# cf.Units

class `cf.``Units`(units=None, calendar=None, formatted=False, names=False, definition=False, _ut_unit=None)

  Bases: `object`

  Store, combine and compare physical units and convert numeric values to different units.

  Units are as defined in UNIDATA’s Udunits-2 package, with a few exceptions for greater consistency with the CF conventions namely support for CF calendars and new units definitions.

  Modifications to the standard Udunits database

  Whilst a standard Udunits-2 database may be used, greater consistency with CF is achieved by using a modified database. The following units are either new to, modified from, or removed from the standard Udunits-2 database (version 2.1.24):

  Unit name | Symbol | Definition | Status
  --- | --- | --- | ---
  practical_salinity_unit | psu | 1e-3 | New unit
  level |  | 1 | New unit
  sigma_level |  | 1 | New unit
  layer |  | 1 | New unit
  decibel | dB | 1 | New unit
  bel |  | 10 dB | New unit
  sverdrup | Sv | 1e6 m3 s-1 | Added symbol
  sievert |  | J kg-1 | Removed symbol

  Plural forms of the new units’ names are allowed, such as `practical_salinity_units`.

  The modified database is in the udunits subdirectory of the etc directory found in the same location as this module.

  Accessing units

  Units may be set, retrieved and deleted via the `units` attribute. Its value is a string that can be recognized by UNIDATA’s Udunits-2 package, with the few exceptions given in the CF conventions.

      >>> u = Units('m s-1')
      >>> u
      <Cf Units: 'm s-1'>
      >>> u.units = 'days since 2004-3-1'
      >>> u
      <CF Units: days since 2004-3-1>

  Equality and equivalence of units

  There are methods for assessing whether two units are equivalent or equal. Two units are equivalent if numeric values in one unit are convertible to numeric values in the other unit (such as `kilometres` and `metres`). Two units are equal if they are equivalent and their conversion is a scale factor of 1 and an offset of 0 (such as `kilometres` and `1000 metres`). Note that equivalence and equality are based on internally stored binary representations of the units, rather than their string representations.

      >>> u = Units('m/s')
      >>> v = Units('m s-1')
      >>> w = Units('km.s-1')
      >>> x = Units('0.001 kilometer.second-1')
      >>> y = Units('gram')

      >>> u.equivalent(v), u.equals(v),  u == v
      (True, True, True)
      >>> u.equivalent(w), u.equals(w)
      (True, False)
      >>> u.equivalent(x), u.equals(x)
      (True, True)
      >>> u.equivalent(y), u.equals(y)
      (False, False)

  Time and reference time units

  Time units may be given as durations of time (time units) or as an amount of time since a reference time (reference time units):

      >>> v = Units()
      >>> v.units = 's'
      >>> v.units = 'day'
      >>> v.units = 'days since 1970-01-01'
      >>> v.units = 'seconds since 1992-10-8 15:15:42.5 -6:00'

  Note

  It is recommended that the units `year` and `month` be used with caution, as explained in the following excerpt from the CF conventions: “The Udunits package defines a year to be exactly 365.242198781 days (the interval between 2 successive passages of the sun through vernal equinox). It is not a calendar year. Udunits includes the following definitions for years: a common_year is 365 days, a leap_year is 366 days, a Julian_year is 365.25 days, and a Gregorian_year is 365.2425 days. For similar reasons the unit `month`, which is defined to be exactly year/12, should also be used with caution.”

  Calendar

  The date given in reference time units is associated with one of the calendars recognized by the CF conventions and may be set with the `calendar` attribute. However, as in the CF conventions, if the calendar is not set then, for the purposes of calculation and comparison, it defaults to the mixed Gregorian/Julian calendar as defined by Udunits:

      >>> u = Units('days since 2000-1-1')
      >>> u.calendar
      AttributeError: Can't get 'Units' attribute 'calendar'
      >>> v = Units('days since 2000-1-1')
      >>> v.calendar = 'gregorian'
      >>> v.equals(u)
      True

  Arithmetic with units

  The following operators, operations and assignments are overloaded:

  Comparison operators:

  > `==, !=`

  Binary arithmetic operations:

  > `+, -, *, /, pow(), **`

  Unary arithmetic operations:

  > `-, +`

  Augmented arithmetic assignments:

  > `+=, -=, *=, /=, **=`

  The comparison operations return a boolean and all other operations return a new units object or modify the units object in place.

      >>> u = Units('m')
      <CF Units: m>

      >>> v = u * 1000
      >>> v
      <CF Units: 1000 m>

      >>> u == v
      False
      >>> u != v
      True

      >>> u **= 2
      >>> u
      <CF Units: m2>

  It is also possible to create the logarithm of a unit corresponding to the given logarithmic base:

      >>> u = Units('seconds')
      >>> u.log(10)
      <CF Units: lg(re 1 s)>

  Modifying data for equivalent units

  Any numpy array or python numeric type may be modified for equivalent units using the `conform` static method.

      >>> Units.conform(2, Units('km'), Units('m'))
      2000.0

      >>> import numpy
      >>> a = numpy.arange(5.0)
      >>> Units.conform(a, Units('minute'), Units('second'))
      array([   0.,   60.,  120.,  180.,  240.])
      >>> a
      array([ 0.,  1.,  2.,  3.,  4.])

  If the inplace keyword is True, then a numpy array is modified in place, without any copying overheads:

      >>> Units.conform(a,
                        Units('days since 2000-12-1'),
                        Units('days since 2001-1-1'), inplace=True)
      array([-31., -30., -29., -28., -27.])
      >>> a
      array([-31., -30., -29., -28., -27.])

  Initialization

  Parameters: | units: `str` or `cf.Units`, optional Set the new units from this string. calendar: `str`, optional Set the calendar for reference time units. formatted: `bool`, optional Format the string representation of the units in a standardized manner. See the `formatted` method. names: `bool`, optional Format the string representation of the units using names instead of symbols. See the `format` method. definition: `bool`, optional Format the string representation of the units using basic units. See the `format` method. _ut_unit: `int`, optional Set the new units from this Udunits binary unit representation. This should be an integer returned by a call to `ut_parse` function of Udunits. Ignored if `units` is set.

## Attributes

`calendar` | The calendar for reference time units.
`isdimensionless` | True if the units are dimensionless, false otherwise.
`islatitude` | True if and only if the units are latitude units.
`islongitude` | True if and only if the units are longitude units.
`ispressure` | True if the units are pressure units, false otherwise.
`isreftime` | True if the units are reference time units, False otherwise.
`istime` | True if the units are time units, False otherwise.
`reftime` | The reference date-time of reference time units.
`units` | The units.

## Methods

`conform` | Conform values in one unit to equivalent values in another, compatible unit.
`copy` | Return a deep copy.
`dump` | Return a string containing a description of the units.
`equals` | Return True if and only if numeric values in one unit are convertible to numeric values in the other unit and their conversion is a scale factor of 1.
`equivalent` | Returns True if numeric values in one unit are convertible to numeric values in the other unit.
`formatted` | Formats the string stored in the `units` attribute in a standardized manner.
`log` | Return the logarithmic unit corresponding to the given logarithmic base.

## Static methods

`conform` | Conform values in one unit to equivalent values in another, compatible unit.