this object. You can read from these might be able to write to some of them.
• lp_collections: List of launchpad objects associated with this object.
• lp_entries: Other Launchpad objects associated with this one.
• lp_operations: The names of Launchpad methods you can call on the object.

   print sorted(bug_one.lp_attributes)
   # ['date_created', 'date_last_message', 'date_last_updated', ... 'tags', 'title']
   print sorted(bug_one.lp_operations)
   # ['addAttachment', 'addWatch', 'subscribe', 'unsubscribe']

If you need more detailed help, you can look the object up in the reference documentation. First, find out the type of the object.

    print repr(bug_one)
    # <bug at https://api.staging.launchpad.net/beta/bugs/1>

This is a 'bug' type object. Now you use the type of the object as an anchor into the reference documentation. To find out the capabilities of this object and what data is stored inside it, you'd visit http://people.ubuntu.com/~flacoste/launchpad-api-doc.html#bug.

As you'll see, the reference documentation still needs some work, and it's geared more towards web service hackers than launchpadlib users, but it will tell you about all of this object's attributes and all the supported operations.

• The "Default representation" section tells you about the available attributes.
• The "Custom POST methods" and "Custom GET methods" sections tell you about methods the object supports other than the default methods described below. The methods take whatever parameters are listed in "Request query parameters". (You can ignore the "ws.op" parameter because you're using launchpadlib; that's just the name of the method.)

The top-level objects

The Launchpad object has attributes corresponding to the major parts of Launchpad. Currently there are two of these attributes, launchpad.bugs, launchpad.people, and launchpad.me. You've already seen launchpad.bugs. launchpad.me is an alias for your own user account.

    me = launchpad.me
    print me.name
    # This should be your user name, e.g. 'salgado'

The launchpad.people attribute gives you access to other people who use Launchpad. This code uses launchpad.people to look up the person with the Launchpad name "salgado".

    people = launchpad.people
    salgado = people['salgado']
    print salgado.display_name
    # Guilherme Salgado

You can search for objects in other ways. Here's another way of finding "salgado".

    salgado = people.getByEmail(email="guilherme.salgado@canonical.com")
    print salgado.display_name
    # Guilherme Salgado

Some searches return more than one object.

    for person in people.find(text="salgado"):
        print person.name
    # agustin-salgado
    # ariel-salgado
    # axel-salgado
    # bruno-salgado
    # camilosalgado
    # ...

Entries

Bugs, people, projects, team memberships, and most other objects published through Launchpad's web service, all work pretty much the same way. We call all these objects "entries". Each corresponds to a single piece of data within Launchpad.

You can use the web service to discover various facts about an entry. The launchpadlib makes the facts available as attributes of the entry object.

'name' and 'display_name' are facts about people.

    print salgado.name
    # salgado

    print salgado.display_name
    # Guilherme Salgado

'private' and 'description' are facts about bugs.

    print bug_one.private
    # False

    print bug_one.description
    # Microsoft has a majority market share in the new desktop PC marketplace.
    # This is a bug, which Ubuntu is designed to fix.
    # ...

Every entry has a 'self_link' attribute. You can treat this as a permanent ID for the entry. If your program needs to keep track of Launchpad objects across multiple runs, a simple way to do it is to keep track of the self_links.

    print salgado.self_link
    # https://api.staging.launchpad.net/beta/~salgado

    bug_one.self_link
    # https://api.staging.launchpad.net/beta/bugs/1

Some of an object's attributes are links to other entries. Bugs have an attribute 'owner', but the owner of a bug is a person, with attributes of its own.

    owner = bug_one.owner
    print repr(owner)
    # <person at https://api.staging.launchpad.net/beta/~sabdfl>
    print owner.name
    # sabdfl
    print owner.display_name
    # Mark Shuttleworth

If you have permission, you can change an entry's attributes and write the data back to the server.

    me = people['my-user-name']
    me.display_name = 'A user who edits through the Launchpad web service.'
    me.lp_save()

    print people['my-user-name'].display_name
    # A user who edits through the Launchpad web service.

Having permission means not only being authorized to perform an operation on the Launchpad side, but using a launchpadlib Credentials object that authorizes the operation. If you've set up your launchpadlib Credentials for read-only access, you won't be able to change the dataset through launchpadlib.

Some entries also support special operations--see the reference documentation for details. A bugtask entry supports an operation called "transitionToAssignee". This operation takes a single argument called "assignee", which should be a Launchpad person. Here it is in action.

    task = list(bug_one.bug_tasks)[0]
    old_assignee = task.assignee
    print old_assignee
    # <team at https://api.staging.launchpad.net/beta/~compscibuntu-bugs>
    task.transitionToAssignee(assignee=me)
    print task.owner.display_name
    # A user who edits through the Launchpad web service.

