~bzr-pqm/bzr/bzr.dev : revision 5416.1.3

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Bazaar Developer Guide to User Interaction

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Getting Input

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=============

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Processing Command Lines

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------------------------

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bzrlib has a standard framework for parsing command lines and calling

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processing routines associated with various commands. See builtins.py

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for numerous examples.

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Standard Parameter Types

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------------------------

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There are some common requirements in the library: some parameters need to be

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unicode safe, some need byte strings, and so on. At the moment we have

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only codified one specific pattern: Parameters that need to be unicode

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should be checked via ``bzrlib.osutils.safe_unicode``. This will coerce the

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input into unicode in a consistent fashion, allowing trivial strings to be

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used for programmer convenience, but not performing unpredictably in the

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presence of different locales.

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Writing Output

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==============

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(The strategy described here is what we want to get to, but it's not

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consistently followed in the code at the moment.)

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bzrlib is intended to be a generically reusable library. It shouldn't

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write messages to stdout or stderr, because some programs that use it

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might want to display that information through a GUI or some other

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mechanism.

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We can distinguish two types of output from the library:

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1. Structured data representing the progress or result of an

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operation. For example, for a commit command this will be a list

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of the modified files and the finally committed revision number

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and id.

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These should be exposed either through the return code or by calls

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to a callback parameter.

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A special case of this is progress indicators for long-lived

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operations, where the caller should pass a ProgressBar object.

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2. Unstructured log/debug messages, mostly for the benefit of the

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developers or users trying to debug problems. This should always

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be sent through ``bzrlib.trace`` and Python ``logging``, so that

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it can be redirected by the client.

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The distinction between the two is a bit subjective, but in general if

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there is any chance that a library would want to see something as

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structured data, we should make it so.

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The policy about how output is presented in the text-mode client

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should be only in the command-line tool.

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Progress and Activity Indications

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---------------------------------

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bzrlib has a way for code to display to the user that stuff is happening

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during a long operation. There are two particular types: *activity* which

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means that IO is happening on a Transport, and *progress* which means that

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higher-level application work is occurring. Both are drawn together by

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the `ui_factory`.

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Transport objects are responsible for calling `report_transport_activity`

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when they do IO.

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Progress uses a model/view pattern: application code acts on a

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`ProgressTask` object, which notifies the UI when it needs to be

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displayed. Progress tasks form a stack. To create a new progress task on

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top of the stack, call `bzrlib.ui.ui_factory.nested_progress_bar()`, then

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call `update()` on the returned ProgressTask. It can be updated with just

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a text description, with a numeric count, or with a numeric count and

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expected total count. If an expected total count is provided the view

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can show the progress moving along towards the expected total.

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The user should call `finish` on the `ProgressTask` when the logical

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operation has finished, so it can be removed from the stack.

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Progress tasks have a complex relationship with generators: it's a very

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good place to use them, but because python2.4 does not allow ``finally``

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blocks in generators it's hard to clean them up properly. In this case

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it's probably better to have the code calling the generator allocate a

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progress task for its use and then call `finalize` when it's done, which

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will close it if it was not already closed. The generator should also

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finish the progress task when it exits, because it may otherwise be a long

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time until the finally block runs.

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Message guidelines

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------------------

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When filenames or similar variables are presented inline within a message,

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they should be enclosed in double quotes (ascii 0x22, not chiral unicode

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quotes)::

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bzr: ERROR: No such file "asdf"

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When we print just a list of filenames there should not be any quoting:

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see `bug 544297`_.

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.. _bug 544297: https://bugs.launchpad.net/bugs/544297

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https://wiki.ubuntu.com/UnitsPolicy provides a good explanation about

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which unit should be used when. Roughly speaking, IEC standard applies

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for base-2 units and SI standard applies for base-10 units:

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* for network bandwidth and disk sizes, use base-10 (Mbits/s, kB/s, GB)

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* for RAM sizes, use base-2 (GiB, TiB)

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Displaying help

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===============

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Bazaar has online help for various topics through ``bzr help COMMAND`` or

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equivalently ``bzr command -h``. We also have help on command options,

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and on other help topics. (See ``help_topics.py``.)

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As for python docstrings, the first paragraph should be a single-sentence

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synopsis of the command. These are user-visible and should be prefixed with

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``__doc__ =`` so help works under ``python -OO`` with docstrings stripped.

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The help for options should be one or more proper sentences, starting with

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a capital letter and finishing with a full stop (period).

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All help messages and documentation should have two spaces between

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sentences.

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Handling Errors and Exceptions

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==============================

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Commands should return non-zero when they encounter circumstances that

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the user should really pay attention to - which includes trivial shell

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pipelines.

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Recommended values are:

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0. OK.

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1. Conflicts in merge-like operations, or changes are present in

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diff-like operations.

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2. Unrepresentable diff changes (i.e. binary files that we cannot show

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a diff of).

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3. An error or exception has occurred.

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4. An internal error occurred (one that shows a traceback.)

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Errors are handled through Python exceptions. Exceptions should be defined

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inside bzrlib.errors, so that we can see the whole tree at a glance.

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We broadly classify errors as either being either internal or not,

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depending on whether ``internal_error`` is set or not. If we think it's our

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fault, we show a backtrace, an invitation to report the bug, and possibly

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other details. This is the default for errors that aren't specifically

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recognized as being caused by a user error. Otherwise we show a briefer

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message, unless -Derror was given.

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Many errors originate as "environmental errors" which are raised by Python

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or builtin libraries -- for example IOError. These are treated as being

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our fault, unless they're caught in a particular tight scope where we know

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that they indicate a user errors. For example if the repository format

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is not found, the user probably gave the wrong path or URL. But if one of

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the files inside the repository is not found, then it's our fault --

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either there's a bug in bzr, or something complicated has gone wrong in

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the environment that means one internal file was deleted.

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Many errors are defined in ``bzrlib/errors.py`` but it's OK for new errors

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to be added near the place where they are used.

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Exceptions are formatted for the user by conversion to a string

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(eventually calling their ``__str__`` method.) As a convenience the

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``._fmt`` member can be used as a template which will be mapped to the

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error's instance dict.

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New exception classes should be defined when callers might want to catch

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that exception specifically, or when it needs a substantially different

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format string.

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#. If it is something that a caller can recover from, a custom exception

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is reasonable.

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#. If it is a data consistency issue, using a builtin like

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``ValueError``/``TypeError`` is reasonable.

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#. If it is a programmer error (using an api incorrectly)

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``AssertionError`` is reasonable.

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#. Otherwise, use ``BzrError`` or ``InternalBzrError``.

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Exception strings should start with a capital letter and should not have a

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final fullstop. If long, they may contain newlines to break the text.