fix: add matplotlib as optional dependency for skymaps plots

CHANGELOG.md

@@ -21,6 +21,10 @@
 * `nested.healpix_to_lonlat`, failed into rust panic for `dx=1` or `dy=1`. This is
   now indicated in the documentation and is catched in a `ValueError` on the python side.
 
+### Changed
+
+* `matplotlib` is now an optional dependency, to plot previews of skymaps.
+
 ## 0.6.5
 
 Released 2023-11-28

pyproject.toml

@@ -18,6 +18,9 @@ description = "A healpix manipulation library."
 readme = "README.md"
 keywords = ["astronomy", "healpix"]
 
+[project.optional-dependencies]
+plot = ["matplotlib"]
+
 [project.urls]
 repository = "https://github.com/cds-astro/cds-healpix-python"
 documentation = "https://cds-astro.github.io/cds-healpix-python/"

python/cdshealpix/skymap/skymap.py

@@ -10,7 +10,10 @@
 from pathlib import Path
 from typing import Union
 
-import matplotlib.pyplot as plt
+try:
+    import matplotlib.pyplot as plt
+except ImportError:
+    _matplotlib_missing = True
 import numpy as np
 
 
@@ -140,6 +143,12 @@ def quick_plot(self, *, size=256, convert_to_gal=True, path=None):
             If different from none, the image will not only be displayed, but also saved
             at the given location. By default None
         """
+        if _matplotlib_missing:
+            raise ModuleNotFoundError(
+                "matplotlib is mandatory to use 'quick_plot'. "
+                "See https://matplotlib.org/ for installation "
+                "instructions."
+            )
         img = cdshealpix.pixels_skymap(self.values, size, convert_to_gal)
         fig = plt.imshow(img)
         plt.axis("off")

requirements-dev.txt

@@ -1,5 +1,6 @@
 astropy<5.3; python_version == '3.8'
 astropy; python_version > '3.8'
+astropy_healpix
+matplotlib
 pre-commit==2.21.*
 pytest
-astropy_healpix