Collections

When Launchpad groups similar entries together, we call it a collection. You've already seen one collection: the list of people you get back when you call launchpad.people.find.

    for person in launchpad.people.find(text="salgado"):
        print person.name

That's a collection of 'people'-type entries. You can iterate over a collection as you can any Python list.

Some of an entry's attributes are links to related collections. Bug #1 has a number of associated bug tasks, represented as a collection of 'bug task' entries.

    tasks = bug_one.bug_tasks
    print len(tasks)
    # 17
    for task in tasks:
        print task.bug_target_display_name
    # Computer Science Ubuntu
    # Ichthux
    # JAK LINUX
    # ...

The person 'salgado' understands two languages, represented here as a collection of two 'language' entries.

    for language in salgado.languages:
        print language.self_link
    # https://api.staging.launchpad.net/beta/+languages/en
    # https://api.staging.launchpad.net/beta/+languages/pt_BR

Because collections can be very large, it's usually a bad idea to iterate over them. Bugs generally have a manageable number of bug tasks, and people understand a manageable number of languages, but Launchpad tracks over 250,000 bugs. If you just iterate over a list, launchpadlib will just keep pulling down entries until it runs out, which might be forever (or, realistically, until your client is banned for making too many requests).

That's why we recommend you slice Launchpad's collections into Python lists, and operate on the lists. Here's code that prints descriptions for the 10 most recently filed bugs.

    bugs = launchpad.bugs[:10]
    for bug in bugs:
        print bug.description    

For performance reasons, we've put a couple restrictions on collection slices that don't apply to slices on regular Python lists. You can only slice from the beginning of a collection, not the end.

    launchpad.bugs[-5:]
    # *** ValueError: Collection slices must have a nonnegative start point.

And your slice needs to have a definite end point: you can't slice to the end of a collection.

    bugs[10:]
    # *** ValueError: Collection slices must have a definite, nonnegative end point.

    bugs[:-5]
    # *** ValueError: Collection slices must have a definite, nonnegative end point.

On the plus side, you can include a step number with your slice, as with a normal Python list:

    every_other_bug = launchpad.bugs[0:10:2]
    len(every_other_bug)
    # 5

Hosted files

Launchpad stores some data in the form of binary files. A good example is peoples' mugshots. With launchpadlib, you can read and write these binary files programatically.

If you have a launchpadlib reference to one of these hosted files, you can read its data by calling the open() method and treating the result as an open filehandle.

    mugshot = launchpad.me.mugshot
    mugshot_handle = mugshot.open()
    mugshot_handle.read()
    # [binary data]
    mugshot_handle.content_type
    # 'image/jpeg'

You'll get an error if the file doesn't exist: for instance, if a person doesn't have a mugshot.

    launchpad.people['has-no-mugshot'].mugshot
    # *** HTTPError: HTTP Error 404: Not Found

To create or overwrite a file, open the hosted file object for write. You'll need to provide the access mode ("w"), and the MIME type of the file you're sending to Launchpad.

    mugshot_handle = mugshot.open("w", "image/jpeg")
    mugshot_handle.write("image data goes here")
    mugshot_handle.close()

If there's something wrong--maybe you provide a file of the wrong type--you'll get an HTTPError with a status code of 400. The 'content' attribute will contain an error message.

    print http_error.content
    # This image is not exactly 192x192 pixels in size.

    print http_error.content
    # The file uploaded was not recognized as an image; please
    # check it and retry.

Persistent references to Launchpad objects

Every entry and collection has a unique ID: its URL. You can get this unique ID by calling str() on the object.

    print str(bug_one)
    # https://api.staging.launchpad.net/beta/bugs/1

If you need to keep track of Launchpad objects over time, or pass references to Launchpad objects to other programs, use these strings. If you've got one of these strings, you can turn it into the corresponding Launchpad object by calling launchpad.load().

    bug_one = launchpad.load("https://api.staging.launchpad.net/beta/bugs/1")
    print bug_one.title
    Microsoft has a majority market share

You're bookmarking the Launchpad objects and coming back to them later, just like you'd bookmark pages in your web browser.

Planned improvements

launchpadlib still has deficiencies. We track bugs in the launchpadlib bug tracker (https://bugs.launchpad.net/launchpadlib) and will be working to improve launchpadlib throughout the limited beta.