Mailman 3 January 2015 - Python-ideas

@classproperty, @abc.abstractclasspropery, etc.
by K. Richard Pixley Dec. 16, 2020

Dec. 16, 2020

There's a whole matrix of these and I'm wondering why the matrix is currently sparse rather than implementing them all. Or rather, why we can't stack them as: class foo(object): @classmethod @property def bar(cls, ...): ... Essentially the permutation are, I think: {'unadorned'|abc.abstract}{'normal'|static|class}{method|property|non-callable attribute}. concreteness implicit first arg type name comments {unadorned} {unadorned} method def foo(): exists now {unadorned} {unadorned} property @property exists now {unadorned} {unadorned} non-callable attribute x = 2 exists now {unadorned} static method @staticmethod exists now {unadorned} static property @staticproperty proposing {unadorned} static non-callable attribute {degenerate case - variables don't have arguments} unnecessary {unadorned} class method @classmethod exists now {unadorned} class property @classproperty or @classmethod;@property proposing {unadorned} class non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract {unadorned} method @abc.abstractmethod exists now abc.abstract {unadorned} property @abc.abstractproperty exists now abc.abstract {unadorned} non-callable attribute @abc.abstractattribute or @abc.abstract;@attribute proposing abc.abstract static method @abc.abstractstaticmethod exists now abc.abstract static property @abc.staticproperty proposing abc.abstract static non-callable attribute {degenerate case - variables don't have arguments} unnecessary abc.abstract class method @abc.abstractclassmethod exists now abc.abstract class property @abc.abstractclassproperty proposing abc.abstract class non-callable attribute {degenerate case - variables don't have arguments} unnecessary I think the meanings of the new ones are pretty straightforward, but in case they are not... @staticproperty - like @property only without an implicit first argument. Allows the property to be called directly from the class without requiring a throw-away instance. @classproperty - like @property, only the implicit first argument to the method is the class. Allows the property to be called directly from the class without requiring a throw-away instance. @abc.abstractattribute - a simple, non-callable variable that must be overridden in subclasses @abc.abstractstaticproperty - like @abc.abstractproperty only for @staticproperty @abc.abstractclassproperty - like @abc.abstractproperty only for @classproperty --rich

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Specify number of items to allocate for array.array() constructor
by Sven Rahmann Feb. 22, 2020

Feb. 22, 2020

At the moment, the array module of the standard library allows to create arrays of different numeric types and to initialize them from an iterable (eg, another array). What's missing is the possiblity to specify the final size of the array (number of items), especially for large arrays. I'm thinking of suffix arrays (a text indexing data structure) for large texts, eg the human genome and its reverse complement (about 6 billion characters from the alphabet ACGT). The suffix array is a long int array of the same size (8 bytes per number, so it occupies about 48 GB memory). At the moment I am extending an array in chunks of several million items at a time at a time, which is slow and not elegant. The function below also initializes each item in the array to a given value (0 by default). Is there a reason why there the array.array constructor does not allow to simply specify the number of items that should be allocated? (I do not really care about the contents.) Would this be a worthwhile addition to / modification of the array module? My suggestions is to modify array generation in such a way that you could pass an iterator (as now) as second argument, but if you pass a single integer value, it should be treated as the number of items to allocate. Here is my current workaround (which is slow): def filled_array(typecode, n, value=0, bsize=(1<<22)): """returns a new array with given typecode (eg, "l" for long int, as in the array module) with n entries, initialized to the given value (default 0) """ a = array.array(typecode, [value]*bsize) x = array.array(typecode) r = n while r >= bsize: x.extend(a) r -= bsize x.extend([value]*r) return x

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Implicit string literal concatenation considered harmful?
by Guido van Rossum March 14, 2018

