1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
|
Combining and Defining Units
****************************
Basic example
=============
.. EXAMPLE START: Combining Units and Quantities
Units and quantities can be combined together using the regular Python
numeric operators::
>>> from astropy import units as u
>>> fluxunit = u.erg / (u.cm ** 2 * u.s)
>>> fluxunit
Unit("erg / (s cm2)")
>>> 52.0 * fluxunit # doctest: +FLOAT_CMP
<Quantity 52. erg / (s cm2)>
>>> 52.0 * fluxunit / u.s # doctest: +FLOAT_CMP
<Quantity 52. erg / (cm2 s2)>
.. EXAMPLE END
Fractional powers
=================
.. EXAMPLE START: Using Fractional Powers with Units
Units support fractional powers, which retain their precision through
complex operations. To do this, it is recommended to use
:class:`fractions.Fraction` objects::
>>> from fractions import Fraction
>>> Franklin = u.g ** Fraction(1, 2) * u.cm ** Fraction(3, 2) * u.s ** -1
.. note::
Floating-point powers that are effectively the same as fractions
with a denominator less than 10 are implicitly converted to
`~fractions.Fraction` objects under the hood. Therefore, the
following are equivalent::
>>> x = u.m ** Fraction(1, 3)
>>> x.powers
[Fraction(1, 3)]
>>> x = u.m ** (1. / 3.)
>>> x.powers
[Fraction(1, 3)]
.. EXAMPLE END
Defining units
==============
.. EXAMPLE START: Defining New Units
Users are free to define new units, either fundamental or compound,
using the :func:`~astropy.units.def_unit` function::
>>> bakers_fortnight = u.def_unit('bakers_fortnight', 13 * u.day)
The addition of a string gives the new unit a name that will show up
when the unit is printed::
>>> 10. * bakers_fortnight # doctest: +FLOAT_CMP
<Quantity 10. bakers_fortnight>
Creating a new fundamental unit is also possible::
>>> titter = u.def_unit('titter')
>>> chuckle = u.def_unit('chuckle', 5 * titter)
>>> laugh = u.def_unit('laugh', 4 * chuckle)
>>> guffaw = u.def_unit('guffaw', 3 * laugh)
>>> rofl = u.def_unit('rofl', 4 * guffaw)
>>> death_by_laughing = u.def_unit('death_by_laughing', 10 * rofl)
>>> (1. * rofl).to(titter) # doctest: +FLOAT_CMP
<Quantity 240. titter>
Users can see the definition of a unit and its :ref:`decomposition
<decomposing>` via::
>>> rofl.represents
Unit("4 guffaw")
>>> rofl.decompose()
Unit("240 titter")
By default, custom units are not searched by methods such as
:meth:`~astropy.units.core.UnitBase.find_equivalent_units`. However, they
can be enabled by calling :func:`~astropy.units.add_enabled_units`::
>>> kmph = u.def_unit('kmph', u.km / u.h)
>>> (u.m / u.s).find_equivalent_units()
There are no equivalent units
>>> u.add_enabled_units([kmph])
<astropy.units.core._UnitContext object at ...>
>>> (u.m / u.s).find_equivalent_units()
Primary name | Unit definition | Aliases
[
kmph | 0.277778 m / s | ,
]
If new units are defined with prefixes enabled, the prefixed units must be
explicitly enabled as well, e.g., by using the ``namespace`` argument::
>>> new_units = dict()
>>> foo = u.def_unit(['Fo', 'foo'], prefixes=True, namespace=new_units)
>>> u.add_enabled_units(new_units)
<astropy.units.core._UnitContext object at ...>
Now, the prefixed units can be parsed etc::
>>> print(u.Unit("megafoo").find_equivalent_units())
Primary name | Unit definition | Aliases
[
Fo | irreducible | foo ,
]
>>> print(u.Unit("megafoo").to(u.Unit("kFo")))
1000.0
.. EXAMPLE END
|
