From 4b501ca8eebaa1ee8bb01f7f7cdcd07bf4031041 Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Tue, 1 Jul 2025 12:31:02 +0200
Subject: [PATCH] Autodetect dimension from units.

This lets one write `ScaleBar(1, "kpc")` instead of the slightly
redundant `ScaleBar(1, "kpc", "astro-length")`.

Note that changes only occur in the constructor; "auto" is not when
later calling set_dimension (directly or via the property) because that
property's mutable semantics seem a bit fuzzy anyways: what happens if
one sets the dimension to something incompatible with the units?  I
would believe `ScaleBar.dimension` should be a readonly property
directly derived from `ScaleBar.units`.
---
 matplotlib_scalebar/scalebar.py | 12 ++++++++++--
 tests/test_scalebar.py          |  5 +++++
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/matplotlib_scalebar/scalebar.py b/matplotlib_scalebar/scalebar.py
index efd5a5d..edfc28d 100644
--- a/matplotlib_scalebar/scalebar.py
+++ b/matplotlib_scalebar/scalebar.py
@@ -174,7 +174,7 @@ def __init__(
         self,
         dx,
         units="m",
-        dimension="si-length",
+        dimension="auto",
         label=None,
         length_fraction=None,
         height_fraction=None,
@@ -230,6 +230,7 @@ def __init__(
                 * ``:const:`pixel-length```: scale bar showing px, kpx, Mpx, etc.
                 * ``:const:`angle```: scale bar showing \u00b0, \u2032 or \u2032\u2032.
                 * a :class:`matplotlib_scalebar.dimension._Dimension` object
+                * ``:const:`auto``` (the default): autodetect dimension based on *units*
         :type dimension: :class:`str` or
             :class:`matplotlib_scalebar.dimension._Dimension`
 
@@ -355,7 +356,14 @@ def __init__(
             raise ValueError("loc and location are specified and not equal")
 
         self.dx = dx
-        self.dimension = dimension  # Should be initialize before units
+        if dimension == "auto":
+            for cls in _DIMENSION_LOOKUP.values():
+                dimension = cls()
+                if dimension.is_valid_units(units):
+                    break
+            else:
+                raise ValueError(f"Invalid unit ({units})")
+        self.dimension = dimension  # Should be initialized before units
         self.units = units
         self.label = label
         self.length_fraction = length_fraction
diff --git a/tests/test_scalebar.py b/tests/test_scalebar.py
index 1093296..6336ead 100644
--- a/tests/test_scalebar.py
+++ b/tests/test_scalebar.py
@@ -398,3 +398,8 @@ def test_info():
 
     plt.close()
     del fig
+
+
+def test_auto_dimension():
+    assert ScaleBar(1, "kpc").dimension.base_units == "pc"
+    assert ScaleBar(1, "1/um").dimension.base_units == "1/m"