March 14, 2018

I just spent a few minutes staring at a bug caused by a missing comma -- I got a mysterious argument count error because instead of foo('a', 'b') I had written foo('a' 'b'). This is a fairly common mistake, and IIRC at Google we even had a lint rule against this (there was also a Python dialect used for some specific purpose where this was explicitly forbidden). Now, with modern compiler technology, we can (and in fact do) evaluate compile-time string literal concatenation with the '+' operator, so there's really no reason to support 'a' 'b' any more. (The reason was always rather flimsy; I copied it from C but the reason why it's needed there doesn't really apply to Python, as it is mostly useful inside macros.) Would it be reasonable to start deprecating this and eventually remove it from the language? -- --Guido van Rossum (python.org/~guido)

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pep 397
by Liam Marsh Feb. 23, 2015

Feb. 23, 2015

hello, *the pep 397 says that any python script is able to choose the language version which will run it, between all the versions installed on the computer, using on windows a launcher in the "C:\windows" folder.* can the idle version be chosen like this too, or can the idle "run" command do it? (except it is already like this and I have a problem) thank you, and have a nice day/evening! (and sorry if this sounds irritating)

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PEP 485: A Function for testing approximate equality
by Chris Barker Feb. 14, 2015

Feb. 14, 2015

Hi folks, After much discussion on this list, I have written up a PEP, and it is ready for review (see below) It is also here: https://www.python.org/dev/peps/pep-0485/ That version is not quite up to date just yet, so please refer to the one enclosed in this email for now. I am managing both the PEP and a sample implementation and tests in gitHub here: https://github.com/PythonCHB/close_pep Please go there if you want to try it out, add some tests, etc. Pull requests welcomed for code, tests, or PEP editing. A quick summary of the decisions I made, and what I think are the open discussion points: The focus is on relative tolerance, but with an optional absolute tolerance, primarily to be used near zero, but it also allows it to be used as a plain absolute difference check. It is using an asymmetric test -- that is, the tolerance is computed relative to one of the arguments. It is perhaps surprising and confusing that you may get a different result if you reverse the arguments, but in this discussion it became clear that there were some use-cases where it was helpful to know exactly what the tolerance is computed relative too, and that in most use cases, if just doesn't matter. I hope this is adequately explained in the PEP. We could add a flag to set a symmetric test (I'd go with what boost calls the "strong" test), but I'd rather not -- it just confuses things, and I expect users will tend to use defaults anyway. It is designed to work mostly with floats, but also supports Integer, Decimal, Fraction, and Complex. I'm not really thrilled with that, though, it turns out to be not quite as easy to duck-type it as I had hoped. To really do it right, there would have to be more switching on type in the code, which I think is ugly to write -- contributions, opinions welcome on this. I used 1e-8 as a default relative tolerance -- arbitrarily because that's about half of the decimal digits in a python float -- suggestions welcome. Other than that, of course, we can bike-shed the names of the function and the parameters. ;-) Fire away! -Chris PEP: 485 Title: A Function for testing approximate equality Version: $Revision$ Last-Modified: $Date$ Author: Christopher Barker <Chris.Barker(a)noaa.gov> Status: Draft Type: Standards Track Content-Type: text/x-rst Created: 20-Jan-2015 Python-Version: 3.5 Post-History: Abstract ======== This PEP proposes the addition of a function to the standard library that determines whether one value is approximately equal or "close" to another value. Rationale ========= Floating point values contain limited precision, which results in their being unable to exactly represent some values, and for error to accumulate with repeated computation. As a result, it is common advice to only use an equality comparison in very specific situations. Often a inequality comparison fits the bill, but there are times (often in testing) where the programmer wants to determine whether a computed value is "close" to an expected value, without requiring them to be exactly equal. This is common enough, particularly in testing, and not always obvious how to do it, so it would be useful addition to the standard library. Existing Implementations ------------------------ The standard library includes the ``unittest.TestCase.assertAlmostEqual`` method, but it: * Is buried in the unittest.TestCase class * Is an assertion, so you can't use it as a general test (easily) * Uses number of decimal digits or an absolute delta, which are particular use cases that don't provide a general relative error. The numpy package has the ``allclose()`` and ``isclose()`` functions. The statistics package tests include an implementation, used for its unit tests. One can also find discussion and sample implementations on Stack Overflow, and other help sites. These existing implementations indicate that this is a common need, and not trivial to write oneself, making it a candidate for the standard library. Proposed Implementation ======================= NOTE: this PEP is the result of an extended discussion on the python-ideas list [1]_. The new function will have the following signature:: is_close_to(actual, expected, tol=1e-8, abs_tol=0.0) ``actual``: is the value that has been computed, measured, etc. ``expected``: is the "known" value. ``tol``: is the relative tolerance -- it is the amount of error allowed, relative to the magnitude of the expected value. ``abs_tol``: is an minimum absolute tolerance level -- useful for comparisons near zero. Modulo error checking, etc, the function will return the result of:: abs(expected-actual) <= max(tol*expected, abs_tol) Handling of non-finite numbers ------------------------------ The IEEE 754 special values of NaN, inf, and -inf will be handled according to IEEE rules. Specifically, NaN is not considered close to any other value, including NaN. inf and -inf are only considered close to themselves. Non-float types --------------- The primary use-case is expected to be floating point numbers. However, users may want to compare other numeric types similarly. In theory, it should work for any type that supports ``abs()``, comparisons, and