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[MNT]: ListedColormap inconsistencies #28763
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There is also an odd behavior that if you pass a non-iterable as colors with a non-None N instantiation succeeds, but it is super broken:
n [3]: ListedColormap(object(), N=2) Out[3]: <matplotlib.colors.ListedColormap at 0x7ca5b00d4ce0> In [4]: ListedColormap(object(), N=2).colors --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) Cell In[4], line 1 ----> 1 ListedColormap(object(), N=2).colors AttributeError: 'ListedColormap' object has no attribute 'colors' In [5]: ListedColormap(object(), N=2)(.5) --------------------------------------------------------------------------- AttributeError Traceback (most recent call last) Cell In[5], line 1 ----> 1 ListedColormap(object(), N=2)(.5) File /usr/lib/python3.12/site-packages/matplotlib/colors.py:725, in Colormap.__call__(self, X, alpha, bytes) 702 r""" 703 Parameters 704 ---------- (...) 722 RGBA values with a shape of ``X.shape + (4, )``. 723 """ 724 if not self._isinit: --> 725 self._init() 727 xa = np.array(X, copy=True) 728 if not xa.dtype.isnative: 729 # Native byteorder is faster. File /usr/lib/python3.12/site-packages/matplotlib/colors.py:1175, in ListedColormap._init(self) 1173 def _init(self): 1174 self._lut = np.zeros((self.N + 3, 4), float) -> 1175 self._lut[:-3] = to_rgba_array(self.colors) 1176 self._isinit = True 1177 self._set_extremes() AttributeError: 'ListedColormap' object has no attribute 'colors'Let's deprecate scalar/string argument. I don't expect is is widely used, and ListedColormap([0.3]*7) is simple enough (we should not make additional APIs just because somebody may not know Python).
Reacted by Jody Klymak and Thomas A CaswellDoes that mean that we should deprecate the N argument as well? As colorlist[:N] is as simple? Or is the islice/cycler combo doing something more clever than that?
I believe N can go as well. Truncating colors by the user is simple. Repeating a short list multiple times should be the very exception (and should possibly have required explicit user consent as duplicate colors make the mapping ambiguous).
- added 2 commits that reference this issue on Nov 12, 2024
The parameter N is deprecated in #29135. When the deprecation expires, N will be gone, which resolves this issue.
I'm closing as there are no further actions needed. The deprecation will guide people away from the API in question. And when expiring the topic is done.
- added a commit that references this issue on Mar 5, 2026
- added 2 commits that reference this issue on Mar 20, 2026
Summary
The documentation to ListedColormap states that the input should be a list or array
matplotlib/lib/matplotlib/colors.py
Lines 1175 to 1177 in b01462c
However, when specifying N, str and float are also supported (if one use the source code)
matplotlib/lib/matplotlib/colors.py
Lines 1199 to 1214 in b01462c
This means that, e.g., ListedColormap("#aabbcc", N=1) works, but ListedColormap("#aabbcc") does not (there will be weird errors later, like N=7 for the latter). Given that there is monochrome attribute and the documentation of N, one may expect the latter to work as well (if the earlier works). Also, ListedColormap(["#aabbcc"]) will not set the monochrome attribute correctly.
Proposed fix
I think there are three(?) possible solutions:
Bonus: document the color and monochrome attributes