testbed/astropy__astropy/docs/units/format.rst

.. _astropy-units-format:

String representations of units
*******************************

.. |quantity| replace:: :class:`~astropy.units.Quantity`

.. |unit| replace:: :class:`~astropy.units.UnitBase`

Converting units to string representations
==========================================

You can control the way that |quantity| and |unit| objects are rendered as
strings using the new `Format String Syntax
<https://docs.python.org/3/library/string.html#format-string-syntax>`__.
New-style format strings use the ``"{}".format()`` syntax.  Most of
the format specifiers are similar to the old ``%``-style formatting,
so things like ``0.003f`` still work, just in the form
``"{:0.003f}".format()``.

For quantities, format specifiers, like ``0.003f`` will be applied to
the |quantity| value, without affecting the unit. Specifiers like
``20s``, which would only apply to a string, will be applied to the
whole string representation of the |quantity|. This means you can do::

    >>> from astropy import units as u
    >>> import numpy as np
    >>> q = 10.5 * u.km
    >>> q
    <Quantity  10.5 km>
    >>> "{0}".format(q)
    '10.5 km'
    >>> "{0:+0.03f}".format(q)
    '+10.500 km'
    >>> "{0:20s}".format(q)
    '10.5 km             '

To format both the value and the unit separately, you can access the |quantity|
class attributes within new-style format strings::

    >>> q = 10.5 * u.km
    >>> q
    <Quantity  10.5 km>
    >>> "{0.value:0.003f} in {0.unit:s}".format(q)
    '10.500 in km'

Because Numpy arrays don't accept most format specifiers, using specifiers like
``0.003f`` will not work when applied to a Numpy array or non-scalar |quantity|.
Use :func:`numpy.array_str` instead. For example::

    >>> q = np.linspace(0,1,10) * u.m
    >>> "{0} {1}".format(np.array_str(q.value, precision=1), q.unit)  # doctest: +FLOAT_CMP
    '[0.  0.1 0.2 0.3 0.4 0.6 0.7 0.8 0.9 1. ] m'

Examine the numpy documentation for more examples with :func:`numpy.array_str`.

Units, or the unit part of a quantity, can also be formatted in a
number of different styles.  By default, the string format used is
referred to as the "generic" format, which is based on syntax of the
FITS standard's format for representing units, but supports all of the
units defined within the `astropy.units` framework, including
user-defined units.  The format specifier (and
`~astropy.units.core.UnitBase.to_string`) functions also take an
optional parameter to select a different format, including
``"latex"``, ``"unicode"``, ``"cds"``, and others, defined below.

    >>> "{0.value:0.003f} in {0.unit:latex}".format(q)  # doctest: +SKIP
    '10.000 in $\\mathrm{km}$'
    >>> fluxunit = u.erg / (u.cm ** 2 * u.s)
    >>> "{0}".format(fluxunit)
    u'erg / (cm2 s)'
    >>> print("{0:console}".format(fluxunit))
     erg
    ------
    s cm^2
    >>> "{0:latex}".format(fluxunit)
    u'$\\mathrm{\\frac{erg}{s\\,cm^{2}}}$'
    >>> "{0:>20s}".format(fluxunit)
    u'       erg / (cm2 s)'

The `~astropy.units.core.UnitBase.to_string` method is an alternative way to
format units as strings, and is the underlying implementation of the
`format`-style usage::

    >>> fluxunit = u.erg / (u.cm ** 2 * u.s)
    >>> fluxunit.to_string('latex')
    u'$\\mathrm{\\frac{erg}{s\\,cm^{2}}}$'

Creating units from strings
===========================

Units can also be created from strings in a number of different
formats using the `~astropy.units.Unit` class::

  >>> from astropy import units as u
  >>> u.Unit("m")
  Unit("m")
  >>> u.Unit("erg / (s cm2)")
  Unit("erg / (cm2 s)")
  >>> u.Unit("erg.s-1.cm-2", format="cds")
  Unit("erg / (cm2 s)")

.. note::

   Creating units from strings requires the use of a specialized
   parser for the unit language, which results in a performance
   penalty if units are created using strings.  Thus, it is much
   faster to use unit objects directly (e.g., ``unit = u.degree /
   u.minute``) instead of via string parsing (``unit =
   u.Unit('deg/min')``).  This parser is very useful, however, if your
   unit definitions are coming from a file format such as FITS or
   VOTable.

