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- String representations of units

# String representations of units

## Converting units to string representations

You can control the way that `Quantity` and `UnitBase` objects are rendered as strings using the new Format String Syntax. New-style format strings use the `"{}".format()` syntax. Most of the format speficiers are simliar 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 * u.km
    >>> q
    <Quantity 10 km>
    >>> "{0}".format(q)
    '10 km'
    >>> "{0:+0.03f}".format(q)
    '+10.000 km'
    >>> "{0:20s}".format(q)
    '10 km               '

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

    >>> q = 10 * u.km
    >>> q
    <Quantity 10 km>
    >>> "{0.value:0.003f} in {0.unit:s}".format(q)
    '10.000 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 `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)
    '[ 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 `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 `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)
    '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 `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 `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. Unlike the “generic” string format, this will only accept or generate units defined in the FITS standard.
> - `"vounit"`: The proposed IVOA 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: This is the standard used by Vizier tables, as well as what is used by VOTable versions 1.2 and earlier.

`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 recommendations for unit presentation. This format is automatically used when printing a unit in the IPython notebook:

>       >>> fluxunit

>   \mathrm{\frac{erg}{s\,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')
>                       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:

    >>> u.Unit("m/s/s")  # The FITS standard only allows one '/'
    Traceback (most recent call last):
      ...
    ValueError: 'm/s/s' did not parse as unit format 'generic': Syntax
    error parsing unit string 'm/s/s'

However, the `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 `UnrecognizedUnit` instance.
> - `'silent'`: return an `UnrecognizedUnit` instance.

So, for example, one can do:

    >>> x = u.Unit("m/s/s", parse_strict="warn")
    WARNING: UnitsWarning: 'm/s/s' did not parse as unit format
    'generic': Syntax error parsing unit string 'm/s/s'
    [astropy.units.core]

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