387 lines
11 KiB
Plaintext
387 lines
11 KiB
Plaintext
PEP: 3126
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Title: Remove Implicit String Concatenation
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Version: $Revision$
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Last-Modified: $Date$
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Author: Jim J. Jewett <JimJJewett@gmail.com>,
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Raymond Hettinger <python@rcn.com>
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Status: Rejected
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 29-Apr-2007
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Post-History: 29-Apr-2007, 30-Apr-2007, 07-May-2007
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Rejection Notice
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================
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This PEP is rejected. There wasn't enough support in favor, the
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feature to be removed isn't all that harmful, and there are some use
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cases that would become harder.
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Abstract
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========
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Python inherited many of its parsing rules from C. While this has
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been generally useful, there are some individual rules which are less
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useful for python, and should be eliminated.
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This PEP proposes to eliminate implicit string concatenation based
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only on the adjacency of literals.
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Instead of::
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"abc" "def" == "abcdef"
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authors will need to be explicit, and either add the strings::
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"abc" + "def" == "abcdef"
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or join them::
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"".join(["abc", "def"]) == "abcdef"
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Motivation
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==========
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One goal for Python 3000 should be to simplify the language by
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removing unnecessary features. Implicit string concatenation should
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be dropped in favor of existing techniques. This will simplify the
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grammar and simplify a user's mental picture of Python. The latter is
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important for letting the language "fit in your head". A large group
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of current users do not even know about implicit concatenation. Of
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those who do know about it, a large portion never use it or habitually
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avoid it. Of those who both know about it and use it, very few could
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state with confidence the implicit operator precedence and under what
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circumstances it is computed when the definition is compiled versus
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when it is run.
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History or Future
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-----------------
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Many Python parsing rules are intentionally compatible with C. This
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is a useful default, but Special Cases need to be justified based on
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their utility in Python. We should no longer assume that python
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programmers will also be familiar with C, so compatibility between
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languages should be treated as a tie-breaker, rather than a
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justification.
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In C, implicit concatenation is the only way to join strings without
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using a (run-time) function call to store into a variable. In Python,
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the strings can be joined (and still recognized as immutable) using
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more standard Python idioms, such ``+`` or ``"".join``.
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Problem
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-------
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Implicit String concatenation leads to tuples and lists which are
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shorter than they appear; this is turn can lead to confusing, or even
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silent, errors. For example, given a function which accepts several
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parameters, but offers a default value for some of them::
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def f(fmt, *args):
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print fmt % args
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This looks like a valid call, but isn't::
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>>> f("User %s got a message %s",
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"Bob"
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"Time for dinner")
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Traceback (most recent call last):
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File "<pyshell#8>", line 2, in <module>
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"Bob"
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File "<pyshell#3>", line 2, in f
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print fmt % args
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TypeError: not enough arguments for format string
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Calls to this function can silently do the wrong thing::
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def g(arg1, arg2=None):
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...
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# silently transformed into the possibly very different
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# g("arg1 on this linearg2 on this line", None)
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g("arg1 on this line"
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"arg2 on this line")
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To quote Jason Orendorff [#Orendorff]
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Oh. I just realized this happens a lot out here. Where I work,
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we use scons, and each SConscript has a long list of filenames::
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sourceFiles = [
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'foo.c'
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'bar.c',
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#...many lines omitted...
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'q1000x.c']
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It's a common mistake to leave off a comma, and then scons
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complains that it can't find 'foo.cbar.c'. This is pretty
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bewildering behavior even if you *are* a Python programmer,
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and not everyone here is.
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Solution
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========
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In Python, strings are objects and they support the __add__ operator,
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so it is possible to write::
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"abc" + "def"
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Because these are literals, this addition can still be optimized away
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by the compiler; the CPython compiler already does so.
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[#rcn-constantfold]_
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Other existing alternatives include multiline (triple-quoted) strings,
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and the join method::
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"""This string
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extends across
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multiple lines, but you may want to use something like
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Textwrap.dedent
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to clear out the leading spaces
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and/or reformat.
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"""
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>>> "".join(["empty", "string", "joiner"]) == "emptystringjoiner"
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True
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>>> " ".join(["space", "string", "joiner"]) == "space string joiner"
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>>> "\n".join(["multiple", "lines"]) == "multiple\nlines" == (
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"""multiple
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lines""")
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True
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Concerns
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========
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Operator Precedence
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-------------------
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Guido indicated [#rcn-constantfold]_ that this change should be
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handled by PEP, because there were a few edge cases with other string
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operators, such as the %. (Assuming that str % stays -- it may be
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eliminated in favor of :pep:`3101` -- Advanced String Formatting.
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[#elimpercent]_)
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The resolution is to use parentheses to enforce precedence -- the same
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solution that can be used today::
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# Clearest, works today, continues to work, optimization is
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# already possible.
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("abc %s def" + "ghi") % var
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# Already works today; precedence makes the optimization more
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# difficult to recognize, but does not change the semantics.