subtraction. The code will be written and tested to accommodate these types: * ``Decimal``: for Decimal, the tolerance must be set to a Decimal type. * ``int`` * ``Fraction`` * ``complex``: for complex, ``abs(z)`` will be used for scaling and comparison. Behavior near zero ------------------ Relative comparison is problematic if either value is zero. In this case, the difference is relative to zero, and thus will always be smaller than the prescribed tolerance. To handle this case, an optional parameter, ``abs_tol`` (default 0.0) can be used to set a minimum tolerance to be used in the case of very small relative tolerance. That is, the values will be considered close if:: abs(a-b) <= abs(tol*expected) or abs(a-b) <= abs_tol If the user sets the rel_tol parameter to 0.0, then only the absolute tolerance will effect the result, so this function provides an absolute tolerance check as well. A sample implementation is available (as of Jan 22, 2015) on gitHub: https://github.com/PythonCHB/close_pep/blob/master/is_close_to.py Relative Difference =================== There are essentially two ways to think about how close two numbers are to each-other: absolute difference: simply ``abs(a-b)``, and relative difference: ``abs(a-b)/scale_factor`` [2]_. The absolute difference is trivial enough that this proposal focuses on the relative difference. Usually, the scale factor is some function of the values under consideration, for instance: 1) The absolute value of one of the input values 2) The maximum absolute value of the two 3) The minimum absolute value of the two. 4) The arithmetic mean of the two Symmetry -------- A relative comparison can be either symmetric or non-symmetric. For a symmetric algorithm: ``is_close_to(a,b)`` is always equal to ``is_close_to(b,a)`` This is an appealing consistency -- it mirrors the symmetry of equality, and is less likely to confuse people. However, often the question at hand is: "Is this computed or measured value within some tolerance of a known value?" In this case, the user wants the relative tolerance to be specifically scaled against the known value. It is also easier for the user to reason about. This proposal uses this asymmetric test to allow this specific definition of relative tolerance. Example: For the question: "Is the value of a within x% of b?", Using b to scale the percent error clearly defines the result. However, as this approach is not symmetric, a may be within 10% of b, but b is not within x% of a. Consider the case:: a = 9.0 b = 10.0 The difference between a and b is 1.0. 10% of a is 0.9, so b is not within 10% of a. But 10% of b is 1.0, so a is within 10% of b. Casual users might reasonably expect that if a is close to b, then b would also be close to a. However, in the common cases, the tolerance is quite small and often poorly defined, i.e. 1e-8, defined to only one significant figure, so the result will be very similar regardless of the order of the values. And if the user does care about the precise result, s/he can take care to always pass in the two parameters in sorted order. This proposed implementation uses asymmetric criteria with the scaling value clearly identified. Expected Uses ============= The primary expected use case is various forms of testing -- "are the results computed near what I expect as a result?" This sort of test may or may not be part of a formal unit testing suite. The function might be used also to determine if a measured value is within an expected value. Inappropriate uses ------------------ One use case for floating point comparison is testing the accuracy of a numerical algorithm. However, in this case, the numerical analyst ideally would be doing careful error propagation analysis, and should understand exactly what to test for. It is also likely that ULP (Unit in the Last Place) comparison may be called for. While this function may prove useful in such situations, It is not intended to be used in that way. Other Approaches ================ ``unittest.TestCase.assertAlmostEqual`` --------------------------------------- ( https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertAlm… ) Tests that values are approximately (or not approximately) equal by computing the difference, rounding to the given number of decimal places (default 7), and comparing to zero. This method was not selected for this proposal, as the use of decimal digits is a specific, not generally useful or flexible test. numpy ``is_close()`` -------------------- http://docs.scipy.org/doc/numpy-dev/reference/generated/numpy.isclose.html The numpy package provides the vectorized functions is_close() and all_close, for similar use cases as this proposal: ``isclose(a, b, rtol=1e-05, atol=1e-08, equal_nan=False)`` Returns a boolean array where two arrays are element-wise equal within a tolerance. The tolerance values are positive, typically very small numbers. The relative difference (rtol * abs(b)) and the absolute difference atol are added together to compare against the absolute difference between a and b In this approach, the absolute and relative tolerance are added together, rather than the ``or`` method used in this proposal. This is computationally more simple, and if relative tolerance is larger than the absolute tolerance, then the addition will have no effect. But if the absolute and relative tolerances are of similar magnitude, then the allowed difference will be about twice as large as expected. Also, if the value passed in are small compared to the absolute tolerance, then the relative tolerance will be completely swamped, perhaps unexpectedly. This is why, in this proposal, the absolute tolerance defaults to zero -- the user will be required to choose a value appropriate for the values at hand. Boost floating-point comparison ------------------------------- The Boost project ( [3]_ ) provides a floating point comparison function. Is is a symetric approach, with both "weak" (larger of the two relative errors) and "strong" (smaller of the two relative errors) options. It was decided that a method that clearly defined which value was used to scale the relative error would be more appropriate for the standard library. References ========== .. [1] Python-ideas list discussion thread (https://mail.python.org/pipermail/python-ideas/2015-January/030947.html) .. [2] Wikipedaia page on relative difference (http://en.wikipedia.org/wiki/Relative_change_and_difference) .. [3] Boost project floating-point comparison algorithms ( http://www.boost.org/doc/libs/1_35_0/libs/test/doc/components/test_tools/fl… ) Copyright ========= This document has been placed in the public domain. -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker(a)noaa.gov

