Astropy

7.2.0 · active · verified Thu Apr 09

Astropy is a community-developed core Python package for astronomy and astrophysics. It provides a wide range of tools for common astronomical tasks, including units, coordinates, FITS file I/O, and cosmology. The library is actively maintained, with major releases annually, minor releases every six months, and bugfix releases as needed. The current version is 7.2.0.

Warnings

breaking In Astropy 8.0, deprecated module-level shim files in `astropy.cosmology` were removed. All cosmology classes and functions must now be imported directly from `astropy.cosmology`.
Fix: Update import statements from `from astropy.cosmology.flrw import FlatLambdaCDM` to `from astropy.cosmology import FlatLambdaCDM`.
gotcha Anaconda users upgrading Astropy with `pip` can result in a corrupted installation. It is strongly recommended to use `conda` for updates in Anaconda environments.
Fix: Always update Astropy in Anaconda using `conda update astropy` instead of `pip install astropy --upgrade`.
gotcha Astropy `Quantity` objects can sometimes lose their units when used with certain NumPy operations or when broadcasted in specific scenarios. Careful attention is needed to ensure unit preservation.
Fix: Explicitly re-apply units or convert to base units before operations where units might be lost. Check `astropy.units` documentation for 'Known Issues' and 'Performance Tips' regarding `Quantity` behavior.
gotcha `numpy.prod` cannot be directly applied to `Quantity` instances because the resulting unit depends on the input shape, making correct unit handling ambiguous.
Fix: Convert `Quantity` to its value using `.value` before passing to `numpy.prod` if units are not critical for the intermediate step, or perform a manual loop for unit-aware multiplication if necessary.
deprecated The `astropy.test` runner and related API (`astropy.tests.runner.keyword`, `TestRunnerBase`, `TestRunner`) are pending deprecation and will eventually be removed. Downstream packages using `packagename.test` generated with `TestRunner` may also be affected.
Fix: Migrate testing workflows to use `pytest` directly or follow updated guidelines in Astropy's developer documentation for testing.
gotcha When defining custom logarithmic units, `u.def_unit` should be avoided as it can lead to unexpected behavior. Logarithmic units should be defined directly.
Fix: Define logarithmic units directly, e.g., `dBW = u.dB(u.W)` instead of `dBW = u.def_unit('dBW', u.dB(u.W))`.

Install

pip install astropy[recommended] Install with recommended dependencies
conda install astropy Install with Conda (recommended for Anaconda users)

Imports

units
```
from astropy import units as u
```
The conventional and recommended alias for units is 'u'.
coordinates
```
from astropy import coordinates as coord
```
The conventional and recommended alias for coordinates is 'coord'.

SkyCoord

from astropy.coordinates import SkyCoord

Table
```
from astropy.table import Table
```
fits
```
from astropy.io import fits
```
Avoid wildcard imports; import the 'fits' sub-package directly or alias it if replacing old 'pyfits'.
constants
```
from astropy import constants as const
```
The conventional and recommended alias for constants is 'const'.
cosmology (classes/functions)
```
from astropy.cosmology import FlatLambdaCDM, z_at_value
```
As of Astropy 8.0, cosmology classes and functions must be imported directly from `astropy.cosmology`.

Quickstart

This quickstart demonstrates key Astropy functionalities: defining celestial coordinates with `SkyCoord`, performing unit-aware calculations using `Quantity` and `astropy.units`, accessing physical constants via `astropy.constants`, and working with tabular data using `astropy.table`.

import numpy as np
from astropy import units as u
from astropy import constants as const
from astropy.coordinates import SkyCoord
from astropy.table import Table

# Define a SkyCoord object
pos = SkyCoord(ra=10.68458 * u.deg, dec=41.26917 * u.deg, frame='icrs')
print(f"Sky position: {pos}\n")

# Create a Quantity and perform a calculation
mass = 5.972e24 * u.kg # Earth's mass
radius = 6371 * u.km # Earth's radius
density = mass / (4/3 * np.pi * radius**3)
print(f"Calculated density: {density.to(u.g / u.cm**3):.2f}\n")

# Access a physical constant
c = const.c
print(f"Speed of light: {c.to(u.km/u.s):.0f}\n")

# Create a simple table
data = [
    (1, 'NGC 10', 12.5),
    (2, 'M 31', 3.4),
    (3, 'Abell 2744', 16.0)
]
t = Table(rows=data, names=('id', 'name', 'magnitude'))
print("Example Table:")
print(t)

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