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"abc" + "def %s ghi" % var
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as opposed to::
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# Already fails because modulus (%) is higher precedence than
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# addition (+)
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("abc %s def" + "ghi" % var)
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# Works today only because adjacency is higher precedence than
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# modulus. This will no longer be available.
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"abc %s" "def" % var
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# So the 2-to-3 translator can automatically replace it with the
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# (already valid):
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("abc %s" + "def") % var
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Long Commands
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-------------
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... build up (what I consider to be) readable SQL queries [#skipSQL]_::
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rows = self.executesql("select cities.city, state, country"
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" from cities, venues, events, addresses"
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" where cities.city like %s"
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" and events.active = 1"
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" and venues.address = addresses.id"
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" and addresses.city = cities.id"
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" and events.venue = venues.id",
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(city,))
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Alternatives again include triple-quoted strings, ``+``, and ``.join``::
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query="""select cities.city, state, country
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from cities, venues, events, addresses
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where cities.city like %s
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and events.active = 1"
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and venues.address = addresses.id
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and addresses.city = cities.id
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and events.venue = venues.id"""
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query=( "select cities.city, state, country"
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+ " from cities, venues, events, addresses"
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+ " where cities.city like %s"
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+ " and events.active = 1"
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+ " and venues.address = addresses.id"
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+ " and addresses.city = cities.id"
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+ " and events.venue = venues.id"
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)
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query="\n".join(["select cities.city, state, country",
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" from cities, venues, events, addresses",
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" where cities.city like %s",
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" and events.active = 1",
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" and venues.address = addresses.id",
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" and addresses.city = cities.id",
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" and events.venue = venues.id"])
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# And yes, you *could* inline any of the above querystrings
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# the same way the original was inlined.
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rows = self.executesql(query, (city,))
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Regular Expressions
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-------------------
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Complex regular expressions are sometimes stated in terms of several
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implicitly concatenated strings with each regex component on a
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different line and followed by a comment. The plus operator can be
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inserted here but it does make the regex harder to read. One
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alternative is to use the re.VERBOSE option. Another alternative is
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to build-up the regex with a series of += lines::
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# Existing idiom which relies on implicit concatenation
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r = ('a{20}' # Twenty A's
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'b{5}' # Followed by Five B's
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)
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# Mechanical replacement
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r = ('a{20}' +# Twenty A's
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'b{5}' # Followed by Five B's
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)
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# already works today
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r = '''a{20} # Twenty A's
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b{5} # Followed by Five B's
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''' # Compiled with the re.VERBOSE flag
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# already works today
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r = 'a{20}' # Twenty A's
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r += 'b{5}' # Followed by Five B's
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Internationalization
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--------------------
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Some internationalization tools -- notably xgettext -- have already
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been special-cased for implicit concatenation, but not for Python's
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explicit concatenation. [#barryi8]_
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These tools will fail to extract the (already legal)::
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_("some string" +
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" and more of it")
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but often have a special case for::
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_("some string"
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" and more of it")
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It should also be possible to just use an overly long line (xgettext
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limits messages to 2048 characters [#xgettext2048]_, which is less
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than Python's enforced limit) or triple-quoted strings, but these
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solutions sacrifice some readability in the code::
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# Lines over a certain length are unpleasant.
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_("some string and more of it")
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# Changing whitespace is not ideal.
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_("""Some string
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and more of it""")
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_("""Some string
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and more of it""")
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_("Some string \
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and more of it")
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I do not see a good short-term resolution for this.
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Transition
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==========
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The proposed new constructs are already legal in current Python, and
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can be used immediately.
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The 2 to 3 translator can be made to mechanically change::
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"str1" "str2"
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("line1" #comment
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"line2")
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into::
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("str1" + "str2")
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("line1" +#comments
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"line2")
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If users want to use one of the other idioms, they can; as these
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idioms are all already legal in python 2, the edits can be made
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to the original source, rather than patching up the translator.
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Open Issues
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===========
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Is there a better way to support external text extraction tools, or at
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least ``xgettext`` [#gettext]_ in particular?
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References
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==========
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.. [#Orendorff] Implicit String Concatenation, Orendorff
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https://mail.python.org/pipermail/python-ideas/2007-April/000397.html
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.. [#rcn-constantfold] Reminder: Py3k PEPs due by April, Hettinger,
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van Rossum
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https://mail.python.org/pipermail/python-3000/2007-April/006563.html
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.. [#elimpercent] ps to question Re: Need help completing ABC pep,
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van Rossum
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https://mail.python.org/pipermail/python-3000/2007-April/006737.html
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.. [#skipSQL] (email Subject) PEP 30XZ: Simplified Parsing, Skip,
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https://mail.python.org/pipermail/python-3000/2007-May/007261.html
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.. [#barryi8] (email Subject) PEP 30XZ: Simplified Parsing
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https://mail.python.org/pipermail/python-3000/2007-May/007305.html
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.. [#gettext] GNU gettext manual
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http://www.gnu.org/software/gettext/
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.. [#xgettext2048] Unix man page for xgettext -- Notes section
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http://www.scit.wlv.ac.uk/cgi-bin/mansec?1+xgettext
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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