Built-in formats
================

`astropy.units` includes support for parsing and writing the following
formats:

  - ``"fits"``: This is the format defined in the Units section of the
    `FITS Standard <https://fits.gsfc.nasa.gov/fits_standard.html>`__.
    Unlike the "generic" string format, this will only accept or
    generate units defined in the FITS standard.

  - ``"vounit"``: The `Units in the VO 1.0
    <http://www.ivoa.net/Documents/VOUnits/>`__ standard for
    representing units in the VO.  Again, based on the FITS syntax,
    but the collection of supported units is different.

  - ``"cds"``: `Standards for astronomical catalogues from Centre de
    Données astronomiques de Strasbourg
    <http://cds.u-strasbg.fr/doc/catstd-3.2.htx>`__: This is the
    standard used by `Vizier tables <http://vizier.u-strasbg.fr/>`__,
    as well as what is used by VOTable versions 1.2 and earlier.

  - ``"ogip"``: A standard for storing units as recommended by the
    `Office of Guest Investigator Programs (OGIP)
    <https://heasarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/general/ogip_93_001/>`_.

`astropy.units` is also able to write, but not read, units in the
following formats:

  - ``"latex"``: Writes units out using LaTeX math syntax using the
    `IAU Style Manual
    <https://www.iau.org/static/publications/stylemanual1989.pdf>`__
    recommendations for unit presentation.  This format is
    automatically used when printing a unit in the IPython notebook::

      >>> fluxunit  # doctest: +SKIP

    .. math::

       \mathrm{\frac{erg}{s\,cm^{2}}}

  - ``"latex_inline"``: Writes units out using LaTeX math syntax using the
    `IAU Style Manual
    <https://www.iau.org/static/publications/stylemanual1989.pdf>`__
    recommendations for unit presentation, using negative powers instead of
    fractions, as required by some journals (e.g., `Apj and AJ
    <https://journals.aas.org/manuscript-preparation/>`_.)
    Best suited for unit representation inline with text::

      >>> fluxunit.to_string('latex_inline')  # doctest: +SKIP

    .. math::

       \mathrm{erg\,s^{-1}\,cm^{-2}}

  - ``"console"``: Writes a multi-line representation of the unit
    useful for display in a text console::

      >>> print(fluxunit.to_string('console'))
       erg
      ------
      s cm^2

  - ``"unicode"``: Same as ``"console"``, except uses Unicode
    characters::

      >>> print(u.Ry.decompose().to_string('unicode'))  # doctest: +SKIP
                      m² kg
      2.1798721×10-¹⁸ ─────
                       s²

Unrecognized Units
==================

Since many files in found in the wild have unit strings that do not
correspond to any given standard, `astropy.units` also has a
consistent way to store and pass around unit strings that did not
parse.

Normally, passing an unrecognized unit string raises an exception::

  >>> # The FITS standard uses 'angstrom', not 'Angstroem'
  >>> u.Unit("Angstroem", format="fits")
  Traceback (most recent call last):
    ...
  ValueError: 'Angstroem' did not parse as fits unit: At col 0, Unit
  'Angstroem' not supported by the FITS standard. Did you mean
  Angstrom or angstrom?

However, the `~astropy.units.Unit` constructor has the keyword
argument ``parse_strict`` that can take one of three values to control
this behavior:

  - ``'raise'``: (default) raise a ValueError exception.

  - ``'warn'``: emit a Warning, and return an
    `~astropy.units.UnrecognizedUnit` instance.

  - ``'silent'``: return an `~astropy.units.UnrecognizedUnit`
    instance.

So, for example, one can do::

   >>> x = u.Unit("Angstroem", format="fits", parse_strict="warn")  # doctest: +SKIP
   WARNING: UnitsWarning: 'Angstroem' did not parse as unit format
   'fits': At col 0, 'Angstroem' is not a valid unit in string
   'Angstroem' [astropy.units.core]

This `~astropy.units.UnrecognizedUnit` object remembers the
original string it was created with, so it can be written back out,
but any meaningful operations on it, such as converting to another
unit or composing with other units, will fail.

   >>> x.to_string()  # doctest: +SKIP
   'Angstroem'
   >>> x.to(u.km)  # doctest: +SKIP
   Traceback (most recent call last):
     ...
   ValueError: The unit 'Angstroem' is unrecognized.  It can not be
   converted to other units.
   >>> x / u.m  # doctest: +SKIP
   Traceback (most recent call last):
     ...
   ValueError: The unit 'Angstroem' is unrecognized, so all arithmetic
   operations with it are invalid.