Specifically, I have a model that has a field like this
pub_date = models.DateField("date published")
I want to be able to easily grab the object with the most recent pub_date. What is the easiest/best way to do this?
Would something like the following do what I want?
Edition.objects.order_by('pub_date')[:-1]
obj = Edition.objects.latest('pub_date')
You can also simplify things by putting get_latest_by in the model's Meta, then you'll be able to do
obj = Edition.objects.latest()
See the docs for more info. You'll probably also want to set the ordering Meta option.
Harley's answer is the way to go for the case where you want the latest according to some ordering criteria for particular Models, as you do, but the general solution is to reverse the ordering and retrieve the first item:
Edition.objects.order_by('-pub_date')[0]
Note:
Normal python lists accept negative indexes, which signify an offset from the end of the list, rather than the beginning like a positive number. However, QuerySet objects will raise AssertionError: Negative indexing is not supported. if you use a negative index, which is why you have to do what insin said: reverse the ordering and grab the 0th element.
Be careful of using
Edition.objects.order_by('-pub_date')[0]
as you might be indexing an empty QuerySet. I'm not sure what the correct Pythonic approach is, but the simplest would be to wrap it in an if/else or try/catch:
try:
last = Edition.objects.order_by('-pub_date')[0]
except IndexError:
# Didn't find anything...
But, as #Harley said, when you're ordering by date, latest() is the djangonic way to do it.
This has already been answered, but for more reference, this is what Django Book has to say about Slicing Data on QuerySets:
Note that negative slicing is not supported:
>>> Publisher.objects.order_by('name')[-1]
Traceback (most recent call last):
...
AssertionError: Negative indexing is not supported.
This is easy to get around, though. Just change the order_by()
statement, like this:
>>> Publisher.objects.order_by('-name')[0]
Refer the link for more such details. Hope that helps!
Related
TLDR: I have 2 arrays indices = numpy.arange(9) and another that contains some of the numbers in indices (maybe none at all, maybe it'll contain [2,4,7]). The output I'd like for this example is [0,1,3,5,6,8]. What method can be used to achieve this?
Edit: I found a method which works somewhat: casting both arrays to a set then taking the difference of the two does give the correct result, but as a set, even if I pass this result to a numpy.array(). I'll update this if I find a solution for that.
Edit2: Casting the result of the subtraction to a list, then casting passing that to a numpy.array() resolved my issue.
I guess I posted this question a little prematurely, given that I found the solution for it myself, but maybe this'll be useful to somebody in future!
You can make use of boolean masking:-
indices[~numpy.isin(indices,[2,4,7])]
Explanation:-
we are using numpy.isin() method to find out the values exists or not in incides array and then using ~ so that this gives opposite result and finally we are passing this boolean mask to indices
Sometimes it is useful to assign a 'tag', which can be a simple string, to a matplotlib artist in order to later find it easily.
If we imagine a scenario where say plt.Line2D had a property called tag which can be retrieved using plt.Line2D.get_tag() it would be very easy to find it later in a complicated plot.
The only thing I can find that looks remotely similar is the group ID: for example line.set_gid() and line.get_gid(). I haven't found any good documentation on this. The only reference is this. Is this meant for such use as described above? Is it reserved for other operations in matplotlib?
This would be very useful for grouping different artists and then performing operations on them later, for example:
for line in ax.get_lines():
if line.get_tag() == 'group A'
line.set_color('red')
# or whatever other operation
Does such a thing exist?
You can use the gid for such purposes. The only side-effect is that those names will appear in a saved svg file as the gid tag.
Alternatively you can assign any attribute to a python object.
line, = plt.plot(...)
line.myid = "group A"
just make sure not to use any existing attribute in such case.
I see that performing a .first() or .last() query on an ActiveRecord query returns an array instead of an ActiveRecord::AssociationRelation. This prevents me from appending an .order() clause as that is not an array method.
