.models.functions import Distance

p1 = Instrument.objects.get(pk=151071000).coordinates
p2 = Instrument.objects.filter(pk=151071008)

for i in p2.annotate(distance=Distance('coordinates', p1)):
    print i.distance
    print i.distance.__class__

Output:
461.10913945 m
<class 'django.contrib.gis.measure.Distance'>

My model:
class Instrument(models.Model):
   ...
    coordinates = gis_models.PointField(null=True, blank=True, dim=3)

But I have only two points so when I try to use Distance() without annotate() it returns instance of class django.contrib.gis.db.models.functions.Distance() rathen than django.contrib.gis.measure.Distance():
p1 = Instrument.objects.get(pk=151071000).coordinates
p2 = Instrument.objects.get(pk=151071008).coordinates
print Distance(p1, p2)

Output:
Distance(Value(SRID=4326;POINT Z (-76.48623600000001 44.260223 0)), GeomValue(SRID=4326;POINT Z (-76.490923 44.262658 0)))

How do I receive the same result as with using annotate()?
Second question
I have to calculate 3d distance that is taking into account depth/elevation. But when I try to do it I receive the same result as with 2d. Below I changed elevation to 200 in the first object:
p1 = Instrument.objects.get(pk=151071000)
p1.coordinates = 'SRID=4326;POINT Z (-76.48623600000001 44.260223 200)'
p2 = Instrument.objects.filter(pk=151071008)

for i in p2.annotate(distance=Distance('coordinates', p1.coordinates)):
    print i.distance

Output:
461.10913945 m


A: Let's break the problem down:


*

*In the Distance class documentation, we can read the following:

Accepts two geographic fields or expressions and returns the distance between them, as a Distance object.

So the Distance(p1, p2) returns a Distance object.
If you do:
p1 = Instrument.objects.get(pk=151071000).coordinates
p2 = Instrument.objects.get(pk=151071008).coordinates
d = Distance(m=p1.distance(p2))
print d.m

You will get the measurement in meters.
I would stick with the annotate solution, which seems more solid! (opinionated response)



*Distance calculates the 2D distance between two points. In order to get a 3D calculation, you need to create one yourself.
You can have a look at my method from this question: Calculating distance between two points using latitude longitude and altitude (elevation)
EDIT 2019: Since the initial answer I have composed a Q&A style example here: How to calculate 3D distance (including altitude) between two points in GeoDjango that uses a far better (and less calculation error-prone) distance calculation between 2 points with altitude.
In sort:
We need to calculate the 2D great-circle distance between 2 points using either the Haversine formula or the Vicenty formula and then we can combine it with the difference (delta) in altitude between the 2 points to calculate the Euclidean distance between them as follows:
dist = sqrt(great_circle((lat_1, lon_1), (lat-2, lon_2).m**2, (alt_1 - alt_2)**2)

The solution assumes that the altitude is in meters and thus converts the great_circle's result into meters as well.

Leaving this here for comment continuation purposes.
 2. Distance calculates the 2D distance between two points. In order to get a 3D calculation, you need to create one yourself. 
You can have a look at my method from this question: Calculating distance between two points using latitude longitude and altitude (elevation)

  
*
  
*Let polar_point_1 = (long_1, lat_1, alt_1) and polar_point_2 =
  (long_2, lat_2, alt_2) 
  
*Translate each point to it's Cartesian equivalent by utilizing this
  formula:
x = alt * cos(lat) * sin(long)
y = alt * sin(lat)
z = alt * cos(lat) * cos(long)

and you will have p_1 = (x_1, y_1, z_1) and p_2 = (x_2, y_2, z_2) points respectively.
  
*Finally use the Euclidean formula:
dist = sqrt((x_2-x_1)**2 + (y_2-y_1)**2 + (z_2-z_1)**2)




A: ** python 3.7, Django 2.2.10
some useful functions ive written to help me with lat/lon coordinates in applications
from django.contrib.gis.geos import Point
from django.contrib.gis.measure import Distance


def get_point(lat, lon):
    try:
        lat = float(lat)
        lon = float(lon)
        if lat and lon:
            return Point(x=float(round(lon, 6)), y=float(round(lat, 6)), srid=4326)
        else:
            return None
    except Exception:
        return None


def get_point_to_point_distance(point_one, point_two, measurement_unit="mile"):
    return Distance(**{measurement_unit: point_one.distance(point_two)})


where measurement_unit can be any of the following:
    STANDARD_UNIT = "m"
    UNITS = {
        'chain': 20.1168,
        'chain_benoit': 20.116782,
        'chain_sears': 20.1167645,
        'british_chain_benoit': 20.1167824944,
        'british_chain_sears': 20.1167651216,
        'british_chain_sears_truncated': 20.116756,
        'cm': 0.01,
        'british_ft': 0.304799471539,
        'british_yd': 0.914398414616,
        'clarke_ft': 0.3047972654,
        'clarke_link': 0.201166195164,
        'fathom': 1.8288,
        'ft': 0.3048,
        'german_m': 1.0000135965,
        'gold_coast_ft': 0.304799710181508,
        'indian_yd': 0.914398530744,
        'inch': 0.0254,
        'km': 1000.0,
        'link': 0.201168,
        'link_benoit': 0.20116782,
        'link_sears': 0.20116765,
       'm': 1.0,
       'mi': 1609.344,
       'mm': 0.001,
        'nm': 1852.0,
        'nm_uk': 1853.184,
        'rod': 5.0292,
       'sears_yd': 0.91439841,
       'survey_ft': 0.304800609601,
        'um': 0.000001,
        'yd': 0.9144,
    }

    # Unit aliases for `UNIT` terms encountered in Spatial Reference WKT.
    ALIAS = {
        'centimeter': 'cm',
        'foot': 'ft',
        'inches': 'inch',
        'kilometer': 'km',
        'kilometre': 'km',
       'meter':'m',
       'metre':'m',
       'micrometer': 'um',
       'micrometre': 'um',
       'millimeter':'mm',
       'millimetre':'mm',
       'mile':'mi',
        'yard': 'yd',
        'British chain (Benoit 1895 B)': 'british_chain_benoit',
        'British chain (Sears 1922)': 'british_chain_sears',
        'British chain (Sears 1922 truncated)': 'british_chain_sears_truncated',
        'British foot (Sears 1922)': 'british_ft',
        'British foot': 'british_ft',
        'British yard (Sears 1922)': 'british_yd',
        'British yard': 'british_yd',
        "Clarke's Foot": 'clarke_