ScalarMappable should copy its input and allow non-arrays · Issue #18841 · matplotlib/matplotlib · GitHub

Problem

Recently I wanted to draw some colormapped vlines. As LineCollection inherits from ScalarMappable, one can indeed write e.g.

from pylab import *
xs = vals = np.arange(10)
plt.vlines(xs, 0, 1, array=vals)
plt.show()

and get

Unfortunately,

The array kwarg only accepts ndarrays, and not e.g. nested lists: plt.vlines(np.arange(10), 0, 1, array=[*range(10)]) crashes (at draw time, which is even worse) with "AttributeError: 'list' object has no attribute 'ndim'". This is unlike most other Axes/pyplot APIs (e.g. imshow), which happily accept lists (nested/2D lists, for imshow).
The array kwarg is not copied, so later changes to it get reflected back into the artist (i.e. doing vals[5] = 45 after the call to vlines still affects the line colors). Again, this is unlike other Axes/pyplot APIs (e.g. imshow), which insulate the artist from later changes to the input.

Proposed Solution

Make a copy of the input and cast it to arrays. (Well, modulo unit handling...)

Additional context and prior art

Behave like imshow.

I'd love to take this up! :)
Initial scanning leads to the following points:

In case of imshow, _ImageBase class inherits from ScalarMappable, but overrides its set_array function as:

matplotlib/lib/matplotlib/image.py

Lines 733 to 743 in 59bfcfa

    
               def set_array(self, A): 
        
                   """ 
        
                   Retained for backwards compatibility - use set_data instead. 
        
                   Parameters 
        
                   ---------- 
        
                   A : array-like 
        
                   """ 
        
                   # This also needs to be here to override the inherited 
        
                   # cm.ScalarMappable.set_array method so it is not invoked by mistake. 
        
                   self.set_data(A)

imshow doesn't raise this error as its set_data (called from set_array) copies the original array and checks for numpy casts:

matplotlib/lib/matplotlib/image.py

Lines 696 to 701 in 59bfcfa

    
           self._A = cbook.safe_masked_invalid(A, copy=True) 
        
           if (self._A.dtype != np.uint8 and 
        
                   not np.can_cast(self._A.dtype, float, "same_kind")): 
        
               raise TypeError("Image data of dtype {} cannot be converted to " 
        
                               "float".format(self._A.dtype))

In case of vlines, ScalarMappable is inherited again, but there's no overriding its set_array function, which evidently just accepts ndarray:

matplotlib/lib/matplotlib/cm.py

Lines 363 to 372 in 59bfcfa

    
               def set_array(self, A): 
        
                   """ 
        
                   Set the image array from numpy array *A*. 
        
                   Parameters 
        
                   ---------- 
        
                   A : ndarray 
        
                   """ 
        
                   self._A = A 
        
                   self._update_dict['array'] = True

To solve this, I added set_array to Collection class, overriding it again. It will need some reviews, I'll issue a PR for the same.

ScalarMappable should copy its input and allow non-arrays · Issue #18841 · matplotlib/matplotlib · GitHub

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ScalarMappable should copy its input and allow non-arrays #18841

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	def set_array(self, A):
	"""
	Retained for backwards compatibility - use set_data instead.

	Parameters
	----------
	A : array-like
	"""
	# This also needs to be here to override the inherited
	# cm.ScalarMappable.set_array method so it is not invoked by mistake.
	self.set_data(A)

	self._A = cbook.safe_masked_invalid(A, copy=True)

	if (self._A.dtype != np.uint8 and
	not np.can_cast(self._A.dtype, float, "same_kind")):
	raise TypeError("Image data of dtype {} cannot be converted to "
	"float".format(self._A.dtype))

	def set_array(self, A):
	"""
	Set the image array from numpy array A.

	Parameters
	----------
	A : ndarray
	"""
	self._A = A
	self._update_dict['array'] = True

ScalarMappable should copy its input and allow non-arrays · Issue #18841 · matplotlib/matplotlib · GitHub

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ScalarMappable should copy its input and allow non-arrays #18841

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Problem

Proposed Solution

Additional context and prior art

Activity

aitikgupta commented on Nov 2, 2020

aitikgupta commented on Nov 2, 2020

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