Stepping back to the goal for a minute, I want to pull the latest five ("live", ie. active) comments related to a post and this is the code that fails:
#post.comments.where(status: "live").last(5).order(id: :desc)
The error is
undefined method `order' for #<Array:0x00000011b096c0>
If I remove the .last() clause, or the .order() clause I get a valid result but obviously not the result set that I am angling for.
What's the best way to get the last five live comments in reverse chronological order? More importantly, perhaps, where is the best documentation / tutorial on this syntax? I find the official guide too brief.
Talking about documentation, I could recommend Rails API. It has a lot of detailed descriptions for each interface method.
What about your question, you could use limit method instead of first or last:
#post.comments.where(status: "live").limit(5).order(id: :desc)
It does not affect the type of result (it stays ActiveRecord::AssociationRelation). Hence, if no result is found, this relation with to_a method will be converted to an empty array. This is different from the behavior of first and last methods which return nil in case of no result.
How about something like:
#post.comments.where(status: 'live').order(created_at: :desc).take(5)
I just plugged in created_at. Maybe you'll want updated_at instead.
Currently I have two data frames. I am trying to get a fuzzy match of client names using fuzzywuzzy's process.extractOne function. When I have run the following script on sample data I get good results and no error, but when I run the following on my current data frames I get both an Attribute and Type error. I am not able to provide the data for security reasons, but if anyone can figure out why I am getting errors based on the script provided I would be much obliged.
names2 = list(dftr3['Common Name'])
names3 = dict(zip(names2,names2))
def get_fuzz_match(row):
match = process.extractOne(row['CLIENT_NAME'],choices = n3.keys(),score_cutoff = 80)
if match:
return n3[match[0]]
return np.nan
dfmi4['Match Name'] = dfmi4.apply(get_fuzz_match, axis=1)
I know not having some examples makes this more difficult to troubleshoot, so I will answer any question and edit the post to help this process along. The specific errors are:
1.AttributeError: 'dict_keys' object has no attribute 'items'
2.TypeError: expected string or buffer
The AttributeError is straightforward and to be expected, I think. Fuzzywuzzy's process.extract function, which does most of the actual work in process.extractOne, uses a try:... except: clause to determine whether to process the choices parameter as dict-like or list-like. I think you are seeing the exception because the TypeError is raised during the except: clause.
The TypeError is trickier to pin down, but I suspect it occurs somewhere in the StringProcessor class, used in the processor module, again called by extract, which uses several string methods and doesn't catch exceptions. So it seems likely that your apply call is passing something that is not a string. Is it possible that you have any empty cells?
I have a model:
class List:
data = ...
previous = models.ForeignKey('List', related_name='r1')
obj = models.ForeignKey('Obj', related_name='nodes')
This is one direction list containing reference to some obj of Obj class. I can reverse relation and get some list's all elements refering to obj by:
obj.nodes
But how Can I get the very last node? Without using raw sql, genering as little SQL queries by django as can.
obj.nodes is a RelatedManager, not a list. As with any manager, you can get the last queried element by
obj.nodes.all().reverse()[0]
This makes sense anyway only if there is any default order defined on the Node's Meta class, because otherwise the semantic of 'reverse' don't make any sense. If you don't have any specified order, set it explicitly:
obj.nodes.order_by('-pk')[0]
len(obj.nodes)-1
should give you the index of the last element (counting from 0) of your list
so something like
obj.nodes[len(obj.nodes)-1]
should give the last element of the list
i'm not sure it's good for your case, just give it a try :)
I see this question is quite old, but in newer versions of Django there are first() and last() methods on querysets now.
Well, you just can use [-1] index and it will return last element from the list. Maybe this question are close to yours:
Getting the last element of a list in Python
for further reading, Django does not support negative indexing and using something like
obj.nodes.all()[-1]
will raise an error.
in newer versions of Django you can use last() function on queryset to get the last item of your list.
obj.nodes.last()
another approach is to use len() function to get the index of last item of a list
obj.nodes[len(obj.nodes)-1]