KeyError using Astropy .differentials code

Question

I keep getting KeyError: 'd_ρ' on my d_rho = ... line. I am trying to run the differentials so that I can put them in a table column (or single column per variable), to use for calculating velocity. Any thoughts on why I keep getting the KeyError?

# Accessing the differentials in table format
eq_cyl_diffs = eq_coord.cylindrical.differentials

# Extract differentials from the cylindrical coordinates
d_rho = eq_coord.cylindrical.differentials['d_ρ']
d_phi = eq_coord.cylindrical.differentials['d_ϕ']
d_z = eq_coord.cylindrical.differentials['d_z']

GAIA1A['d_ρ'] = d_rho
GAIA1A['d_ϕ'] = d_phi
GAIA1A['d_z'] = d_z
GAIA1A['d_ρ', 'd_ϕ', 'd_z'][0:1]

#d_rho, d_phi, d_z = eq_coord.cylindrical.differentials.d_rho, eq_coord.cylindrical.differentials.d_phi, eq_coord.cylindrical.differentials.d_z

Without knowing what exactly is eq_coord and eq_coord.cylindrical (provide at least the class names), and without seeing the exact error (i.e., the traceback), this is hard to answer. — 9769953
– 9769953, Commented Jul 21 at 12:20

Matt Pitkin · Accepted Answer · 2025-08-21 13:34:05Z

0

From looking at the documentation for the CylindricalDifferential class, my educated guess would be that you need to use:

d_rho = eq_coord.cylindrical.differentials['d_rho']
d_phi = eq_coord.cylindrical.differentials['d_phi']
d_z = eq_coord.cylindrical.differentials['d_z']

as the classes attributes do not use the Greek symbols ρ and ϕ in their names.

answered Aug 21 at 13:34

Matt Pitkin

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