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Adding a subprocess.CompletedProcess class
by Thomas Kluyver Feb. 2, 2015

Feb. 2, 2015

The subprocess module provides some nice tools to control the details of running a process, but it's still rather awkward for common use cases where you want to execute a command in one go. * There are three high-level functions: call, check_call and check_output, which all do very similar things with different return/raise behaviours * Their naming is not very clear (check_output doesn't check the output, it checks the return code and captures the output) * You can't use any of them if you want stdout and stderr separately. * You can get stdout and returncode from check_output, but it's not exactly obvious: try: stdout = check_output(...) returncode = 0 except CalledProcessError as e: stdout = e.output returncode = e.returncode I think that what these are lacking is a good way to represent a process that has already finished (as opposed to Popen, which is mostly designed to handle a running process). So I would: 1. Add a CompletedProcess class: * Attributes stdout and stderr are bytes if the relevant stream was piped, None otherwise, like the return value of Popen.communicate() * Attribute returncode is the exit status * ? Attribute cmd is the list of arguments the process ran with (not sure if this should be there or not) * Method cp.check_returncode() raises CalledProcessError if returncode != 0, inspired by requests' Response.raise_for_status() 2. Add a run() function - like call/check_call/check_output, but returns a CompletedProcess instance 3. Deprecate call/check_call/check_output, but leave them around indefinitely, since lots of existing code relies on them. Thanks, Thomas

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"atexit" equivalent for functions in contextlib
by Nikolaus Rath Feb. 1, 2015

