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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
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to the user is usually less than optimal. Characters outside the standard
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set are printed as escapes, rather than the real character, and local
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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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====================
added
removed
HACKING
