Constraints - #61

@eteq – The travis build failed on some Python 3 runs because Time is not a hashable type. Any idea why this would affect py3 but not py2? Should I make a tuple of the JDs instead?

Nice! I'm digging this.

I was talking to @astrofrog earlier about a use case that would be nice to have: given an observer, a single time, a list of targets, constraints, and (optionally) a list of exposure times (or is this an attribute of Target?), which targets can I observe? This would be super nice to have when observing -- not for scheduling, but for more free form observing :).

Amateur astronomers would probably also find this useful -- we could imagine building a way to, given a catalog name, return all objects that are visible (e.g., what Messier objects are observable right now from XX?) Thinking a bit more long term, it would also be straightforward to build a web front-end with Flask and yadda yadda...

This looks good! I like the is_observable/is_always_observable methods in particular--exactly the questions I was trying to answer in my Summer Triangle tutorial.

A wishlist of constraints that could/should be added is developing on my wiki. Contribute if there's anything missing!

@astroplanners/astroplanners-contributors: @jberlanga brought up one type of constraint that we haven't discussed in a while – target-intrinsic constraints. For example: give me all targets in this list with 11<V<14. Of course, by themselves these target-intrinsic constraints could be sorted out by the user in a simple for loop before they hit the constraints module, but it would be more natural to write them into a subclass of constraint.

One option for doing this could be to allow users to add their own kwargs to FixedTarget which gets stored on the object, and then have a generic TargetPropertyConstraint(property_keyword, [min, max]) that loops through the targets and applies the cut by the supplied keyword. That might look like this:

from astroplan import FixedTarget
from astroplan.constraints import Constraint

polaris = FixedTarget.from_name("Polaris", Bmag=2.62)
aldebaran = FixedTarget.from_name("Aldebaran", Bmag=2.4)
algol = FixedTarget.from_name("Algol", Bmag=2.07)

class TargetPropertyConstraint(Constraint):
    def __init__(self, property_keyword, min=None, max=None):
        self.min = min
        self.max = max
        self.property_keyword = property_keyword

    def _compute_constraint(self, time_range, observer, targets):
        if self.min is None:
            mask = [self.min < target[self.property_keyword]
                    for target in targets]
        elif self.max is None:
            mask = [self.max > target[self.property_keyword]
                    for target in targets]
        elif self.min is not None and self.max is not None:
            mask = [self.min < target[self.property_keyword]
                    for target in targets] & [self.max > target[self.property_keyword]
                    for target in targets]
        return mask

How would you like to see this?

@cdeil and others: there are some high level docs for the constraints module here. I'll make a commit with those docs included in the PR once @jberlanga 's high level docs get posted (#42), so I can link to the tutorial from her tutorials page.

I've left a bunch of inline comments.

Two more big-picture comments:

• Is the user-defined constraint tested via doctest? If no, do you think it's worth copying over to test_constraints.py (with a note that it is the example from the docs)?
• I think the constraints section can stand on it's own, it's nice! But I'd add a sentence at the start, saying something like "this is a start ... more high-level functionality to compute observability and do scheduling will be added to astroplan soon ... feature requests and contributions welcome!"

I forgot to say: IMO this is nice and it's been open for a while, so @bmorris3, if you want to merge it, I'd say go ahead! Or leave it open for one more day and ping the others for final review one more time, as you like.

Ok, I'm done with my review - mostly doc suggestions (although one possibly-complicated bit about user-defined constraints... we may want to discuss that more on slack)

This has already been discussed some on slack, but I think the whole constraints tutorial should be doctest-ed. Annoyingly the easiest way is probably to add >>> in front of basically everything. But if we don't want to have to figure out how to do this right for 0.1 it could also be moved to the test suite as @cdeil suggested.

Updates done, tests passed, merging! Feel free to leave more comments in issues if I jumped the gun on anything.

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