Feb. 1, 2015

Hello, The other day, I was looking for an "atexit" equivalent at the function level. I was hoping to replace code like this: def my_function(bla, fasel): with ExitStack() as cm: ... cm.push(...) ... with something like: @handle_on_return def my_function(bla, fasel, on_return): ... on_return.push(...) ... It seems that contextlib *almost* offers a way to do this with ExitStack and ContextDecorator - but as far as I can tell the final piece is missing, because ContextDecorator does not offer a way to pass the context manager to the decorated function. However, the following decorator does the job: def handle_on_return(fn): @functools.wraps(fn) def wrapper(*a, **kw): with contextlib.ExitStack() as on_return: kw['on_return'] = on_return return fn(*a, **kw) return wrapper It's not a lot of code, but my feeling is that not anyone who might be interested in an "on_return" functionality would be able to come up with this right away. Would it make sense to add something along these lines to contextlib? Maybe instead of a new decorator, ContextDecorator could also take an additional keyword argument that tells it to pass the context manager to the decorated function? Best, -Nikolaus -- GPG encrypted emails preferred. Key id: 0xD113FCAC3C4E599F Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F »Time flies like an arrow, fruit flies like a Banana.«

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datetime: Support infinity
by Thomas Güttler Jan. 29, 2015

Jan. 29, 2015

Hi, postgreSQL supports infinity for datetime: http://www.postgresql.org/docs/current/static/datatype-datetime.html#AEN6027 {{{ infinity date, timestamp later than all other time stamps -infinity date, timestamp earlier than all other time stamps }}} Mapping this to python is not possible at the moment. See: http://initd.org/psycopg/docs/usage.html#infinite-dates-handling {{{ PostgreSQL can store the representation of an “infinite” date, timestamp, or interval. Infinite dates are not available to Python, so these objects are mapped to date.max, datetime.max, interval.max. Unfortunately the mapping cannot be bidirectional so these dates will be stored back into the database with their values, such as 9999-12-31. }}} I don't know the internals of the datetime module. I guess it is not possible to support infinity. What do you think? Thomas Güttler

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python on mobile
by Fetchinson . Jan. 28, 2015

Jan. 28, 2015

Hi all, maybe the best list would be python-dev but I don't dare waking the sleeping lions there :) So my question is this: is there a coherent mobile strategy among core dev people? I mean you guys release python for linux/macos/windows and the question is if there are any plans to do the same for a mobile platform. It doesn't have to be android or ios just anything that the core dev team chooses and sticks with. I've been developing python apps for smartphones (mostly hobby projects though) using sl4a but that seems like is dead. Now people suggest me using kivy which seems to be alive but who knows how long. There are some other projects qpython, etc, which are small and equally not so reliable at least looking from the outside. Is kivy now an officially blessed distribution? Since google was so integral to both python (through employing Guido) and android I'd think it would make sense for google to have an official python environment for android in cooperation with the python dev team. Does the PSF has an opinion on this? It would be great if there would be something for mobile phones that we could rely on not going away just as with linux/macos/windows. Or there are some issues which preclude this from the start? Cheers, Daniel -- Psss, psss, put it down! - http://www.cafepress.com/putitdown

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Fwd: PEP 485: A Function for testing approximate equality
by Chris Barker Jan. 27, 2015

