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Sample implementation allowing Quantity initialisation from Columns #1540

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Sample implementation allowing Quantity initialisation from Columns #1540

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@mhvk mhvk commented Oct 8, 2013

This follows a question from @taldcroft in #732 whether one could set a Quantity with a table column. With this:

from astropy.table import Table; import astropy.units as u
t = Table([[1.,2.],[3.,4.]], names=('a','b'))
t['a'].unit = u.km
u.Quantity(t['a']), u.Quantity(t['a'], u.m), u.Quantity(t['b']), u.Quantity(t['b'], u.Mpc/u.m)

yields

(<Quantity [ 1., 2.] km>,
 <Quantity [ 1000., 2000.] m>,
 <Quantity [ 3., 4.] >,
 <Quantity [ 3., 4.] Mpc / m>)

Hence, an unset unit is taken as "treat this as a float array" rather than dimensionless unscaled.

Notes:

  1. This trial implementation goes the route of making quantity initialisation duck type; instead of looking whether one is dealing with a Quantity instance, it looks whether there is an unit attribute; the one extra part required was for Column to have a value attribute (just equal to data).
  2. Miraculously, it doesn't break any test cases!

taldcroft
taldcroft reviewed Oct 8, 2013
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astropy/table/table.py
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Make this a property like data so it is strictly read only.

@mhvk mhvk mentioned this pull request Oct 15, 2013
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@astrofrog astrofrog mentioned this pull request Nov 16, 2013
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astrofrog commented Nov 26, 2013

@mhvk - could you rebase, and add tests?

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mhvk commented Nov 26, 2013

@astrofrog - I'm not quite sure any more this is the right approach towards solving the general problem of interacting with Quantity, Time, and Coordinates. I've been working with @wkerzendorf on an alternative and think it would be best to hold off on this PR until that has been explored a bit further (especially since this PR affects Quantity and there still a few others for Quantity that have higher priority).

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taldcroft commented Nov 27, 2013

@mhvk @astrofrog - I agree this is likely to be overtaken by approaches suggested in #1833 and #1835. We should leave this PR open though. For instance there may end up being a table attribute like use_quantity which might be set to false. In this case the approach use in this PR would still be appropriate.

@astrofrog astrofrog modified the milestones: v1.0.0, v0.4.0 May 3, 2014
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eteq commented Jul 6, 2014

@mhvk - this has been overtaken by some later PR, right? In master, right now, it appears your example at the top works. So should this be closed?

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mhvk commented Jul 6, 2014

@eteq - yes, indeed Quantity now duck types right, so no need for this any more. I'll close it.

@mhvk mhvk closed this Jul 6, 2014
@mhvk mhvk deleted the quantity-input-from-column branch July 6, 2014 10:19
@embray embray removed this from the v1.0.0 milestone Jul 6, 2014
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@mhvk @astrofrog @taldcroft @eteq @embray