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 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. * u.km
>>> q
<Quantity 10.0 km>
>>> "{0}".format(q)
'10.0 km'
>>> "{0:+0.03f}".format(q)
'+10.000 km'
>>> "{0:20s}".format(q)
'10.0 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 10.0 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:

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  
    
    \[\mathrm{\frac{erg}{s\,cm^{2}}}\]
  • "latex_inline": Writes units out using LaTeX math syntax using the IAU Style Manual recommendations for unit presentation, using negative powers instead of fractions, as required by some journals (e.g., Apj and AJ.) Best suited for unit representation inline with text:

    >>> fluxunit.to_string('latex_inline')  
    
    \[\mathrm{erg\,s^{-1}\,cm^{-2}}\]
  • "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'))  
                    m² kg
    2.1798721×10-¹⁸ ─────
    
    

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 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("Angstroem", format="fits", parse_strict="warn")  
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 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()  
'Angstroem'
>>> x.to(u.km)  
Traceback (most recent call last):
  ...
ValueError: The unit 'Angstroem' is unrecognized.  It can not be
converted to other units.
>>> x / u.m  
Traceback (most recent call last):
  ...
ValueError: The unit 'Angstroem' is unrecognized, so all arithmetic
operations with it are invalid.