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indexes into the branch's revision history.
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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
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directory, and it allows various operations on files within it. You can
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*clone* a transport to get a new Transport connected to a subdirectory or
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Transports are not used for access to the working tree. At present
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working trees are always local and they are accessed through the regular
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Python file io mechanisms.
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Transports work in URLs. Take note that URLs are by definition only
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ASCII - the decision of how to encode a Unicode string into a URL must be
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taken at a higher level, typically in the Store. (Note that Stores also
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escape filenames which cannot be safely stored on all filesystems, but
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this is a different level.)
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The main reason for this is that it's not possible to safely roundtrip a
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URL into Unicode and then back into the same URL. The URL standard
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gives a way to represent non-ASCII bytes in ASCII (as %-escapes), but
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doesn't say how those bytes represent non-ASCII characters. (They're not
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guaranteed to be UTF-8 -- that is common but doesn't happen everywhere.)
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For example if the user enters the url ``http://example/%e0`` there's no
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way to tell whether that character represents "latin small letter a with
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grave" in iso-8859-1, or "latin small letter r with acute" in iso-8859-2
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or malformed UTF-8. So we can't convert their URL to Unicode reliably.
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Equally problematic if we're given a url-like string containing non-ascii
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characters (such as the accented a) we can't be sure how to convert that
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to the correct URL, because we don't know what encoding the server expects
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for those characters. (Although this is not totally reliable we might still
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accept these and assume they should be put into UTF-8.)
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A similar edge case is that the url ``http://foo/sweet%2Fsour`` contains
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one directory component whose name is "sweet/sour". The escaped slash is
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not a directory separator. If we try to convert URLs to regular Unicode
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paths this information will be lost.
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This implies that Transports must natively deal with URLs; for simplicity
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they *only* deal with URLs and conversion of other strings to URLs is done
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elsewhere. Information they return, such as from ``list_dir``, is also in
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the form of URL components.
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Unicode and Encoding Support
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============================
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This section discusses various techniques that Bazaar uses to handle
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characters that are outside the ASCII set.
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When a ``Command`` object is created, it is given a member variable
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accessible by ``self.outf``. This is a file-like object, which is bound to
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``sys.stdout``, and should be used to write information to the screen,
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rather than directly writing to ``sys.stdout`` or calling ``print``.
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This file has the ability to translate Unicode objects into the correct
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representation, based on the console encoding. Also, the class attribute
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``encoding_type`` will effect how unprintable characters will be
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handled. This parameter can take one of 3 values:
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Unprintable characters will be represented with a suitable replacement
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marker (typically '?'), and no exception will be raised. This is for
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any command which generates text for the user to review, rather than
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for automated processing.
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For example: ``bzr log`` should not fail if one of the entries has text
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that cannot be displayed.
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Attempting to print and unprintable character will cause a UnicodeError.
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This is for commands that are intended more as scripting support, rather
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than plain user review.
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For exampl: ``bzr ls`` is designed to be used with shell scripting. One
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use would be ``bzr ls --null --unknows | xargs -0 rm``. If ``bzr``
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printed a filename with a '?', the wrong file could be deleted. (At the
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very least, the correct file would not be deleted). An error is used to
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indicate that the requested action could not be performed.
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Do not attempt to automatically convert Unicode strings. This is used
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for commands that must handle conversion themselves.
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For example: ``bzr diff`` needs to translate Unicode paths, but should
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not change the exact text of the contents of the files.
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``bzrlib.urlutils.unescape_for_display``
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----------------------------------------
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Because Transports work in URLs (as defined earlier), printing the raw URL
427
to the user is usually less than optimal. Characters outside the standard
428
set are printed as escapes, rather than the real character, and local
429
paths would be printed as ``file://`` urls. The function
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``unescape_for_display`` attempts to unescape a URL, such that anything
431
that cannot be printed in the current encoding stays an escaped URL, but
432
valid characters are generated where possible.
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Merge/review process
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436
====================
added
removed
HACKING