Jan. 27, 2015

OOPS, sent to only Paul by mistake the first time. -Chris On Sun, Jan 25, 2015 at 11:07 PM, Paul Moore <p.f.moore(a)gmail.com> wrote: > And to many users (including me, apparently - I expected the first one > to give False), the following is "floating point arcana": > > >>> 0.1*10 == 1.0 > True > >>> sum([0.1]*10) > 0.9999999999999999 > >>> sum([0.1]*10) == 1 > False Any of the approaches on the table will do something reasonable in this case: In [4]: is_close_to.is_close_to(sum([0.1]*10), 1) testing: 0.9999999999999999 1 Out[4]: True Note that the 1e-8 d3fault I chose (which I am not committed to) is not ENTIRELY arbitrary -- it's about half the digits carried by a python float (double) -- essentially saying the values are close to about half of the precision available. And we are constrained here, the options are between 0.1 (which would be crazy, if you ask me!) and 1e-14 -- any larger an it would meaningless, and any smaller, and it would surpass the precision of a python float. PIcking a default near the middle of that range seems quite sane to me. This is quite different than setting a value for an absolute tolerance -- saying something is close to another number if the difference is less than 1e-8 would be wildly inappropriate when the smallest numbers a float can hold are on order of 1e-300! > This does seem relatively straightforward, though. Although in the > second case you glossed over the question of X% of *what* which is the > root of the "comparison to zero" question, and is precisely where the > discussion explodes into complexity that I can't follow, so maybe > that's precisely the bit of "floating point arcana" that the naive > user doesn't catch on to. arcana, maybe, not it's not a floating point issue -- X% of zero is zero absolutely precisely. But back to a point made earlier -- the idea here is to provide something better than naive use of x == y for floating point. A default for the relative tolerance provides that. There is no sane default for absolute tolerance, so I dont think we should set one. Note that the zero_tolerance idea provides a way to let is_close_to(something, 0.0) work out of the box, but I'm not sure we can come up with a sane default for that either -- in which case, what's the point? > I'm not sure what you're saying here - by "not setting defaults" do > you mean making it mandatory for the user to supply a tolerance, as I > suggested above? I for one, think making it mandatory to set one would be better than just letting the zeros get used. I've used the numpy version of this a lot for tests, , and my work flow is usually: write a test with the defaults if it pases, I'm done. If it fails, then I look and see if my code is broken, or if I can accept a larger tolerance. So I'm quite happy to have a default. > I really think that having three tolerances, once of which is nearly > > always ignored, is poor API design. The user usually knows when they are > > comparing against an expected value of zero and can set an absolute > > error tolerance. > > Agreed. > also agreed -- Nathanial -- can you live with this? Note that Nathaniel found a LOT of examples of people using assertAlmostEqual to compare to zero -- I think that's why he thinks it's important to have defaults that do something sane for that case. However, that is an absolute comparison function -- inherently different anyway -- it's also tied to "number of digits after the decimal place", so only appropriate for values near 1 anyway -- so you can define a sensible default there. not so here. > - Absolute tolerance defaults to zero (which is equivalent to > > exact equality). > yup > > - Relative tolerance defaults to something (possibly zero) to be > > determined after sufficient bike-shedding. > > Starting the bike-shedding now, -1 on zero. Having is_close default to > something that most users won't think of as behaving like their naive > expectation of "is close" (as opposed to "equals") would be confusing. 1e-8 -- but you already know that ;-) -- anything between 1e-8 and 1e-12 would be fine with me. > Just make it illegal to set both. What happens when you have both set > is another one of the things that triggers discussions that make my > head explode. sorry about the head -- but setting both is a pretty useful use-case -- you have a bunch of values you want to do a realive test on. Some of them maybe exactly zero (note -- it could be either expected or actual) -- and you know what "close to zero" means to you. so you set that for abs_tolerance. Done. And I actually didn't think anyone objected to that approach -- though maybe the exploded heads weren't able to write ;-) In fact, an absolute tolerance is so easy that I wouldn't write a function for it: abs(expected - actual) <= abs_tolerance I ended up adding it to my version to deal with the zero case. > Having said that, I don't think the name "is_close" makes the > asymmetry clear enough. Maybe "is_close_to" would work better (there's > still room for bikeshedding over which of the 2 arguments is implied > as the "expected" value in that case)? I now have "expected" and "actual", but I think those makes are too unclear -- I like "expected" -- anyone have a better idea for the other one? > > abs(actual - expected) <= relative_tolerance*expected > > > > Now if expected is zero, the condition is true if and only if > > actual==expected. > > I would call out this edge case explicitly in the documentation. It is called out, but I guess I need to make that clearer. Overall, I think it would be better to simplify the proposed function > in order to have it better suit the expectations of its intended > audience, rather than trying to dump too much functionality in it on > the grounds of making it "general". right -- that is why I've been resisting adding flags for all the various options -Chris -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker(a)noaa.gov -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/OR&R (206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception Chris.Barker(a)noaa.gov

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