String Representations of Units and Quantities

Converting to Strings

You can control the way that Quantity and UnitBase objects are rendered as strings using the Python Format String Syntax (demonstrated below using f-strings).

For a Quantity, format specifiers that are names of Built-In Formats are applied to the Quantity unit, and if possible also to the value. Format specifiers for numerical values, like .3f, will be applied to the Quantity value, without affecting the unit. Finally, specifiers like ^20s, which would apply to a string, will be applied to the string representation of the Quantity as a whole. Format specifiers that apply to the unit part of a Quantity are also applicable to a UnitBase instance.

Examples

To render Quantity or UnitBase objects as strings:

>>> from astropy import units as u
>>> q = 10.5 * u.km
>>> q
<Quantity  10.5 km>
>>> f"{q}"
'10.5 km'
>>> f"{q:latex}"
'$10.5 \\; \\mathrm{km}$'
>>> f"{q:+.3f}"
'+10.500 km'
>>> f"{q:^20}"
'        10.5         km'
>>> f"{q:^20s}"
'     10.5 km        '

To format both the value and the unit separately, you can access the Quantity attributes within format strings:

>>> q = 10.5 * u.km
>>> q
<Quantity  10.5 km>
>>> f"{q.value:.3f} in {q.unit}"
'10.500 in km'

This might not work well with LaTeX strings, in which case it would be better to use the Quantity.to_string() method:

>>> q = 1.2478e12 * u.pc/u.Myr
>>> f"{q:latex}"  # Might not have the number of digits we would like
'$1.2478 \\times 10^{12} \\; \\mathrm{\\frac{pc}{Myr}}$'
>>> f"{q.value:.3e} {q.unit:latex}"  # The value is not in LaTeX
'1.248e+12 $\\mathrm{\\frac{pc}{Myr}}$'
>>> q.to_string(format="latex", precision=4)  # Right number of LaTeX digits
'$1.248 \\times 10^{12} \\; \\mathrm{\\frac{pc}{Myr}}$'

Because numpy.ndarray does not accept most format specifiers, using specifiers like .3f will not work when applied to a numpy.ndarray or non-scalar Quantity. Use numpy.array_str() instead. For instance:

>>> import numpy as np
>>> q = np.linspace(0,1,10) * u.m
>>> f"{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().

A UnitBase, or the unit part of a Quantity, can also be formatted in a number of different styles. By default, the string format used is the “generic” format, which is based on syntax of the FITS standard format for representing units, but supports all of the units defined within the astropy.units framework, including user-defined units. The format specifier (and UnitBase.to_string()) functions also take an optional parameter to select a different format:

>>> q = 10 * u.km
>>> f"{q:latex}"
'$10 \\; \\mathrm{km}$'
>>> fluxunit = u.erg / (u.cm ** 2 * u.s)
>>> f"{fluxunit}"
'erg / (cm2 s)'
>>> print(f"{fluxunit:console}")
 erg
------
s cm^2
>>> f"{fluxunit:latex}"
'$\\mathrm{\\frac{erg}{s\\,cm^{2}}}$'
>>> f"{fluxunit:>20s}"
'       erg / (cm2 s)'

The 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')
'$\\mathrm{\\frac{erg}{s\\,cm^{2}}}$'

Converting from Strings

Units can also be created from strings in a number of different formats using the Unit class:

>>> 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)")

It is also possible to create a scalar Quantity from a string:

>>> u.Quantity("3m/s")
<Quantity 3. m / s>

Note

Converting from strings requires the use of a specialized parser for the unit language, which results in a performance penalty. It is much faster to use UnitBase 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:

    >>> f"{fluxunit:latex}"
'$\\mathrm{\\frac{erg}{s\\,cm^{2}}}$'

    which renders as

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

  • "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')
'$\\mathrm{erg\\,s^{-1}\\,cm^{-2}}$'

    which renders as

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

  • "console": Writes a multiline 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
2.1798724×10⁻¹⁸ ─────
                 s²

Dealing with Unrecognized Units

Since many files 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. In addition, it provides tools for transforming non-standard, legacy or misspelt unit strings into their standardized form, preventing the further propagation of these unit strings.

By default, 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? If this is meant to be a custom unit, define it with
'u.def_unit'. To have it recognized inside a file reader or other
code, enable it with 'u.add_enabled_units'. For details, see
https://docs.astropy.org/en/latest/units/combining_and_defining.html

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

By either adding additional unit aliases for the misspelt units with set_enabled_aliases() (e.g., ‘Angstroms’ for ‘Angstrom’; as demonstrated below), or defining new units via def_unit() and add_enabled_units(), we can use parse_strict='raise' to rapidly find issues with the units used, while also being able to read in older datasets where the unit usage may have been less standard.

Examples

To set unit aliases, pass set_enabled_aliases() a dict mapping the misspelt string to an astropy unit. The following code snippet shows how to set up Angstroem -> Angstrom:

>>> u.set_enabled_aliases({"Angstroem": u.Angstrom})
<astropy.units.core._UnitContext object at 0x...>
>>> u.Unit("Angstroem")
Unit("Angstrom")
>>> u.Unit("Angstroem") == u.Angstrom
True

You can also set multiple aliases up at once or add to existing ones:

>>> u.set_enabled_aliases({"Angstroem": u.Angstrom, "Angstroms": u.Angstrom})
<astropy.units.core._UnitContext object at 0x...>
>>> u.add_enabled_aliases({"angstroem": u.Angstrom})
<astropy.units.core._UnitContext object at 0x...>
>>> u.Unit("Angstroem") == u.Unit("Angstroms") == u.Unit("angstroem") == u.Angstrom
True

The aliases can be reset by passing an empty dictionary:

>>> u.set_enabled_aliases({})
<astropy.units.core._UnitContext object at 0x...>

You can use both set_enabled_aliases() and add_enabled_aliases() as a context manager, limiting where a particular alias is used:

>>> with u.add_enabled_aliases({"Angstroem": u.Angstrom}):
...     print(u.Unit("Angstroem") == u.Angstrom)
True
>>> u.Unit("Angstroem") == u.Angstrom
Traceback (most recent call last):
  ...
ValueError: 'Angstroem' did not parse as unit: At col 0, Angstroem is not
a valid unit. Did you mean Angstrom, angstrom, mAngstrom or mangstrom? If
this is meant to be a custom unit, define it with 'u.def_unit'. To have it
recognized inside a file reader or other code, enable it with
'u.add_enabled_units'. For details, see
https://docs.astropy.org/en/latest/units/combining_and_defining.html

To pass an unrecognized unit string:

>>> x = u.Unit("Angstroem", format="fits", parse_strict="warn")  
UnitsWarning: 'Angstroem' did not parse as fits unit: At col 0, Unit
'Angstroem' not supported by the FITS standard. Did you mean Angstrom or
angstrom? If this is meant to be a custom unit, define it with 'u.def_unit'.
To have it recognized inside a file reader or other code, enable it with
'u.add_enabled_units'. For details, see
https://docs.astropy.org/en/latest/units/combining_and_defining.html

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.