194
194
may not catch every case but it's still useful sometimes.
200
In some places we have variables which point to callables that construct
201
new instances. That is to say, they can be used a lot like class objects,
202
but they shouldn't be *named* like classes:
204
> I think that things named FooBar should create instances of FooBar when
205
> called. Its plain confusing for them to do otherwise. When we have
206
> something that is going to be used as a class - that is, checked for via
207
> isinstance or other such idioms, them I would call it foo_class, so that
208
> it is clear that a callable is not sufficient. If it is only used as a
209
> factory, then yes, foo_factory is what I would use.
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To make startup time faster, we use the ``bzrlib.lazy_import`` module to
216
delay importing modules until they are actually used. ``lazy_import`` uses
217
the same syntax as regular python imports. So to import a few modules in a
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from bzrlib.lazy_import import lazy_import
221
lazy_import(globals(), """
232
import bzrlib.transport
236
At this point, all of these exist as a ``ImportReplacer`` object, ready to
237
be imported once a member is accessed.
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Modules versus Members
241
~~~~~~~~~~~~~~~~~~~~~~
243
While it is possible for ``lazy_import()`` to import members of a module
244
wehn using the ``from module import member`` syntax, it is recommended to
245
only use that syntax to load sub modules ``from module import submodule``.
246
This is because variables and classes can frequently be used without
247
needing a sub-member for example::
249
lazy_import(globals(), """
250
from module import MyClass
254
return isinstance(x, MyClass)
256
This will incorrectly fail, because ``MyClass`` is a ``ImportReplacer``
257
object, rather than the real class.
260
Passing to other variables
261
~~~~~~~~~~~~~~~~~~~~~~~~~~
263
It also is incorrect to assign ``ImportReplacer`` objects to other variables.
264
Because the replacer only knows about the original name, it is unable to
265
replace other variables. The ``ImportReplacer`` class will raise an
266
``IllegalUseOfScopeReplacer`` exception if it can figure out that this
267
happened. But it requires accessing a member more than once from the new
268
variable, so some bugs are not detected right away.
294
386
indexes into the branch's revision history.
392
The ``Transport`` layer handles access to local or remote directories.
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Each Transport object acts like a logical connection to a particular
394
directory, and it allows various operations on files within it. You can
395
*clone* a transport to get a new Transport connected to a subdirectory or
398
Transports are not used for access to the working tree. At present
399
working trees are always local and they are accessed through the regular
400
Python file io mechanisms.
405
Transports work in URLs. Take note that URLs are by definition only
406
ASCII - the decision of how to encode a Unicode string into a URL must be
407
taken at a higher level, typically in the Store. (Note that Stores also
408
escape filenames which cannot be safely stored on all filesystems, but
409
this is a different level.)
411
The main reason for this is that it's not possible to safely roundtrip a
412
URL into Unicode and then back into the same URL. The URL standard
413
gives a way to represent non-ASCII bytes in ASCII (as %-escapes), but
414
doesn't say how those bytes represent non-ASCII characters. (They're not
415
guaranteed to be UTF-8 -- that is common but doesn't happen everywhere.)
417
For example if the user enters the url ``http://example/%e0`` there's no
418
way to tell whether that character represents "latin small letter a with
419
grave" in iso-8859-1, or "latin small letter r with acute" in iso-8859-2
420
or malformed UTF-8. So we can't convert their URL to Unicode reliably.
422
Equally problematic if we're given a url-like string containing non-ascii
423
characters (such as the accented a) we can't be sure how to convert that
424
to the correct URL, because we don't know what encoding the server expects
425
for those characters. (Although this is not totally reliable we might still
426
accept these and assume they should be put into UTF-8.)
428
A similar edge case is that the url ``http://foo/sweet%2Fsour`` contains
429
one directory component whose name is "sweet/sour". The escaped slash is
430
not a directory separator. If we try to convert URLs to regular Unicode
431
paths this information will be lost.
433
This implies that Transports must natively deal with URLs; for simplicity
434
they *only* deal with URLs and conversion of other strings to URLs is done
435
elsewhere. Information they return, such as from ``list_dir``, is also in
436
the form of URL components.
439
Unicode and Encoding Support
440
============================
442
This section discusses various techniques that Bazaar uses to handle
443
characters that are outside the ASCII set.
448
When a ``Command`` object is created, it is given a member variable
449
accessible by ``self.outf``. This is a file-like object, which is bound to
450
``sys.stdout``, and should be used to write information to the screen,
451
rather than directly writing to ``sys.stdout`` or calling ``print``.
452
This file has the ability to translate Unicode objects into the correct
453
representation, based on the console encoding. Also, the class attribute
454
``encoding_type`` will effect how unprintable characters will be
455
handled. This parameter can take one of 3 values:
458
Unprintable characters will be represented with a suitable replacement
459
marker (typically '?'), and no exception will be raised. This is for
460
any command which generates text for the user to review, rather than
461
for automated processing.
462
For example: ``bzr log`` should not fail if one of the entries has text
463
that cannot be displayed.
466
Attempting to print and unprintable character will cause a UnicodeError.
467
This is for commands that are intended more as scripting support, rather
468
than plain user review.
469
For exampl: ``bzr ls`` is designed to be used with shell scripting. One
470
use would be ``bzr ls --null --unknows | xargs -0 rm``. If ``bzr``
471
printed a filename with a '?', the wrong file could be deleted. (At the
472
very least, the correct file would not be deleted). An error is used to
473
indicate that the requested action could not be performed.
476
Do not attempt to automatically convert Unicode strings. This is used
477
for commands that must handle conversion themselves.
478
For example: ``bzr diff`` needs to translate Unicode paths, but should
479
not change the exact text of the contents of the files.
482
``bzrlib.urlutils.unescape_for_display``
483
----------------------------------------
485
Because Transports work in URLs (as defined earlier), printing the raw URL
486
to the user is usually less than optimal. Characters outside the standard
487
set are printed as escapes, rather than the real character, and local
488
paths would be printed as ``file://`` urls. The function
489
``unescape_for_display`` attempts to unescape a URL, such that anything
490
that cannot be printed in the current encoding stays an escaped URL, but
491
valid characters are generated where possible.
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494
Merge/review process
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495
====================
added
removed
HACKING
